Magnet Wire

The wire used in winding magnets is, of course, an important part of the electromagnet. It is always nece ary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pa around the core through all the length of wire in each turn rather than allowing it to take the shorter and easier path from one turn to the next, as would be the case if the turns were not insulated. For this purpose the wire is usually covered with a coating of some insulating material. There are, however, methods of winding magnet coils with bare wire and taking care of the insulation between the turns in another way, as will be pointed out. Insulated wire for the purpose of winding magnet coils is termed magnet wire . Copper is the material almost universally employed for the conductor. Its high conductivity, great ductility, and low cost are the factors which make it superior to all other metals. However, in special cases, where exceedingly high conductivity is required with a limited winding space, silver wire is sometimes employed, and on the other hand, where very high resistance is desired within a limited winding space, either iron or German silver or some other high-resistance alloy is used. Wire Gauges . Wire for electrical purposes is drawn to a number of different standard gauges. Each of the so-called wire gauges consists of a series of graded sizes of wire, ranging from approximately one-half an inch in diameter down to about the finene of a lady's hair. In certain branches of telephone work, such as line construction, the existence of the several wire gauges or standards is very likely to lead to confusion. Fortunately, however, so far as magnet wire is concerned, the so-called Brown and Sharpe, or American, wire gauge is almost universally employed in this country. The abbreviations for this gauge are B. . or A.W.G. TABLE III Copper Wire Table Giving weights, lengths, and resistances of wire @ 68° F., of Matt hies sen's Standard Conductivity. | A.W.G. B. . | DIAMETER Mils | AREA Circular Mils | RESISTANCE | LENGTH | WEIGHT | | Ohms per Pound | Ohms Per Foot | Feet per Pound | Feet per Ohm | Pounds per Foot | Pounds per Ohm | | 0000 | 460. | 211,600. | 0.00007639 | 0.0000489 | 1.561 | 20,440. | 0.6405 | 13,090. | | 000 | 409.6 | 167,800. | 0.0001215 | 0.0000617 | 1.969 | 16,210. | 0.5080 | 8,232. | | 00 | 364.8 | 133,100. | 0.0001931 | 0.0000778 | 2.482 | 12,850. | 0.4028 | 5,177. | | 0 | 324.9 | 105,500. | 0.0003071 | 0.0000981 | 3.130 | 10,190. | 0.3195 | 3,256. | | 1 | 289.3 | 83,690. | 0.0004883 | 0.0001237 | 3.947 | 8,083. | 0.2533 | 2,048. | | 2 | 257.6 | 66,370. | 0.0007765 | 0.0001560 | 4.977 | 6,410. | 0.2009 | 1,288. | | 3 | 229.4 | 52,630. | 0.001235 | 0.0001967 | 6.276 | 5,084. | 0.1593 | 810.0 | | 4 | 204.3 | 41,740. | 0.001963 | 0.0002480 | 7.914 | 4,031. | 0.1264 | 509.4 | | 5 | 181.9 | 33,100. | 0.003122 | 0.0003128 | 9.980 | 3,197. | 0.1002 | 320.4 | | 6 | 162.0 | 26,250. | 0.004963 | 0.0003944 | 12.58 | 2,535. | 0.07946 | 201.5 | | 7 | 144.3 | 20,820. | 0.007892 | 0.0004973 | 15.87 | 2,011. | 0.06302 | 126.7 | | 8 | 128.5 | 16,510. | 0.01255 | 0.0006271 | 20.01 | 1,595. | 0.04998 | 79.69 | | 9 | 114.4 | 13,090. | 0.01995 | 0.0007908 | 25.23 | 1,265. | 0.03963 | 50.12 | | 10 | 101.9 | 10,380. | 0.03173 | 0.0009273 | 31.82 | 1,003. | 0.03143 | 31.52 | | 11 | 90.74 | 8,234. | 0.05045 | 0.001257 | 40.12 | 795.3 | 0.02493 | 19.82 | | 12 | 80.81 | 6,530. | 0.08022 | 0.001586 | 50.59 | 630.7 | 0.01977 | 12.47 | | 13 | 71.96 | 5,178. | 0.1276 | 0.001999 | 63.79 | 500.1 | 0.01568 | 7.840 | | 14 | 64.08 | 4,107. | 0.2028 | 0.002521 | 80.44 | 396.6 | 0.01243 | 4.931 | | 15 | 57.07 | 3,257. | 0.3225 | 0.003179 | 101.4 | 314.5 | 0.009858 | 3.101 | | 16 | 50.82 | 2,583. | 0.5128 | 0.004009 | 127.9 | 249.4 | 0.007818 | 1.950 | | 17 | 45.26 | 2,048. | 0.8153 | 0.005055 | 161.3 | 197.8 | 0.006200 | 1.226 | | 18 | 40.30 | 1,624. | 1.296 | 0.006374 | 203.4 | 156.9 | 0.004917 | 0.7713 | | 19 | 35.89 | 1,288. | 2.061 | 0.008038 | 256.5 | 124.4 | 0.003899 | 0.4851 | | 20 | 31.96 | 1,022. | 3.278 | 0.01014 | 323.4 | 98.66 | 0.003092 | 0.3051 | | 21 | 28.46 | 810.1 | 5.212 | 0.01278 | 407.8 | 78.24 | 0.002452 | 0.1919 | | 22 | 25.35 | 642.4 | 8.287 | 0.01612 | 514.2 | 62.05 | 0.001945 | 0.1207 | | 23 | 22.57 | 509.5 | 13.18 | 0.02032 | 648.4 | 49.21 | 0.001542 | 0.07589 | | 24 | 20.10 | 404.0 | 20.95 | 0.02563 | 817.6 | 39.02 | 0.001223 | 0.04773 | | 25 | 17.90 | 320.4 | 33.32 | 0.03231 | 1,031. | 30.95 | 0.0009699 | 0.03002 | | 26 | 15.94 | 254.1 | 52.97 | 0.04075 | 1,300. | 24.54 | 0.0007692 | 0.1187 | | 27 | 14.2 | 201.5 | 84.23 | 0.05138 | 1,639. | 19.46 | 0.0006100 | 0.01888 | | 28 | 12.64 | 159.8 | 133.9 | 0.06479 | 2,067. | 15.43 | 0.0004837 | 0.007466 | | 29 | 11.26 | 126.7 | 213.0 | 0.08170 | 2,607. | 12.24 | 0.0003836 | 0.004696 | | 30 | 10.03 | 100.5 | 338.6 | 0.1030 | 3,287. | 9.707 | 0.0003042 | 0.002953 | | 31 | 8.928 | 79.70 | 538.4 | 0.1299 | 4,145. | 7.698 | 0.0002413 | 0.001857 | | 32 | 7.950 | 63.21 | 856.2 | 0.1638 | 5,227. | 6.105 | 0.0001913 | 0.001168 | | 33 | 7.080 | 50.13 | 1,361. | 0.2066 | 6,591. | 4.841 | 0.0001517 | 0.0007346 | | 34 | 6.305 | 39.75 | 2,165. | 0.2605 | 8,311. | 3.839 | 0.0001203 | 0.0004620 | | 35 | 5.615 | 31.52 | 3,441. | 0.3284 | 10,480. | 3.045 | 0.00009543 | 0.0002905 | | 36 | 5.0 | 25.0 | 5,473. | 0.4142 | 13,210. | 2.414 | 0.00007568 | 0.0001827 | | 37 | 4.453 | 19.83 | 8,702. | 0.5222 | 16,660. | 1.915 | 0.00006001 | 0.0001149 | | 38 | 3.965 | 15.72 | 13,870. | 0.6585 | 21,010. | 1.519 | 0.00004759 | 0.00007210 | | 39 | 3.531 | 12.47 | 22,000. | 0.8304 | 26,500. | 1.204 | 0.00003774 | 0.00004545 | | 40 | 3.145 | 9.888 | 34,980. | 1.047 | 33,410. | 0.9550 | 0.00002993 | 0.00002858 | In the Brown and Sharpe gauge the sizes, beginning with the largest, are numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger than about No. 16 B. . gauge are seldom used as magnet wire in telephony, but for the purpose of making the list complete, Table III is given, including all of the sizes of the B. . gauge. In Table III there is given for each gauge number the diameter of the wire in mils (thousandths of an inch); the cro -sectional area in circular mils (a unit area equal to that of a circle having a diameter of one one-thousandth of an inch); the resistance of the wire in various units of length and weight; the length of the wire in terms of resistance and of weight; and the weight of the wire in terms of its length and resistance. It is to be understood that in Table III the wire referred to is bare wire and is of pure copper. It is not commercially practicable to use absolutely pure copper, and the ordinary magnet wire has a conductivity equal to about 98 per cent of that of pure copper. The figures given in this table are sufficiently accurate for all ordinary practical purposes. Silk and Cotton Insulation . The insulating material usually employed for covering magnet wire is of silk or cotton. Of these, silk is by far the better material for all ordinary purposes, since it has a much higher insulating property than cotton, and is very much thinner. Cotton, however, is largely employed, particularly in the larger sizes of magnet wire. Both of these materials po e the disadvantage of being hygroscopic, that is, of readily absorbing moisture. This disadvantage is overcome in many cases by saturating the coil after it is wound in some melted insulating compound, such as wax or varnish or asphaltum, which will solidify on cooling. Where the coils are to be so saturated the best practice is to place them in a vacuum chamber and exhaust the air, after which the hot insulating compound is admitted and is thus drawn into the innermost rece es of the winding space. Silk-insulated wire, as regularly produced, has either one or two layers of silk. This is referred to commercially as single silk wire or as double silk wire. The single silk has a single layer of silk fibers wrapped about it, while the double silk has a double layer, the two layers being put on in reverse direction. The same holds true of cotton insulated wire. Frequently, also, there is a combination of the two, consisting of a single or a double wrapping of silk next to the wire with an outer wrapping of cotton. Where this is done the cotton serves principally as a mechanical protection for the silk, the principal insulating properties residing in the silk. Enamel . A later development in the insulation of magnet wire has resulted in the so-called enamel wire. In this, instead of coating the wire with some fibrous material such as silk or cotton, the wire is heated and run through a bath of fluid insulating material or liquid enamel, which adheres to the wire in a very thin coating. The wire is then run through baking ovens, so that the enamel is baked on. This proce is repeated several times so that a number of these thin layers of the enamel are laid on and baked in succe ion. The characteristics sought in good enamel insulation for magnet wire may be thus briefly set forth: It is desirable for the insulation to po e the highest insulating qualities; to have a glo y, flawle surface; to be hard without being brittle; to adhere tenaciously and stand all reasonable handling without cracking or flaking; to have a coefficient of elasticity greater than the wire itself; to withstand high temperatures; to be moisture-proof and inert to corrosive agencies; and not to "dry out" or become brittle over a long period of time. Space Utilization . The utilization of the winding space in an electromagnet is an important factor in design, since obviously the copper or other conductor is the only part of the winding that is effective in setting up magnetizing force. The space occupied by the insulation is, in this sense, waste space. An ideally perfect winding may be conceived as one in which the space is all occupied by wire; and this would nece arily involve the conception of wire of square cro -section and insulation of infinite thinne . In such a winding there would be no waste of space and a maximum amount of metal employed as a conductor. Of course, such a condition is not po ible to attain and in practice some insulating material must be introduced between the layers of wire and between the adjacent convolutions of wire. The ratio of the space occupied by the conductor to the total space occupied by the winding, that is, by the conductor and the insulation, is called the coefficient of space utilization of the coil . For the ideal coil just conceived the coefficient of space utilization would be 1. Ordinarily the coefficient of space utilization is greater for coarse wire than for fine wire, since obviously the ratio of the diameter of the wire to the thickne of the insulation increases as the size of the wire grows larger. The chief advantage of enamel insulation for magnet wire is its thinne , and the high coefficient of space utilization which may be secured by its use. In good enamel wire the insulation will average about one-quarter the thickne of the standard single silk insulation, and the dielectric strength is equal or greater. Where economy of winding space is desirable the advantages of this may readily be seen. For instance, in a given coil wound with No. 36 single silk wire about one-half of the winding space is taken up with the insulation, whereas when the same coil is wound with No. 36 enameled wire only about one-fifth of the winding space is taken up by the insulation. Thus the coefficient of space utilization is increased from.50 to.80. The practical result of this is that, in the case of any given winding space where No. 36 wire is used, about 60 per cent more turns can be put on with enameled wire than with single silk insulation, and of course this ratio greatly increases when the comparison is made with double silk insulation or with cotton insulation. Again, where it is desired to reduce the winding space and keep the same number of turns, an equal number of turns may be had with a corresponding reduction of winding space where enameled wire is used in place of silk or cotton. In the matter of heat-resisting properties the enameled wire po e es a great advantage over silk and cotton. Cotton or silk insulation will char at about 260° Fahrenheit, while good enameled wire will stand 400° to 500° Fahrenheit without deterioration of the insulation. It is in the matter of liability to injury in rough or carele handling, or in winding coils having irregular shapes, that enamel wire is decidedly inferior to silk or cotton-covered wire. It is likely to be magnet spool during winding, or run over the edge of a hard surface while it is being fed on to the spool. Coils having other than round cores, or having sharp corners on their spool heads, should not ordinarily be wound with enamel wire. The dielectric strength of enamel insulation is much greater than that of either silk or cotton insulation of equal thickne . This is a distinct advantage and frequently a combination of the two kinds of insulation results in a superior wire. If wire insulated with enamel is given a single wrapping of silk or of cotton, the insulating and dielectric properties of the enamel is secured, while the presence of the silk and cotton affords not only an additional safeguard against bare spots in the enamel but also a certain degree of mechanical protection to the enamel.