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Inline speed skating

Inline speed skating is the roller sport of racing on inline skates. The sport may also be called inline racing or speed skating by participants. Although it primarily evolved from racing on traditional roller skates, the sport is similar enough to ice speed skating that many competitors are known to switch between inline and ice speed skating according to the season.

Skate
An inline speed skate is a specialized shoe version of the inline skate. The boot or shoe is close-fitting, without much padding and usually made of leather, carbon fiber, and/or fiberglass composites. For best performance, the boot must conform closely to the shape of the foot, so most inline speed skating boots are custom-fitted or else heat-moldable.

Speed skating boots are low-cut and offer little ankle support, allowing the skater extra ankle movement. Skin blisters due to friction can be a problem, and common solutions include neoprene or silicone "ankle bootee" such as "Ezeefit" or "Bunga Pads"; double thin synthetic socks; smaller boots; improving technique; re-moulding the boots; sports tape; and use of "advanced healing" plasters to help recovery.

The frame (sometimes called the chassis or plate) that holds the wheels may be made of aircraft-quality aluminum, magnesium, or possibly carbon fiber. Frames flex during skating, and the amount of flex can be a personal factor in which frame choice to use. Very "stiff" frames may be favored by heavy skaters. A frame that is too stiff for a particular skater may feel unstable on corners, while a frame that is not stiff enough will be slower. Frame stiffness also works along with boot and wheel stiffness, so there are many possible variations. Nevertheless, a light frame is desirable. Ideal frame length is affected by foot size and wheel size. A slightly shorter frame may be preferred for the tight curves of smaller tracks; a longer frame may be preferred for longer distance events.

The frame position can usually be adjusted with respect to the skate to adjust for a skater's individual foot, ankle and leg characteristics. Frame positioning is very critical as even a minor change from the skater's actual frame position can lead to severe foot pain. Also, many times it leads to 'locking' of the skater's ankle and/or calf muscle, thereby restricting its movements. It may take a skater several days to weeks to test and adjust the frame position of his new skates. The common inline mounting is 195mm, which is different from the ice mounting of 165mm. The frame usually mounts three, four, or five polyurethane wheels. The three wheel frames are used by skaters with small feet, otherwise four-wheel frames are commonly used, with 90 mm to 110 mm diameter wheels. Five-wheel frames with smaller wheel have lost favor. Each wheel contains two ball bearings with an aluminum spacer, held in place with an axle screwed into the frame.

Larger wheels require better skating technique, so skaters generally progress upwards in wheel size as they gain experience. "Hi-Lo" arrangements are also available, which usually have three larger wheels and one smaller wheel under the ball of the foot, allowing a lower and shorter overall frame design.

In 2014 Powerslide (a German inline skate company) introduced a 125mm wheel for use on a three-wheeled frame varying in sizes from 11.8" to 13.0". Much controversy surrounded this development since FIRS did not allow 125mm wheels at the 2014 and 2015 world championships. On January 18 of 2016 FIRS released a press release that stated: "Dear Friends, Considering the evolution and growth that our sport has attained in the last years, the FIRS and the Speed Technical Committee have decided to allow, starting from February 1, 2016, the use of the wheels up to a maximum size of 125mm but only for the Marathons (JUNIOR and SENIOR) and the MASTER Category (MEN and LADIES). We will be grateful for the spread of this information and we take this opportunity to send you. Kind regards, Jorge Roldan, FIRS Speed Technical Committee, Chairman & Robert Marotta FIRS Secretary General."

Harder wheels minimize elastic hysteresis energy absorption, due to skater's weight deforming the solid polyurethane "tyre". So, speed skaters tend to select the hardest possible wheels, with the highest polyurethane durometer for their skating condition, limited by either wheel slip or surface roughness. Durometer selection is also affected by skater weight, and temperature. Wheels for indoor use are hardest with a durometer of 88–97. They tend to last well, but can be easily damaged if used outdoors. Wheels for outdoor use are softer with a durometer of 82–87, and tend to wear more quickly. Harder outdoor wheels can also be used effectively indoors. Skaters sometimes combine different hardness wheels on the same skate in an attempt to achieve the best combination.

Skaters also consider wheel "rebound". This refers to the relative height to which a dropped wheel rebounds. It is a reasonable comparative indicator of the relative energy absorbed by elastic hysteresis of a wheel during skating.

Bearing sizes have been standardized around the popular 608 series. A smaller and lighter 688 series has had limited acceptance. Bearing manufacturing precision generally run from ABEC-1 to ABEC-11, and some skate bearings are additionally designed to be "loose" to minimize ball rolling friction.

Various grades of steel offer better hardness, rust resistance etc. Bearings with ceramic balls (and races) have been available since the late 1990s. They are lighter and longer lasting, however significantly more expensive. Black silicon nitride ceramic is superior to white zirconium dioxide ceramic, since it is considerably harder and tougher. At the modest rotational speeds encountered in skates, manufacturer data suggests negligible difference in friction performance between the various bearing materials. At these speeds, ball bearing friction tends to be dominated by seals and lubricants.

Bearing shields reduce the entry of dirt into the bearing. Metal and rubber non-contact shields are commonly used, of which rubber shields are slightly more effective. Neither shield type is totally effective, often resulting in the need for bearing maintenance. The ball retainer is usually made of either metal, plastic, or glass. Plastic types are preferred since they are quieter.

Bearing lubrication is usually either light oil or grease. Synthetic types last longer before breaking down. Grease assists in holding dirt away, and stays in the bearing longer, reducing maintenance and increasing bearing life. The lifetime of bearings used for outdoor speed skating is often quite limited due to damage caused by dirt ingress. These bearings are usually cleaned by soaking them in petrol overnight and then cleaning the dirt.

In search of the maximum speed the principal goal is to minimize wind resistance, hence the use of skinsuits, special helmets and techniques. The second issue is elastic hysteresis energy absorption in the wheel. The distant third is bearing internal friction, a set of bearings in good condition, properly inserted and lubricated is normally enough.

Technique and control
Mechanically, strokes in speed skating are deeper and faster (to a sharper angle, closer to the point of losing traction) than recreational skating but not as deep or as fast as in ice speed skating. This is because of the greater frictional forces in the direction of travel and lesser ability to apply friction without slipping of wheels on a hard surface compared to a blade on ice.

Speedskaters move each foot across the center line of travel, leading to the double push, a method named by United States skater Chad Hedrick. The technique allows two pushes in each stroke of the skate. However, it can be tiring for inexperienced skaters who have improper technique, and they will often save it until needed, such as the latter stages or final sprint of a distance race. With proper execution, the double push is an energy saver. The double push is mostly used in outdoor racing and the straightaways of indoor skating.

During sprints, skaters need to initially push their body with force to get a jump start. For this they tend to take quick and sharp strides. Once they have achieved a good pace, they move to taking comparatively longer strides. Speedskaters usually bend from their knees to maintain a low posture. This has two advantages. First of all, it increases the reach of the skater's legs, which results into even more powerful strides. Secondly, a low posture reduces the total surface area of the skater's body that comes in contact with the resistive air, thus providing an aerodynamic advantage.

Turning is significantly more difficult with inline speed skates than recreational skates because of more and larger wheels, creating a longer wheelbase. The wheel profile, that is, the cross-section, is parabolic, with a sharper shape than recreational or aggressive wheels, allowing the skater to essentially skate on a smaller, and hence more agile, wheel when leaned over in a turn.

Brakes are not used on speed skates, so various other techniques to slow down are used, such as slaloming (skating s-curves) or v-plowing (or "snow-plowing"), where the heels are pushed outward and the toes inward. The v-plow is often the stop used in situations where there is little lateral and forward room to stop. One technique is the T-stop, essentially dragging one foot perpendicular to and behind the other, however this wears the wheels of that skate quickly. Another stop involves picking up one foot and setting it down quickly and repeatedly somewhat perpendicular to the forward motion while keeping weight on the other foot. Hockey stops are possible on speed skates, but require a very deep lean in order to cause the wheels to lose traction and slide, also the fact that wheels are sliding means that the wheels are also wearing down very quickly. Grass runouts are always a last option, given an adjacent grassy area.

When switching over to ice, speed skaters are generally faster. While there are some small differences in technique, speed skaters already have the muscle they need to skate on ice.

An inline speedskater takes much time to stop and often has few options in an emergency. It typically takes several hundred feet on a level surface to come to a stop at a full, controlled deceleration. Thus, a skater should be familiar with and proficient in stopping techniques before attempting difficult situations such as heavily travelled roads or hills.

Training
Inline speed skating requires professional athletes to go through intense physical training. A strict diet and a rigorous training schedule has to be followed. The training schedule is mainly designed to build and maintain strong thighs and calves. But skating, just like swimming, requires the use of the whole body. Therefore, it is critical that the whole schedule is well balanced to attain and maintain a sturdy upper-body too. Also, a flexible upper body is preferred, which could help in maintaining the balance of the body in a better way. A heavy, protein-rich diet is required to be followed.

Usually, skaters have two sets of skates/wheels, one for training and the other for races and competitions. Wheels and bearings used for practice generally require much more efforts to gain some momentum and speed, as compared to the ones used in competitions. Ultra distance training requires years of training, time, and dedication to reach world class levels.

Tactics
In outdoor inline racing events, team tactics may apply. If so, tactics are similar to those of marathon ice speed skating and of road bicycle racing, in which members of the team perform specified roles.

Skaters tend to form packs or "pacelines", or "pelotons", in which skaters line up behind a lead skater and match their stride, thereby saving energy by skating in their draft. Sportsmanship requires that skaters in the paceline share the duty as paceline leader. Those who never "take a pull" at the front are likely to find other skaters tactically working together to defeat them.

During the course of a race, skaters may make "attacks", speeding up the pace in an effort to weed out the weaker and slower competition. These attacks may include "breakaways" and "fliers", in which skaters try to create new smaller and faster packs or else to escape entirely from the other skaters. Depending on the length of the race and the skills and the cooperative effort of the chasers, these breakaways may or may not prove successful. If a skater escapes a pack in order to join a successful breakaway group, it is known as "bridging up".

When skaters who are member of teams participate in a race together, they often have pre-determined roles. One or two would be designated attackers whose role it is to tire out the competition. Another skater may be the designated winner for the team, and he may avoid chasing any breakaways until late in a race, possibly until the final sprint if the lead pack has never broken up.

Quad speed skating
Quad roller-skating racing is the precursor to the popularity and acclaim received by competitive racing on in-line skates. Up until 1991, all World Championships were held on quad skates. Most events at the 1992 World Championships were specific to quads, however, some events were classed as "open" giving the athlete the option of choosing either quads or in-lines. The same criteria were applied for the 1993 World Championships. In 1994 all events were declared as "open". Despite this, it had soon become evident that in-lines were predominantly quicker than quads on all surfaces and all tracks and to this end athletes opted for in-lines over quads, as is still the case today.

Race venues and formats
Inline speed skating races are held in a variety of formats and on a variety of surfaces.

Indoor races are most common in the United States, which has a long tradition of racing on skates at rinks. The competitions are generally held at roller skating rinks with plastic-coated wood floors and, less commonly, a plastic coated cement floor. The track is about 100 m in circumference. At USA Roller Sports (USARS) events, tracks are marked by four pylons set in a parabolic oval, while at NIRA (National Inline Racing Association) events, tracks are marked by multiple pylons that create an oval shaped track. Events, or meets, are typically structured so that members of numerous age groups race in three or four distances. For the more populous divisions, there may be a series of heats in order to qualify for the final race. To some extent, indoor inline races are similar to short track speed skating.

Outdoor races may be held on regular pavement on city streets or park roads, or they may be held at specialized venues similar to velodromes, sometimes called patinodromes. A patinodrome is generally about 200 m in circumference and may be surfaced with asphalt, concrete or similar material. The curves may be banked. Such specialized skating tracks are relatively common in Europe but rare in the United States. The international governing body for World Roller Sports, Federation Internationale de Roller Sports (FIRS) and its technical committee, Committee International de Course (CIC), are making strides to commonise tracks used specifically for World Championships that have the same size, shape and surface. Plans for such tracks are available from FIRS upon request.

Race formats include:

 * Time trials: Held "against the clock", each skater races individually or in pairs over a distance of 100 m to 300 m, attempting to establish the best time. Time trials are occasionally held over longer distances, up to 100k which is 62 miles for fastest time. Another format measures how many miles one can achieve alone in 24 hours or 48 hours. All time trials are very physically demanding and not as popular as group racing. The time trial is the hardest of races, called "The Truth" because it's the true test of the individual with no help from drafting, teammates, or other competitors to achieve the fastest times. Bicycling has time trials in the Olympics and the Tour de France, for example. Sometimes time trial races can use individual pursuit format.
 * Sprint races: Four to eight competitors races on short distance (100m to 1000m) in a knock-out tournament.
 * Elimination races: In these moderate-distance races, also known as last man out, the hindmost skater is eliminated from the competition each time the pack of skaters complete a lap or when they complete certain specified lap numbers. At one or two laps to before the finish, the group has usually been pared down to four or five skaters. At this point, the first across the finish line is the winner.

 * Points races: In these moderate-distance races, the first, second and third skaters to cross the start/finish line at certain specified laps are awarded points. Laps late in the race are worth more points, with the final lap worth the most points of all. It is possible to win a points race without actually being the first to cross the finish line at the end.

 * Points-elimination races: A combination of elimination races and points races.

 * Relays: Relay events include teams of two to four skaters each. Indoor meets may include "mixed" relay events in which teams have either one female and one male OR two females and two males, but outdoor relays (usually held on tracks) are usually if not always single-sex events. In a mix relay, it is traditional that a female goes to the starting line as the first skater to race.

 * Criterium races: Instead of racing a specified distance or number laps, the skaters skate for a certain amount of time, then plus a (small) number of laps. The time is typically between 15 and 45 minutes, after which a bell is rung and the skaters informed the race is over when they skate one or two more laps around the course. The portion of the race skated after the bell is rung is known as the bell lap (or laps).

 * Distance races: Although events such as points-elimination races and criteriums may cover a distance of 10 to 25 km, a distance race usually refers to a race over a set distance of about 5 km or longer and without specialized points or elimination rules. The event may be truly point-to-point or may be held on a repeating course with a circumference of at least 1 km. Distance races are often marketed to the general populace and not just to members of inline racing clubs.

 * Marathons: Lately there is a new movement of skaters bringing big masses to events, these events are the skate marathons, 42.195 km. The most popular marathons in the USA are: The North Shore Marathon  and Saint Paul Inline Marathon, however they are now taking place all over the world including the Goodwood Roller Marathon in the UK.  These races gain more popularity everyday as skaters form friendships and bonds at these events.

 * Ultra Marathons: Ultra Marathons draw relatively large numbers. Given the time needed to complete such events, one could say that they are the equivalent to a running marathon. These events were very popular in the late 1990s but declined after the year 2001; there is a new movement of people keeping this events alive and bringing them back to the forefront of the speed skating world.

 * There are two very old and popular ultra marathons in the USA:
 * The New York City Skate Marathon and New York 100K The New York City Skate Marathon & NY 100K has been held since the early 1990s, with Brooklyn's Prospect Park hosting the event since the late 1990s. Various distances are competed at the event, but the event always includes a race of approximately 42 kilometers and another of 100 kilometers.
 * Athens to Atlanta Road Skate (The A2A) This is the longest running point to point event in the USA, with a maximum distance of 86.7 mi covering the distance from Athens, Georgia to Atlanta, Georgia.

 * In the early days of inline racing, sponsors of distance races were often also running event organizers, and the races they organized were commonly the same distances as those of running races, about 5–10 km. By the mid-1990s such events were proving to not be very popular and in the United States, where sales of inline skates were also beginning to slip, there was a decline in participation at races. However, at about that time in Europe, where inline skate sales were beginning to rise, race sponsors began to regularly organize longer events, particularly inline marathons. Such events proved to be enormously popular among fitness skaters, with some events such as the Berlin Inline Marathon (with more than 11.000 at its peak) and the Engadin Inline Marathon in St. Moritz, Switzerland, regularly attracting over 5000 skaters each year.

 * In about 2000 American event sponsors followed suit, and inline half-marathons and marathons were scheduled more and more frequently around the country. As in Europe the events proved a big draw with fitness skaters looking for events which would give their training a focus. However, by 2005 this surge was tempered as some major events were either postponed for a year or cancelled permanently. In the United States the most popular inline marathon has continued to be the NorthShore Inline Marathon in Duluth, Minnesota.

 * In 1999, a team of six British men led by Paul Robinson skated from Land's End to John O'Groats, a distance of 886 mi. This is the only known long-distance skating event held in the world to date.

 * Dryland triathlons: Occasionally organized by triathlon sponsors, these events substitute inline skating for the swimming component of the race. These events were infrequent even during the mid-1990s boom in inline skating participation. Today they are rare to non-existent.

 * Downhill races: An event most popular in the Alpine countries of Europe, these races are timed events down a steep course. The use of concrete bobsleigh courses in summertime is not uncommon. Racers usually skate alone and the event commonly uses the best time of two heats to establish the winner. Downhill inline racers usually wear skates much more like "regular" inline skates than inline speed skates, along with extensive body covering and protective gear, and strong helmets. They may reach speeds of up to 130 km/h. The International Inline Downhill Association (IIDA) is the largest organization for inline downhill racing, holding races on several continents.

Olympic status
Attempts by the world governing body for roller sports, the International Roller Sports Federation (FIRS), to gain Olympic status for any of its disciplines were distinctly insufficient in the closing decades of the 20th century. Most notably, it failed to capitalize when rink hockey (a form of roller hockey) appeared as a demonstration sport at the 1992 Summer Olympics in Barcelona.

Efforts by FIRS to obtain Olympic status became more coherent in about 2000, with inline speed skating promoted as the roller sport best suited for the Olympics. However, the federation faces competition from approximately 20 other sports also seeking entry into the Olympics, while at the same time the president of the International Olympic Committee has expressed a desire to reduce the size of the summer Olympic Games. Roller sports was a candidate sport for the 2016 Summer Olympics, following the drop of baseball and softball, but the Olympic Committee eventually chose rugby sevens and golf instead.

Notably, roller speed or in-line speed skating has been an included sport at the World Games since their inception in 1981.

title: Inline speed skating


text:
DICT

DICT is a dictionary network protocol created by the DICT Development Group in 1997, described by RFC 2229. Its goal is to surpass the Webster protocol to allow clients to access a variety of dictionaries via a uniform interface.

In section 3.2 of the DICT protocol RFC, queries and definitions are sent in clear-text, meaning that there is no encryption. Nevertheless, according to section 3.1 of the RFC, various forms of authentication (sans encryption) are supported, including Kerberos version 4.

The protocol consists of a few commands a server must recognize so a client can access the available data and lookup word definitions. DICT servers and clients use TCP port 2628 by default. Queries are captured in the following URL scheme: dict:// ; @ : /: : will compile a Unicode-compatible DICT file called mydict, with heading My Dictionary, from mydict.txt which is in Jargon File format i.e.:

:word2:definition 2 etc.

Once the dictionary file has been produced, it can be easily installed on a server with commands similar to this:

mv mydict.dict mydict.index /usr/share/dictd/ /usr/sbin/dictdconfig—write /etc/init.d/dictd restart

Format converters

 * Linguae Software is able to convert from/to wb, dict (stardict and dictd) csv, xdxf, txt, ini and ling (native) file formats, Linux, Windows and Mac OS X.
 * XDXF XML Dictionary Exchange Format converts between various dictionary formats using pluggable codec architecture.

dictzip
In order to efficiently store dictionary data, dictzip, an extension to the gzip compression format (also the name of the utility), can be used to compress a .dict file. Dictzip compresses file in chunks and stores the chunk index in the gzip file header, thus allowing random access to the data.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: DICT


text:
La Bolduc

Mary Rose-Anne Bolduc, born Travers, (June 4, 1894 – February 20, 1941) was a musician and singer of French Canadian music. She was known as Madame Bolduc or La Bolduc. During the peak of her popularity in the 1930s, she was known as the Queen of Canadian Folk Singers. Bolduc is often considered to be Quebec's first singer-songwriter. Her style combined the traditional folk music of Ireland and Quebec, usually in upbeat, comedic songs.

Childhood
Mary Rose-Anne Travers "La Bolduc" was born in Newport, Quebec, in the Gaspé region. Her father, Lawrence Travers, was an Anglophone of Irish heritage, and her mother, Adéline Cyr, was of mixed French Canadian and Mi'kmaq heritage. Her family included five full siblings, and an additional six half-siblings from her father's first marriage. Bolduc and her eleven siblings spoke English at home, but also spoke French fluently. The family was extremely poor, but Bolduc attended school for a time, becoming literate in French.

Her only music teacher was her father, who taught her how to play the instruments that were traditional in Quebec culture of the era: the fiddle, accordion, harmonica, spoons and jaw harp. She learned traditional music from the two heritages, both Irish melodies and French-Canadian folk tunes. The family did not own a record player, piano or sheet music, so Bolduc learned jigs and folk songs from memory or by ear. She was giving casual public performances by the spring of 1908, when she played the accordion at the logging camp where she worked as a cook and her father as a lumberjack.

Early working life
In 1908, at the age of thirteen, Bolduc was sent to live with her half-sister Mary-Ann in Montreal, Quebec. Mary-Ann worked as a maid and had secured Bolduc a job as a maid in the house of Dr Lesage. She was paid $15 per month, in addition to room and board. A few years later she took a job at a textile mill, which paid $15 weekly for 60 hours of work per week.

On August 17, 1914, she married Édouard Bolduc, a plumber. The couple's first child was stillborn. Denise Bolduc was born in July 1916, Jeannette Bolduc in July 1917 and Roger Bolduc in August 1918. Roger died at the age of ten months and Jeanette at two years. The couple experienced more bad fortune with pregnancies and children; of the twelve or thirteen pregnancies, only four children reached adulthood. The family was quite poor, and in 1921 when Édouard had difficulty finding work they decided to move to Springfield, Massachusetts. Édouard Bolduc's sister was living there at the time. They returned to Montreal a year later, as Édouard had been unable to secure work in Springfield. During this time Mary Bolduc entertained both friends and family with her musical talents. Among her friends were amateur musicians who sometimes performed with the Veillées du bon vieux temps at the Monument-National under Conrad Gauthier.

Musical career

When Conrad Gauthier's troupe was missing a folk violinist for a performance, one of Bolduc's friends arranged for her to fill in for the absent performer. Gauthier was suitably impressed by her performance and asked her to return for subsequent productions. The family was always in need of money and the small income she earned this way was useful. Bolduc became a regular player with Gauthier's troupe by 1928, playing the violin or the jaw harp. Her work with them expanded to include other instrumental work and even some comic acting.

Bolduc was recommended by folk singer Ovila Légaré to musical producer Roméo Beaudry of the Compo Company. Beaudry signed musicians for French language recordings on the Starr Records label. Beaudry signed her to a recording contract to make four 78 rpm records, paying her $25 per side. She made her first recording in April 1929, the French folk song Y'a longtemps que je couche par terre on side A, and an instrumental reel on side B. The record was a commercial flop. Her next two recordings also had meagre sales.

Bolduc's second recording was released for Christmas of 1929. The first side had an original song of Bolduc's, La Cuisinière. Side B was an adaptation of an English folk song titled Johnny Monfarleau. The record sold more than twelve thousand copies, which was unprecedented in Quebec. Bolduc earned a total of $450 from the sales and became a household name in Quebec. With this success, Beaudry had Bolduc releasing a double-sided record every month. Bolduc recorded an additional four songs in January 1930. Nine more songs were recorded in April that year. By the end of 1930, she had recorded more than 30 songs. During this time, she collaborated on no less than fifty-six recordings of other artists. Most of these recordings did not credit her. Bolduc sang accompaniments or played instruments for recordings by Juliette Béliveau, Eugène Daignault, Ovila Légaré, Alfred Montmarquette, Adélard St. Jean, and possibly others.

Bolduc's first headlining performance came in November 1930, in Lachute at a costume ball. The audience was extremely receptive to her music and she was inspired to start a show that focused on her own songs. In March 1931 she took an offer from a burlesque company at the Théâtre Arlequin de Québec to perform as their main act. From this, she embarked on a three-month tour of Quebec with Juliette d'Argère. Starting in Hull in May 1931, they travelled western Quebec and Montreal, finishing in Sept-Îles in July. In 1931, her rising popularity increased the cost of her sheet music from four for one dollar to three for one dollar.

Bolduc formed her own touring troupe in 1932, named La Troupe du bon vieux temps. She hired Jean Grimaldi to direct the tours. The performances contained elements of both vaudeville and traditional folk music. Their first tours were around the Montreal area, and from August through December 1932 they gave fifty shows. The tours were a great success in Quebec with Bolduc earning $2000 from her first tour, compared to $500 – $1000 annually she got from royalties. The troupe went on a tour of New England from April through June 1934, and a second tour of New England that autumn. They toured across Quebec, and in 1935 toured the French speaking areas of northern Ontario. They went on additional tours of New England in 1937 and 1939.

As the 1930s progressed, Bolduc's record sales began to slump, but her income from touring remained good. Bolduc stuck to her folk music style as the record buying public turned increasingly to jazz and popular music. She produced eighteen records in 1930 and 10 in 1931, but with her declining sales she recorded nothing from July 1932 until she released a single album in March 1935, and then four more in 1936. In 1936, the family was able to afford a nanny to attend to the children while Bolduc toured.

Beginning in 1935, her daughter Denise appeared with her as a pianist. Other children occasionally appeared as backup singers, and her daughter Lucienne recorded L'Enfant volé.

Bolduc was seriously injured in June 1937 in Rivière-du-Loup when her tour company's car was in a head-on collision. She suffered a broken leg, a broken nose and a concussion. She was sent to a hospital in Rimouski for treatment, where doctors discovered a cancerous tumour. She began radiation treatment at the Radium Institute in Montreal, and engaged in practically no musical endeavours at this point, making no stage appearances for a full year. Her insurance company refused to pay for her damages, notably the concussion that caused memory loss and loss of concentration, which prevented her from writing songs. The suit ended badly as Bolduc did not use banks and had no record of her income to prove loss of income. Of her total damages and lost income, Bolduc recovered only $1500.

Bolduc began limited touring again in the summer of 1938, only in the Montreal area. She made a radio broadcast in January 1939, and made two recordings in February 1939. One of those songs, Les Souffrances de mon accident (French "The sufferings of my accident") was on her accident. She died of cancer on February 20, 1941, in Montreal and was buried in the Cimetière Notre-Dame-des-Neiges.

Musical style

Bolduc never had any formal music lessons, and developed her own style under the influence of her father's teaching and the musical traditions of Irish folk music and Québécois folk tunes. Her songs tended to be happy and comical with lively rhythms. Her self-written songs often used existing melodies from folk tunes or dances, combined with lyrics she wrote herself. For instance, she wrote the song Les Cinq Jumelles about the Dionne Quintuplets, which was set to the tune of "Little Brown Jug". In other cases, she adapted popular contemporary American songs.

Bolduc often used the technique of the enumerative song, which lists something such as foods or tasks. This technique was traditional in French-Canadian folk songs, derived from similar French traditions. Bolduc also employed the traditional French folk song style of the dialogue song, usually a duet with a man, where the song is a conversation or debate between the man and the woman. One such song was Mademoiselle, dites-moi donc, which she recorded with Ovila Légaré and featured the two of them bantering and flirting comedically. She often wrote in the style derived from traditional English broadside ballads, which tell current news to the tunes of traditional songs. One such song by Bolduc is La chanson du bavard, which notably employs an introduction inviting the listener to hear a tale, as is common in broadside ballads. Other topical ballads by Bolduc include Les Américains about Americans coming to Montreal during Prohibition to obtain liquor and the unrecorded Si je pouvais tenir Hitler, which she wrote a few days after the outbreak of World War II.

Her music relied heavily upon the harmonica and the fiddle, the traditional instruments of reels in Quebec. Her singing also adopted a nasal style, and her pitch was relaxed, both of which are found traditionally. Her singing often featured turlutage, which derives from Irish and Scottish musical traditions. Most often she employed this technique in reels, such as her song Reel turluté.

Her touring troupe La Troupe du bon vieux temps gave fairly consistent performances. Mary Bolduc opened the show with her newest songs. The troupe then performed comedy sketches, ensemble songs, folk songs and vaudeville routines. Most performances included a segment where amateurs performed, sometimes for cash prizes. Bolduc closed with some of her newest or most topical songs.

Bolduc's lyrics are predominantly French, but frequently include a few words or lines in English. This was reflective of her country upbringing in Gaspésie, where the two languages mingle. The recordings were marketed to working class francophone audiences, in small towns and rural areas where people had traditional values.

Professional image

Singing and stage careers were not well regarded in Quebec society of the 1920s and 1930s, especially for women. To avoid gossip and to keep a good reputation, Bolduc was always credited under her married name Madame Édouard Bolduc, both at live performances and on recordings. She attempted to include her family in her activities as much as possible. Her husband Édouard accompanied her troupe on their 1932 and 1934 tours. Her eldest daughter Denise joined the troupe in 1935 as a pianist.

Recordings
Recordings of about 100 of her songs survive. Many of those that have not survived were written for special occasions.
 * April 12, 1929: Ya longtemps que je couche par terre and La Gaspésienne.
 * August 13, 1929: Gendre and belle-mère and Quand on s'est vu.
 * November 22, 1929: Valse Denise and Reel de la goélette.
 * December 6, 1929: La cuisinière and Johnny Monfarleau.
 * January 15, 1930: La servante and Regardez donc mouman.
 * January 29, 1930: Arthimise marie le bedeau and Tourne ma roulette.
 * March 11, 1930: Le bonhomme et la bonne femme and Si vous avez une fille qui veut se marier.
 * March 18, 1930: Reel comique and Galop des pompiers.
 * April 3, 1930: Le joueur de violon and Ton amour, ma Catherine.
 * April 30, 1930: Reel turluté, Gigue des commères (Duet with Alfred Monmarquette), Fantaisie écossaise (Duet with Alfred Monmarquette) and Reel Balmoral (Duet with Alfred Monmarquette).
 * May 14, 1930: Fricassez-vous, Valse turlutée and La morue.
 * May 20, 1930: Clogue à Ti-Zeph Parent (with Alfred Monmarquette and Adélard Saint-Jean), Reel des barbouillés (with Alfred Monmarquette and Adélard Saint-Jean).
 * June 18, 1930: Mon vieux est jaloux and La pitoune.
 * June 27, 1930: Un petit bonhomme avec un nez pointu and Chez ma tante Gervais.
 * August 21, 1930: Toujours "L'R-100" and Les maringouins.
 * September 23, 1930: Ça va venir découragez-vous pas and Fin Fin Bigaouette.
 * October 27, 1930: La bastringue and Mademoiselle, dites-moi donc.
 * November 4, 1930: Les agents d'assurance.
 * November 5, 1930: Chapleau fait son Jour de l'An (Duet with Ovila Légaré) and C'est comme ça qu'ça s'passe (Duet with Ovila Légaré).
 * November 13, 1930: Rouge carotte.
 * November 14, 1930: Le Jour de lAn and Le bas de Noël.
 * December 9, 1930: La petite boulangère (Duet with Ovila Légaré) and Mechetagouine (Duet with Ovila Légaré).
 * December 10, 1930: La grocerie du coin and La gigueuse (with Albertine Villeneuve and Thomas).
 * December 12, 1930: Le propriétaire.
 * January 15, 1931: Fêtons le Mardi-gras and Un vieux garçon gêné.
 * February 3, 1931: Les filles de campagne and Nos braves habitants.
 * March 26, 1931: Le sauvage du nord and Jean-Baptiste Beaufouette.
 * April 9, 1931: L'ouvrage aux Canadiens and La chanson du bavard.
 * July 7, 1931: C'est la fille du vieux Roupi and Il va m'faire mourir c'gars-là.
 * July 8, 1931: La côte Nord and Aux chauffeurs d'automobile.
 * September 15, 1931: Ah! C'qu'il est slow 'Tit Joe, Chanson de la bourgeoise and Le commerçant des rues.
 * October 8, 1931: Tit Noir a le mal imaginaire.
 * November 6, 1931: R'garde donc c'que t'as d'l'air and Danse en souliers d'boeufs.
 * November 7, 1931: Bien vite c'est le Jour de l'an and Voilà le père Noël qui nous arrive.
 * January 20, 1932: J'ai un bouton sur la langue, Rose cherche à se marier, Quand j'étais chez mon père and Les femmes.
 * May 5, 1932: L'enfant volé, Si les saucisses pouvaient parler, Les policemen and Les Américains.
 * July 2, 1932: En revenant des foins, Les conducteurs de chars, Les vacances and Sans travail.
 * March 6, 1935: Les cinq jumelles and La Gaspésienne pure laine.
 * March 20, 1936: Les colons Canadiens and La lune de miel.
 * April 15, 1936: Les médecins, Gédéon amateur and Les pompiers de St-Éloi.
 * April 27, 1936: Arrête donc, Mary.
 * August 24, 1936: Les belles-mères and Quand J'ai vingt ans.
 * February 23, 1939: Tout le monde a la grippe, Le voleur de poule, Je m'en vais au marché and Les souffrances de mon accident.

Unrecorded original songs
Songs known to have been written and performed by Mary Bolduc but never recorded include:
 * La Reine des Bières
 * As-tu vu l'éclipse?
 * Le nouveau gouvernement
 * La visite royale
 * Si je pouvais tenir Hitler
 * Roosevelt est un peu là
 * On déménage
 * Le secours direct

Legacy
Some debate exists among historians as to whether Mary Bolduc or Félix Leclerc should be identified as Quebec's first singer-songwriter. Either way, both had significant influence on the development of Quebec's folk music culture from the 1930s onwards. Bolduc was the most widely known folk music singer of Quebec in the 1930s. Her humorous images of daily life, her realism in depicting the society of the time, and her satirical characters all appear in the work of subsequent singer-songwriters. Although it was received poorly by critics of the day, Bolduc's use of colloquialisms and working-class vocabulary influenced future musicians like Gilles Vigneault and Clémence DesRochers. Other Québécois musicians with notable influences from Bolduc include Oscar Thiffault, whose style was descended from Bolduc, André Gagnon, whose composition Les Turluteries is based on Bolduc's use of turlutage, and Robert Charlebois, who also sang in the dual French Canadian and English derived style and used the everyday slang of Quebec.

On August 12, 1994, a stamp was released that honoured her with her portrait on a Canadian postage stamp. The stamp was designed by Pierre Fontaine based on images from Bernard Leduc. Seven and a half million copies were printed. The same year, a park was created in her hometown of Newport named Mary Travers Park.

In 2002, Mary Bolduc was made a MasterWorks honoree by the Audio-Visual Preservation Trust of Canada.

La Bolduc, a biographical film about Bolduc by director François Bouvier, was released in Quebec in April 2018. The film stars Debbie Lynch-White as Bolduc.

title: La Bolduc


text:
User:Coma

I am a sysop at the german wikipedia. I have written several articles about TV, graph theory, datastructure there.

title: User:Coma


text:
Skin effect

In electromagnetism, skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the conductor. It is caused by opposing eddy currents induced by the changing magnetic field resulting from the alternating current. The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth.

Skin depth depends on the frequency of the alternating current; as frequency increases, current flow becomes more concentrated near the surface, resulting in less skin depth. Skin effect reduces the effective cross-section of the conductor and thus increases its effective resistance. At 60 Hz in copper, skin depth is about 8.5 mm. At high frequencies, skin depth becomes much smaller.

Increased AC resistance caused by skin effect can be mitigated by using a specialized multistrand wire called litz wire. Because the interior of a large conductor carries little of the current, tubular conductors can be used to save weight and cost.

Skin effect has practical consequences in the analysis and design of radio-frequency and microwave circuits, transmission lines (or waveguides), and antennas. It is also important at mains frequencies (50–60 Hz) in AC electric power transmission and distribution systems. It is one of the reasons for preferring high-voltage direct current for long-distance power transmission.

The effect was first described in a paper by Horace Lamb in 1883 for the case of spherical conductors, and was generalized to conductors of any shape by Oliver Heaviside in 1885.

Cause

Conductors, typically in the form of wires, may be used to transmit electrical energy or signals using an alternating current flowing through that conductor. The charge carriers constituting that current, usually electrons, are driven by an electric field due to the source of electrical energy. A current in a conductor produces a magnetic field in and around the conductor. When the intensity of current in a conductor changes, the magnetic field also changes. The change in the magnetic field, in turn, creates an electric field that opposes the change in current intensity. This opposing electric field is called counter-electromotive force (back EMF). The back EMF is strongest at the center of the conductor, allowing current only near the outside skin of the conductor, as shown in the diagram on the right.

Regardless of the driving force, the current density is found to be greatest at the conductor's surface, with a reduced magnitude deeper in the conductor. That decline in current density is known as the skin effect and the skin depth is a measure of the depth at which the current density falls to 1/e of its value near the surface. Over 98% of the current will flow within a layer 4 times the skin depth from the surface. This behavior is distinct from that of direct current which usually will be distributed evenly over the cross-section of the wire.

An alternating current may also be induced in a conductor due to an alternating magnetic field according to the law of induction. An electromagnetic wave impinging on a conductor will therefore generally produce such a current; this explains the attenuation of electromagnetic waves in metals. Although the term skin effect is most often associated with applications involving transmission of electric currents, skin depth also describes the exponential decay of the electric and magnetic fields, as well as the density of induced currents, inside a bulk material when a plane wave impinges on it at normal incidence.

Formula
The AC current density $J$ in a conductor decreases exponentially from its value at the surface $J$$S$ according to the depth $d$ from the surface, as follows: $$J = J_\mathrm{S} \,e^{-{(1+j)d/\delta }}$$ where $$\delta$$ is called the skin depth which is defined as the depth below the surface of the conductor at which the current density has fallen to 1/e (about 0.37) of $J$S. The imaginary part of the exponent indicates that the phase of the current density is delayed 1 radian for each skin depth of penetration. One full wavelength in the conductor requires 2$\pi$ skin depths, at which point the current density is attenuated to e−2π (1.87×, or −54.6 dB) of its surface value. The wavelength in the conductor is much shorter than the wavelength in vacuum, or equivalently, the phase velocity in a conductor is very much slower than the speed of light in a vacuum. For example, a 1 MHz radio wave has a wavelength in vacuum $λ$$o$ of about 300 m, whereas in copper, the wavelength is reduced to only about 0.5 mm with a phase velocity of only about 500 m/s. As a consequence of Snell's law and this very tiny phase velocity in a conductor, any wave entering a conductor, even at grazing incidence, refracts essentially in the direction perpendicular to the conductor's surface.

The general formula for skin depth when there is no dielectric or magnetic loss is: $$\delta= \sqrt{ \frac{\, 2\rho \,}{\omega\mu } \;} \; \sqrt{\, \sqrt{1 + \left({\rho\omega\varepsilon}\right)^2 \;} + \rho\omega\varepsilon \;} ~$$

where

$ \rho = $ resistivity of the conductor

$ \omega =$ angular frequency of current $= 2\pi f,$ where $f$ is the frequency.

$ \mu =$ permeability of the conductor, $ \mu_r \, \mu_0 $

$ \mu_r =$ relative magnetic permeability of the conductor

$ \mu_0 =$ the permeability of free space

$ \varepsilon =$ permittivity of the conductor, $ \varepsilon_r \, \varepsilon_0 $

$ \varepsilon_r =$ relative permittivity of the conductor

$ \varepsilon_0 =$ the permittivity of free space. At frequencies much below $$1/(\rho \varepsilon)$$ the quantity inside the large radical is close to unity and the formula is more usually given as: $$\delta=\sqrt{\frac{\, 2\rho \,}{\omega\mu} \,} ~.$$

This formula is valid at frequencies away from strong atomic or molecular resonances (where $$\varepsilon$$ would have a large imaginary part) and at frequencies that are much below both the material's plasma frequency (dependent on the density of free electrons in the material) and the reciprocal of the mean time between collisions involving the conduction electrons. In good conductors such as metals all of those conditions are ensured at least up to microwave frequencies, justifying this formula's validity. For example, in the case of copper, this would be true for frequencies much below $Hz$.

However, in very poor conductors, at sufficiently high frequencies, the factor under the large radical increases. At frequencies much higher than $$1/(\rho \varepsilon)$$ it can be shown that skin depth, rather than continuing to decrease, approaches an asymptotic value: $$\delta \approx {2 \rho} \sqrt{\frac{\, \varepsilon \,}{ \mu }\,} ~.$$

This departure from the usual formula only applies for materials of rather low conductivity and at frequencies where the vacuum wavelength is not much larger than the skin depth itself. For instance, bulk silicon (undoped) is a poor conductor and has a skin depth of about 40 meters at 100 kHz ($λ$ = 3 km). However, as the frequency is increased well into the megahertz range, its skin depth never falls below the asymptotic value of 11 meters. The conclusion is that in poor solid conductors, such as undoped silicon, skin effect does not need to be taken into account in most practical situations: Any current is equally distributed throughout the material's cross-section, regardless of its frequency.

Round wire
When skin depth is not small with respect to the radius of the wire, current density may be described in terms of Bessel functions. The current density inside round wire away from the influences of other fields, as function of distance from the axis is given by:

$$ \mathbf{J}(r) = \frac{ k \mathbf{I} }{ 2 \pi R } \frac{ J_0(k r) }{ J_1(k R) } = \mathbf{J}(R) \frac{ J_0(k r) }{ J_0(k R) }$$

where $ \omega ={}$angular frequency of current = 2π × frequency

$ r = {} $distance from the axis of the wire

$ R = {} $radius of the wire

$ \mathbf{J}(r) = {} $current density phasor at distance, r, from the axis of the wire

$ \mathbf{J}(R) = {} $current density phasor at the surface of the wire

$ \mathbf{I} = $ total current phasor

$ J_0 = {}$Bessel function of the first kind, order 0

$ J_1 = {}$Bessel function of the first kind, order 1

$ k = \sqrt{ \frac { -j \omega \mu } { \rho }   } = \frac {  1-j } { \delta } $ the wave number in the conductor

$ \delta = \sqrt{ \frac {  2 \rho } { \omega \mu  }   }  $ also called skin depth.

$ \rho = {} $resistivity of the conductor

$ \mu_r = {} $relative magnetic permeability of the conductor

$ \mu_0 = {} $the permeability of free space = 4π × 10−7 H/m

$ \mu = \mu_r \mu_0 $ Since $$ k $$ is complex, the Bessel functions are also complex. The amplitude and phase of the current density varies with depth.

Derivation
Combining the electromagnetic wave equation and Ohm's law produces $$ \nabla^2\mathbf{J}(r) + k^2\mathbf{J}(r) = \frac{\partial^2}{\partial r^2}\mathbf{J}(r) + \frac{1}{r}\frac{\partial}{\partial r}\mathbf{J}(r) + k^2\mathbf{J}(r) = 0. $$ The solution to this equation is, for finite current in the center of the conductor, $$ \mathbf{J}(r) = \mathbf{C}J_0(kr), $$ where $$J_0$$ is a Bessel function of the first kind of order $$0$$ and $$\mathbf{C}$$ is a constant phasor. To satisfy the boundary condition for the current density at the surface of the conductor, $$\mathbf{J}(R),$$ $$\mathbf{C}$$ must be $$\frac{\mathbf{J}(R)}{J_0(kR)}.$$ Thus, $$ \mathbf{J}(r) = \mathbf{J}(R)\frac{J_0(kr)}{J_0(kR)}. $$

Resistance
The most important effect of skin effect on the impedance of a single wire is the increase of the wire's resistance, and consequent losses. The effective resistance due to a current confined near the surface of a large conductor (much thicker than $δ$) can be solved as if the current flowed uniformly through a layer of thickness $δ$ based on the DC resistivity of that material. The effective cross-sectional area is approximately equal to $δ$ times the conductor's circumference. Thus a long cylindrical conductor such as a wire, having a diameter $D$ large compared to $δ$, has a resistance approximately that of a hollow tube with wall thickness $δ$ carrying direct current. The AC resistance of a wire of length $ℓ$ and resistivity $$\rho$$ is: $$R\approx {{\ell \rho} \over {\pi (D-\delta) \delta}} \approx {{\ell \rho} \over {\pi D \delta}} $$

The final approximation above assumes $$D \gg \delta$$.

A convenient formula (attributed to F.E. Terman) for the diameter $D$$W$ of a wire of circular cross-section whose resistance will increase by 10% at frequency $f$ is: $$D_\mathrm{W} = {\frac{200~\mathrm{mm}}{\sqrt{f/\mathrm{Hz}}}}$$

This formula for the increase in AC resistance is accurate only for an isolated wire. For nearby wires, e.g. in a cable or a coil, the AC resistance is also affected by proximity effect, which can cause an additional increase in the AC resistance. The internal impedance per unit length of a segment of round wire is given by: $$ \mathbf{Z}_\text{int} =  \frac { k \rho } { 2 \pi R }      \frac {  J_0(k R) } { J_1(k R) }.$$

This impedance is a complex quantity corresponding to a resistance (real) in series with the reactance (imaginary) due to the wire's internal self-inductance, per unit length.

Inductance
A portion of a wire's inductance can be attributed to the magnetic field inside the wire itself which is termed the internal inductance; this accounts for the inductive reactance (imaginary part of the impedance) given by the above formula. In most cases this is a small portion of a wire's inductance which includes the effect of induction from magnetic fields outside of the wire produced by the current in the wire. Unlike that external inductance, the internal inductance is reduced by skin effect, that is, at frequencies where skin depth is no longer large compared to the conductor's size. This small component of inductance approaches a value of $$ \frac \mu { 8 \pi } $$ (50 nH/m for non-magnetic wire) at low frequencies, regardless of the wire's radius. Its reduction with increasing frequency, as the ratio of skin depth to the wire's radius falls below about 1, is plotted in the accompanying graph, and accounts for the reduction in the telephone cable inductance with increasing frequency in the table below.

Refer to the diagram below showing the inner and outer conductors of a coaxial cable. Since skin effect causes a current at high frequencies to flow mainly at the surface of a conductor, it can be seen that this will reduce the magnetic field inside the wire, that is, beneath the depth at which the bulk of the current flows. It can be shown that this will have a minor effect on the self-inductance of the wire itself; see Skilling or Hayt for a mathematical treatment of this phenomenon.

The inductance considered in this context refers to a bare conductor, not the inductance of a coil used as a circuit element. The inductance of a coil is dominated by the mutual inductance between the turns of the coil which increases its inductance according to the square of the number of turns. However, when only a single wire is involved, then in addition to the external inductance involving magnetic fields outside the wire (due to the total current in the wire) as seen in the white region of the figure below, there is also a much smaller component of internal inductance due to the portion of the magnetic field inside the wire itself, the green region in figure B. That small component of the inductance is reduced when the current is concentrated toward the skin of the conductor, that is, when skin depth is not much larger than the wire's radius, as will become the case at higher frequencies.

For a single wire, this reduction becomes of diminishing significance as the wire becomes longer in comparison to its diameter, and is usually neglected. However, the presence of a second conductor in the case of a transmission line reduces the extent of the external magnetic field (and of the total self-inductance) regardless of the wire's length, so that the inductance decrease due to skin effect can still be important. For instance, in the case of a telephone twisted pair, below, the inductance of the conductors substantially decreases at higher frequencies where skin effect becomes important. On the other hand, when the external component of the inductance is magnified due to the geometry of a coil (due to the mutual inductance between the turns), the significance of the internal inductance component is even further dwarfed and is ignored.

Coaxial cable
Let the dimensions a, b, and c be the inner conductor radius, the shield (outer conductor) inside radius and the shield outer radius respectively, as seen in the crossection of figure A below.

For a given current, the total energy stored in the magnetic fields must be the same as the calculated electrical energy attributed to that current flowing through the inductance of the coax; that energy is proportional to the cable's measured inductance.

The magnetic field inside a coaxial cable can be divided into three regions, each of which will therefore contribute to the electrical inductance seen by a length of cable.

 * The inductance $$ L_\text{cen} \, $$ is associated with the magnetic field in the region with radius $$ r < a \, $$, the region inside the center conductor.

 * The inductance $$ L_\text{ext} \, $$ is associated with the magnetic field in the region $$ a < r < b \, $$, the region between the two conductors (containing a dielectric, possibly air).

 * The inductance $$ L_\text{shd} \, $$ is associated with the magnetic field in the region $$ b < r < c \, $$, the region inside the shield conductor.

The net electrical inductance is due to all three contributions: $$ L_\text{total} = L_\text{cen} + L_\text{shd} + L_\text{ext}\, $$

$$ L_\text{ext} \, $$ is not changed by the skin effect and is given by the frequently cited formula for inductance L per length D of a coaxial cable: $$ L/D = \frac{\mu_0}{2 \pi} \ln \left( \frac {b}{a}  \right)   \, $$

At low frequencies, all three inductances are fully present so that $$ L_\text{DC} = L_\text{cen} + L_\text{shd} + L_\text{ext}\, $$.

At high frequencies, only the dielectric region has magnetic flux, so that $$ L_\infty = L_\text{ext}\, $$.

Most discussions of coaxial transmission lines assume they will be used for radio frequencies, so equations are supplied corresponding only to the latter case.

As skin effect increases, the currents are concentrated near the outside the inner conductor (r = a) and the inside of the shield (r = b). Since there is essentially no current deeper in the inner conductor, there is no magnetic field beneath the surface of the inner conductor. Since the current in the inner conductor is balanced by the opposite current flowing on the inside of the outer conductor, there is no remaining magnetic field in the outer conductor itself where $$ b < r < c \, $$. Only $$ L_\text{ext} $$ contributes to the electrical inductance at these higher frequencies.

Although the geometry is different, a twisted pair used in telephone lines is similarly affected: at higher frequencies, the inductance decreases by more than 20% as can be seen in the following table.

Telephone cable
Representative parameter data for 24 gauge PIC telephone cable at 21 C. More extensive tables and tables for other gauges, temperatures and types are available in Reeve. Chen gives the same data in a parameterized form that he states is usable up to 50 MHz.

Chen gives an equation of this form for telephone twisted pair: $$ L(f) = \frac {\ell_0 + \ell_\infty \left(\frac{f}{f_m}\right)^b }{1 + \left(\frac{f}{f_m}\right)^b} \, $$

Material effect on skin depth
In a good conductor, skin depth is proportional to square root of the resistivity. This means that better conductors have a reduced skin depth. The overall resistance of the better conductor remains lower even with the reduced skin depth. However the better conductor will show a higher ratio between its AC and DC resistance, when compared with a conductor of higher resistivity. For example, at 60 Hz, a 2000 MCM (1000 square millimeter) copper conductor has 23% more resistance than it does at DC. The same size conductor in aluminum has only 10% more resistance with 60 Hz AC than it does with DC.

Skin depth also varies as the inverse square root of the permeability of the conductor. In the case of iron, its conductivity is about 1/7 that of copper. However being ferromagnetic its permeability is about 10,000 times greater. This reduces the skin depth for iron to about 1/38 that of copper, about 220 micrometers at 60 Hz. Iron wire is impractical for AC power lines (except to add mechanical strength by serving as a core to a non-ferromagnetic conductor like aluminum). Skin effect also reduces the effective thickness of laminations in power transformers, increasing their losses.

Iron rods work well for direct-current (DC) welding but it is difficult to use them at frequencies much higher than 60 Hz. At a few kilohertz, an iron welding rod would glow red hot as current flows through the greatly increased AC resistance resulting from skin effect, with relatively little power remaining for the arc itself. Only non-magnetic rods are used for high-frequency welding.

At 1 megahertz skin effect depth in wet soil is about 5.0 m; in seawater it is about 0.25 m.

Mitigation
A type of cable called litz wire (from the German Litzendraht, braided wire) is used to mitigate skin effect for frequencies of a few kilohertz to about one megahertz. It consists of a number of insulated wire strands woven together in a carefully designed pattern, so that the overall magnetic field acts equally on all the wires and causes the total current to be distributed equally among them. With skin effect having little effect on each of the thin strands, the bundle does not suffer the same increase in AC resistance that a solid conductor of the same cross-sectional area would due to skin effect.

Litz wire is often used in the windings of high-frequency transformers to increase their efficiency by mitigating both skin effect and proximity effect. Large power transformers are wound with stranded conductors of similar construction to litz wire, but employing a larger cross-section corresponding to the larger skin depth at mains frequencies. Conductive threads composed of carbon nanotubes have been demonstrated as conductors for antennas from medium wave to microwave frequencies. Unlike standard antenna conductors, the nanotubes are much smaller than the skin depth, allowing full use of the thread's cross-section resulting in an extremely light antenna.

High-voltage, high-current overhead power lines often use aluminum cable with a steel reinforcing core; the higher resistance of the steel core is of no consequence since it is located far below the skin depth where essentially no AC current flows.

In applications where high currents (up to thousands of amperes) flow, solid conductors are usually replaced by tubes, eliminating the inner portion of the conductor where little current flows. This hardly affects the AC resistance, but considerably reduces the weight of the conductor. The high strength but low weight of tubes substantially increases span capability. Tubular conductors are typical in electric power switchyards where the distance between supporting insulators may be several meters. Long spans generally exhibit physical sag but this does not affect electrical performance. To avoid losses, the conductivity of the tube material must be high.

In high current situations where conductors (round or flat busbar) may be between 5 and 50 mm thick skin effect also occurs at sharp bends where the metal is compressed inside the bend and stretched outside the bend. The shorter path at the inner surface results in a lower resistance, which causes most of the current to be concentrated close to the inner bend surface. This causes an increase in temperature at that region compared with the straight (unbent) area of the same conductor. A similar skin effect occurs at the corners of rectangular conductors (viewed in cross-section), where the magnetic field is more concentrated at the corners than in the sides. This results in superior performance (i.e. higher current with lower temperature rise) from wide thin conductors (for example, ribbon conductors) in which the effects from corners are effectively eliminated.

It follows that a transformer with a round core will be more efficient than an equivalent-rated transformer having a square or rectangular core of the same material.

Solid or tubular conductors may be silver-plated to take advantage of silver's higher conductivity. This technique is particularly used at VHF to microwave frequencies where the small skin depth requires only a very thin layer of silver, making the improvement in conductivity very cost effective. Silver plating is similarly used on the surface of waveguides used for transmission of microwaves. This reduces attenuation of the propagating wave due to resistive losses affecting the accompanying eddy currents; skin effect confines such eddy currents to a very thin surface layer of the waveguide structure. Skin effect itself is not actually combatted in these cases, but the distribution of currents near the conductor's surface makes the use of precious metals (having a lower resistivity) practical. Although it has a lower conductivity than copper and silver, gold plating is also used, because unlike copper and silver, it does not corrode. A thin oxidized layer of copper or silver would have a low conductivity, and so would cause large power losses as the majority of the current would still flow through this layer.

Recently, a method of layering non-magnetic and ferromagnetic materials with nanometer scale thicknesses has been shown to mitigate the increased resistance from skin effect for very high-frequency applications. A working theory is that the behavior of ferromagnetic materials in high frequencies results in fields and/or currents that oppose those generated by relatively nonmagnetic materials, but more work is needed to verify the exact mechanisms. As experiments have shown, this has potential to greatly improve the efficiency of conductors operating in tens of GHz or higher. This has strong ramifications for 5G communications.

Examples
[[File:Skin depth by Zureks-en.svg|thumb|350px|Skin depth vs. frequency for some materials at room temperature, red vertical line denotes 50 Hz frequency:Mn-Zn – magnetically soft ferrite

Al – metallic aluminum

Cu – metallic copper

steel 410 – magnetic stainless steel

Fe-Si – grain-oriented electrical steel

Fe-Ni – high-permeability permalloy (80%Ni-20%Fe) ]]

We can derive a practical formula for skin depth as follows: $$\begin{align} \delta &= \frac{1}{\alpha} = \sqrt{{2\rho }\over{(2 \pi f) (\mu_0 \mu_r)}} \\ &= \frac{1}{\sqrt{\pi f \mu \sigma}} \approx 503\,\sqrt{\frac{\rho}{\mu_r f}} \approx 503\,\frac{1}{\sqrt{\mu_r f \sigma}}, \end{align}$$

where $\delta = $ the skin depth in meters

$\alpha = $ the attenuation in $\frac{Np}{m}$

$\mu_0 = $ the permeability of free space

$\mu_r = $ the relative permeability of the medium (for copper, $\mu_r$ = $1$)

$\mu = $ the permeability of the medium

$\rho = $ the resistivity of the medium in Ω·m, also equal to the reciprocal of its conductivity: $\rho = \frac{1}{\sigma}$ (for copper, ρ = $1.68 Ω·m$)

$\sigma = $ the conductivity of the medium (for copper, $\sigma \approx $ $58.5 S/m$)

$f = $ the frequency of the current in Hz Gold is a good conductor with a resistivity of $2.44 Ω·m$ and is essentially nonmagnetic: $$\mu_r = $$ 1, so its skin depth at a frequency of 50 Hz is given by $$\delta = 503 \,\sqrt{\frac{2.44 \cdot 10^{-8}}{1 \cdot 50}}= 11.1\,\mathrm{mm} $$

Lead, in contrast, is a relatively poor conductor (among metals) with a resistivity of $2.2 Ω·m$, about 9 times that of gold. Its skin depth at 50 Hz is likewise found to be about 33 mm, or $$\sqrt{9} = 3 $$ times that of gold.

Highly magnetic materials have a reduced skin depth owing to their large permeability $$\mu_r$$ as was pointed out above for the case of iron, despite its poorer conductivity. A practical consequence is seen by users of induction cookers, where some types of stainless steel cookware are unusable because they are not ferromagnetic.

At very high frequencies skin depth for good conductors becomes tiny. For instance, skin depths of some common metals at a frequency of 10 GHz (microwave region) are less than a micrometer:

Thus at microwave frequencies, most of the current flows in an extremely thin region near the surface. Ohmic losses of waveguides at microwave frequencies are therefore only dependent on the surface coating of the material. A layer of silver 3 μm thick evaporated on a piece of glass is thus an excellent conductor at such frequencies.

In copper, skin depth can be seen to fall according to the square root of frequency:

In Engineering Electromagnetics, Hayt points out that in a power station a busbar for alternating current at 60 Hz with a radius larger than one-third of an inch (8 mm) is a waste of copper, and in practice bus bars for heavy AC current are rarely more than half an inch (12 mm) thick except for mechanical reasons.

Electromagnetic waves
In electromagnetic waves, the skin depth is the depth at which the amplitude of the electric and magnetic fields have reduced by $$\frac{1}{e}$$. The intensity of the wave is proportional to the square of the amplitude, and thus the depth at which the intensity has diminished by $$\frac{1}{e}$$ is $$\frac{\delta}2.$$ In waveguides, losses due to induced currents occur mostly within one skin depth of the surface. Thus, plating the surface of a waveguide with a material which has a low skin depth reduces losses.

Anomalous skin effect
For high frequencies and low temperatures, the usual formulas for skin depth break down. This effect was first noticed by Heinz London in 1940, who correctly suggested that it is due to the mean free path length of the electrons reaching the range of the classical skin depth. Mattis–Bardeen theory was developed for this specific case for metals and superconductors.

title: Skin effect


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Breast pump

A breast pump is a mechanical device that lactating women use to extract milk from their breasts. They may be manual devices powered by hand or foot movements or automatic devices powered by electricity.

History
On June 20, 1854, the United States Patent Office issued Patent No. 11,135 to O.H. Needham for a breast pump. Scientific American (1863) credits L.O. Colbin as the inventor and patent applicant of a breast pump. In 1921–23, engineer and chess master Edward Lasker produced a mechanical breast pump that imitated an infant's sucking action and was regarded by physicians as a marked improvement on existing hand-operated breast pumps, which failed to remove all the milk from the breast. The U.S. Patent Office issued for Lasker's breast pump. In 1956 Einar Egnell published his groundbreaking work, "Viewpoints on what happens mechanically in the female breast during various methods of milk collection". This article provided insight into the technical aspects of milk extraction from the breast. Many Egnell SMB breast pumps designed through this research are still in operation over 50 years after publication.

Archaeologists working at a glass factory site in Philadelphia, Pennsylvania, excavated a 19th-century breast pipe that matches breast pumping instruments in period advertisements.

Reasons for use
Breast pumps are used for many reasons. Many parents use them to continue breastfeeding after they return to work. They express their milk at work, which is later bottle-fed to their child by a caregiver. This use of breast milk is widespread in the United States, where paid family leave is one of the shortest in the developed world. American historian Jill Lepore argues that the need for so-called "lactation rooms" and breast pumps is driven by the corporate desire for parents to return to work immediately rather than mothers' wishes or babies' needs.

A breast pump may also be used to stimulate lactation for women with a low milk supply or those who have not just given birth.

A breast pump may be also used to address a range of challenges parents may encounter breast feeding, including difficulties latching, separation from an infant in intensive care, to feed an infant who cannot extract sufficient milk itself from the breast, to avoid passing medication through breast milk to the baby, or to relieve engorgement, a painful condition whereby the breasts are overfull. Pumping may also be desirable to continue lactation and its associated hormones to aid in recovery from pregnancy even if the pumped milk is not used.

In a 2012 policy statement, the American Academy of Pediatrics recommended feeding preterm infants human milk, finding "significant short- and long-term beneficial effects," including lower rates of necrotizing enterocolitis (NEC). When infants are unable to suckle, mothers can pump if they wish their babies to be fed (via naso-gastric tube) with the mothers' own milk.

Expressing milk for donation is another use for breast pumps. Donor milk may be available from milk banks for babies who are not able to receive their mothers' milk.

"Pump and dump" refers to the practice of disposing of breast milk rather than pumping it to be used. This may be done because the mother produces too much milk, or because the mother will be away from the baby too long without a way to store the milk, or because of concern over alcohol or some other substance that the baby should not have.

Efficiency
The breast pump is not as efficient at removing milk from the breast as most nursing babies or hand expression.

Research done at Stanford University in 2009 showed the correlation of various factors with the volume of milk production in mothers of preterm babies (born before the 31st week of gestation). The research found that hand expression in addition to a breast pump (a technique called "hands-on pumping", or HOP), along with other factors correlated to higher milk production. The study found that mothers who used massage techniques and hand expression more than 5 times a day in the first 3 days after birth increased their milk production 8 weeks later, milk production increased 48%. The authors produced a video showing the technique and states that this technique is good for both mothers of premature infants as well as mothers that return to work or pump for other purposes.

A second article on the same study found that the combination of HOP techniques increased the fat content of the milk expressed.

Mechanical properties
Mechanically, a breast pump triggers the milk ejection response or "letdown". A misconception is that the breast pump suctions milk out of the breast. Pumps achieve letdown by using suction to pull the nipple into the tunnel of the breast shield or flange, then release, which counts as one cycle. Thirty to sixty cycles per minute can be expected with better-quality electric breast pumps. This suction-release cycle can also be accomplished with hand pumps, which the nursing parent operates by compressing a handle.

Multiple sizes of flanges ranging from 13mm to 36mm are available to purchase from various manufacturers. Most pump manufacturers, however, supply their pumps with only standard 24 and 27 mm flanges. What leads to unawareness in pumping women and Lactation consultants about the need to be fit with the right size flange for efficient emptying during pumping.

There are several pump mechanisms. Piston pumps draw a piston through a cylinder to create suction. These tend to operate at low speed, have high reliability, low noise, and long life. Rotary vane pumps use a cam with retractable vanes to create suction and are no longer widely used. Fast-diaphragm pumps use a diaphragm that is acted on by a lever and can operate at thousands of cycles per minute. They tend to be noisy. Slow-diaphragm pumps use a larger diaphragm operated by a cam or lever to generate suction with each stroke. Pumps have also been designed that use venturi effects powered by a faucet or water stream, wall suction in hospitals, or oral suctioning.

Manual breast pumps
Manual breast pumps are operated by squeezing or pulling a handle in a repetitive fashion, allowing the user to directly control the pressure and frequency of milk expression. Though manual pumps are small and inexpensive, they can require significant effort and can be tiring because the user provides all the power. These pumps may not provide sufficient stimulation and emptying of the breast. "Bicycle-horn" style manual pumps can damage breast tissue and harbor bacteria in the rubber suction bulb, which is difficult to clean.

Foot-powered breast pumps use the same collection tubing and breast horns as electric breast pumps, but are powered by a foot pedal. This eliminates the work of pumping by hand or the need for finding an electrical outlet with privacy.

Electric breast pumps
There are two types of electric breast pumps, hospital-grade and personal-use. Hospital-grade pumps are larger and intended for multiple users. Personal-use pumps are generally intended for one user. Electric breast pumps are powered by a motor which supplies suction through plastic tubing to a horn that fits over the nipple. The portions of the pump that come into direct contact with the expressed milk must be sterilized to prevent contamination. This style provides more suction, making pumping significantly faster, and allows users to pump both their breasts at the same time. Electric breast pumps are larger than manual ones, but portable models are available (e.g. in a backpack or shoulder bag). Some models include battery packs or built-in batteries to enable portable operation of the pump. Some electric pumps allow multi-user operation but recommend an accessory kit for each user to maintain cleanliness.

Rental pumps may be recommended when medical conditions temporarily preclude breastfeeding.

Some breast pumps are designed to be part of a "feeding system" so that the milk storage portion of the pump is the baby bottle used to feed the infant. This allows the milk to be collected in the same bottle that will feed the baby eliminating the need to transfer breastmilk. Freezable breastmilk storage bags are available that connect directly to some breast pumps and can then be used with disposable bottle feeding systems.

Pressure range and safety

Egnell in 1956 established a 220 mmHg safe maximum for automatic cycling pumps; however, there have been reports of sensitive breasts and nipples at much lower pressures. Hartman et al., in a 2008 study, showed that the maximum comfortable vacuum enhances milk flow and milk yield.

Open collection systems vs. closed collection systems
Pump designs are referred to as either open or closed based on whether there is a barrier between where the tubing connects to the pump and where milk flows into the pump. The plastic tubing and horn of an electric breast pump are commonly referred to as the collection system and typically supply the pump's suction.

A closed collection system has a barrier or diaphragm that separates the pump tubing from the horn. In this design, the suction of the pump motor lifts the diaphragm to create a vacuum within the collection system to extract milk.

In contrast, an open system does not have a barrier between the tubing and the horn. Bacteria and viral filters may be present to prevent contamination or overflow into pump motor. The pump motor's suction is directly passed to the breast versus indirectly as with closed-diaphragm systems.

Open-collection systems can cause milk to overflow into the collection system tubing, and milk particles being drawn into the pump motor. If milk leaks into the pump's tubing, the tubes should be washed, sterilized and air-dried prior to using the pump again. Failure to thoroughly clean collection tubing may lead to mould growth within the tubing. Some models of pumps have bacteria and overflow filters which prevent milk from entering the tubing.

A subtype of the open-collection system is the single-user suction source. These pumps have added hygienic benefit in that all the parts that generate the suction and come in contact with breast milk stay with the mother. The parts that generate the suction are external to the pump and can be removed, preventing cross-contamination. These pumps are considered "hospital grade" and virtually eliminate the chance of cross-contamination of the pump from user to user. However, it is important to clean the diaphragm as condensation or milk reaching this part can cause bacterial contamination.

A disadvantage of the diaphragm is that it may limit the amount of air/suction available to extract milk from the breast. It may also not be able to compensate for larger shield sizes.

There are no studies comparing the open- versus closed-system design. Most information in marketing materials by breast-pump manufacturers is offered without studies to back them up.

Milk collection and storage
Most breast pumps direct the pumped milk into a container that can be used for storage and feeding. Some manufacturers offer adapters to fit a variety of types and sizes of bottles, enabling more flexibility to mix and match products of different brands.

The expressed breast milk (EBM) may be stored and later fed to a baby by bottle. It can either be frozen directly in the bottle, or stored in disposable breast milk bags which are more compact when frozen, thus saving space in a freezer. Expressed milk may be kept at room temperature for up to six hours (at 66-72 degrees Fahrenheit, around 20 degrees Celsius), in an insulated cooler with ice packs for up to one day, refrigerated at the back of the refrigerator for up to 5 days (optimal: use or freeze the milk within 3 days), or frozen for 12 months in a deep freeze separate from a refrigerator maintained at a temperature of 0 degrees Fahrenheit or −18 degrees Celsius (optimal: use this milk within 6 months). If using frozen milk, the oldest milk should be thawed and used first. Thawing can be done by placing the frozen milk in the refrigerator the night before intended use, or by placing the milk in a bowl of warm water. Breast milk should never be microwaved as this can produce dangerous hot areas and may also destroy the milk's antibodies. Many experts recommend discarding thawed milk that is not used within 24 hours. Breast milk changes to meet a baby's needs so that breast milk expressed when a baby is a newborn will not as completely meet the same baby's needs when he or she is a few months older. Also, storage guidelines might differ for preterm, sick or hospitalized infants.

Expressed milk may be donated to milk banks, which provide human breast milk to premature infants and other children whose mothers cannot provide for them.

title: Breast pump