The most usual form of magnet construction employed in telephony is shown in Fig. 98. On the core, which is of soft Norway iron, usually cylindrical in form, are forced two washers of either fiber or hard rubber. Fiber is ordinarily to be preferred because it is tougher and less liable to breakage. Around the core, between the two heads, are then wrapped several layers of paper or specially prepared cloth in order that the wire forming the winding may be thoroughly insulated from the core. One end of the wire is then pa.s.sed through a hole in one of the spool heads or washers, near the core, and the wire is then wound on in layers. Sometimes a thickness of paper is placed around each layer of wire in order to further guard against the breaking down of the insulation between layers. When the last layer is wound on, the end of the wire is pa.s.sed out through a hole in the head, thus leaving both ends projecting.
[Ill.u.s.tration: Fig. 98 Construction of Electromagnet]
Magnet Wire. The wire used in winding magnets is, of course, an important part of the electromagnet. It is always necessary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pa.s.s 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 s.p.a.ce, silver wire is sometimes employed, and on the other hand, where very high resistance is desired within a limited winding s.p.a.ce, 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 fineness 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.&S. or A.W.G.
TABLE III
Copper Wire Table
Giving weights, lengths, and resistances of wire @ 68 F., of Matthiessen"s Standard Conductivity.
+-------+----------+----------+-----------------------+--------------------+-----------------------+RESISTANCELENGTHWEIGHTA.W.G.DIAMETERAREA +-----------------------+--------------------+-----------------------+B.&S.MILSCIRCULAROHMS PEROHMS PERFEET PERFEET PERPOUNDS PERPOUNDS PERMILSPOUNDFOOTPOUNDOHMFOOTOHM+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+0000460.211,600.0.000076390.00004891.56120,440.0.640513,090.000409.6167,800.0.00012150.00006171.96916,210.0.50808,232.00364.8133,100.0.00019310.00007782.48212,850.0.40285,177.0324.9105,500.0.00030710.00009813.13010,190.0.31953,256.+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+1289.383,690.0.00048830.00012373.9478,083.0.25332,048.2257.666,370.0.00077650.00015604.9776,410.0.20091,288.3229.452,630.0.0012350.00019676.2765,084.0.1593810.04204.341,740.0.0019630.00024807.9144,031.0.1264509.45181.933,100.0.0031220.00031289.9803,197.0.1002320.46162.026,250.0.0049630.000394412.582,535.0.07946201.57144.320,820.0.0078920.000497315.872,011.0.06302126.78128.516,510.0.012550.000627120.011,595.0.0499879.699114.413,090.0.019950.000790825.231,265.0.0396350.1210101.910,380.0.031730.000927331.821,003.0.0314331.52+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+1190.748,234.0.050450.00125740.12795.30.0249319.821280.816,530.0.080220.00158650.59630.70.0197712.471371.965,178.0.12760.00199963.79500.10.015687.8401464.084,107.0.20280.00252180.44396.60.012434.9311557.073,257.0.32250.003179101.4314.50.0098583.1011650.822,583.0.51280.004009127.9249.40.0078181.9501745.262,048.0.81530.005055161.3197.80.0062001.2261840.301,624.1.2960.006374203.4156.90.0049170.77131935.891,288.2.0610.008038256.5124.40.0038990.48512031.961,022.3.2780.01014323.498.660.0030920.3051+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+2128.46810.15.2120.01278407.878.240.0024520.19192225.35642.48.2870.01612514.262.050.0019450.12072322.57509.513.180.02032648.449.210.0015420.075892420.10404.020.950.02563817.639.020.0012230.047732517.90320.433.320.032311,031.30.950.00096990.030022615.94254.152.970.040751,300.24.540.00076920.11872714.2201.584.230.051381,639.19.460.00061000.018882812.64159.8133.90.064792,067.15.430.00048370.0074662911.26126.7213.00.081702,607.12.240.00038360.0046963010.03100.5338.60.10303,287.9.7070.00030420.002953+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+318.92879.70538.40.12994,145.7.6980.00024130.001857327.95063.21856.20.16385,227.6.1050.00019130.001168337.08050.131,361.0.20666,591.4.8410.00015170.0007346346.30539.752,165.0.26058,311.3.8390.00012030.0004620355.61531.523,441.0.328410,480.3.0450.000095430.0002905365.025.05,473.0.414213,210.2.4140.000075680.0001827374.45319.838,702.0.522216,660.1.9150.000060010.0001149383.96515.7213,870.0.658521,010.1.5190.000047590.00007210393.53112.4722,000.0.830426,500.1.2040.000037740.00004545403.1459.88834,980.1.04733,410.0.95500.000029930.00002858+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
[Ill.u.s.tration: SOUTH OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO]
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.&S. 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.&S. gauge.
In Table III there is given for each gauge number the diameter of the wire in mils (thousandths of an inch); the cross-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 possess 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 recesses of the winding s.p.a.ce.
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 princ.i.p.ally as a mechanical protection for the silk, the princ.i.p.al 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 process is repeated several times so that a number of these thin layers of the enamel are laid on and baked in succession.
The characteristics sought in good enamel insulation for magnet wire may be thus briefly set forth: It is desirable for the insulation to possess the highest insulating qualities; to have a glossy, flawless 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.
_s.p.a.ce Utilization_. The utilization of the winding s.p.a.ce 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 s.p.a.ce occupied by the insulation is, in this sense, waste s.p.a.ce. An ideally perfect winding may be conceived as one in which the s.p.a.ce is all occupied by wire; and this would necessarily involve the conception of wire of square cross-section and insulation of infinite thinness. In such a winding there would be no waste of s.p.a.ce and a maximum amount of metal employed as a conductor. Of course, such a condition is not possible 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 s.p.a.ce occupied by the conductor to the total s.p.a.ce occupied by the winding, that is, by the conductor and the insulation, is called the _coefficient of s.p.a.ce utilization of the coil_. For the ideal coil just conceived the coefficient of s.p.a.ce utilization would be 1. Ordinarily the coefficient of s.p.a.ce utilization is greater for coa.r.s.e wire than for fine wire, since obviously the ratio of the diameter of the wire to the thickness of the insulation increases as the size of the wire grows larger.
The chief advantage of enamel insulation for magnet wire is its thinness, and the high coefficient of s.p.a.ce utilization which may be secured by its use. In good enamel wire the insulation will average about one-quarter the thickness of the standard single silk insulation, and the dielectric strength is equal or greater. Where economy of winding s.p.a.ce 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 s.p.a.ce 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 s.p.a.ce is taken up by the insulation. Thus the coefficient of s.p.a.ce utilization is increased from .50 to .80. The practical result of this is that, in the case of any given winding s.p.a.ce 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 s.p.a.ce and keep the same number of turns, an equal number of turns may be had with a corresponding reduction of winding s.p.a.ce where enameled wire is used in place of silk or cotton.
In the matter of heat-resisting properties the enameled wire possesses 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 careless 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 damaged if it is allowed to strike against the sharp corners of the 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 thickness. 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.
Winding Methods. In winding a coil, the spool, after being properly prepared, is placed upon a spindle which may be made to revolve rapidly.
Ordinarily the wire is guided on by hand; sometimes, however, machinery is used, the wire being run over a tool which moves to and fro along the length of the spool, just fast enough to lay the wire on at the proper rate. The movement of this tool is much the same as that of the tool in a screw cutting lathe.
Unless high voltages are to be encountered, it is ordinarily not necessary to separate the layers of wire with paper, in the case of silk-or cotton-insulated magnet wire; although where especially high insulation resistance is needed this is often done. It is necessary to separate the successive layers of a magnet that is wound with enamel wire, by sheets of paper or thin oiled cloth.
[Ill.u.s.tration: Fig. 99. Electromagnet with Bare Wire]
In Fig. 99 is shown a method, that has been used with some success, of winding magnets with bare wire. In this the various adjacent turns are separated from each other by a fine thread of silk or cotton wound on beside the wire. Each layer of wire and thread as it is placed on the core is completely insulated from the subsequent layer by a layer of paper. This is essentially a machine-wound coil, and machines for winding it have been so perfected that several coils are wound simultaneously, the paper being fed in automatically at the end of each layer.
Another method of winding the bare wire omits the silk thread and depends on the permanent positioning of the wire as it is placed on the coil, due to the slight sinking into the layer of paper on which it is wound. In this case the feed of the wire at each turn of the spool is slightly greater than the diameter of the wire, so that a small distance will be left between each pair of adjacent turns.
Upon the completion of the winding of a coil, regardless of what method is used, it is customary to place a layer of bookbinders" cloth over the coil so as to afford a certain mechanical protection for the insulated wire.
_Winding Terminals_. The matter of bringing out the terminal ends of the winding is one that has received a great deal of attention in the construction of electromagnets and coils for various purposes. Where the winding is of fine wire, it is always well to reinforce its ends by a short piece of larger wire. Where this is done the larger wire is given several turns around the body of the coil, so that the finer wire with which it connects may be relieved of all strain which may be exerted upon it from the protruding ends of the wire. Great care is necessary in the bringing out of the inner terminal--_i.e._, the terminal which connects with the inner layer--that the terminal wire shall not come in contact with any of the subsequent layers that are wound on.
[Ill.u.s.tration Fig. 100. Electromagnet with Terminals]
Where economy of s.p.a.ce is necessary, a convenient method of terminating the winding of the coil consists in fastening rigid terminals to the spool head. This, in the case of a fiber spool head, may be done by driving heavy metal terminals into the fiber. The connections of the two wires leading from the winding are then made with these heavy rigid terminals by means of solder. A coil having such terminals is shown in its finished condition in Fig. 100.
_Winding Data_. The two things princ.i.p.ally affecting the manufacture of electromagnets for telephone purposes are _the number of turns in a winding_ and _the resistance of the wound wire_. The latter governs the amount of current which may flow through the coil with a given difference of potential at its end, while the former control the amount of magnetism produced in the core by the current flowing. While a coil is being wound, it is a simple matter to count the turns by any simple form of revolution counter. When the coil has been completed it is a simple matter to measure its resistance. But it is not so simple to determine in advance how many turns of a given size wire may be placed on a given spool, and still less simple to know what the resistance of the wire on that spool will be when the desired turns shall have been wound.
TABLE IV
Winding Data for Insulated Wires--Silk and Cotton Covering
A.W.G. B & S20 21 22 23 24 25 --------------------------------------------------------------------- DIAMETERMils31.961 28.462 25.347 22.571 20.100 17.900 --------------------------------------------------------------------- AREACircular Mils1021.20 810.10 642.70 509.45 404.01 320.40 --------------------------------------------------------------------- DIAMETER OVERINSULATIONSINGLECOTTON37.861 34.362 31.247 28.471 26.000 23.800DOUBLECOTTON42.161 38.662 35.547 32.771 30.300 28.100SINGLE SILK34.261 30.762 27.647 24,871 22.401 20.200DOUBLE SILK36.161 32.662 29.547 26.771 24.300 22.100 --------------------------------------------------------------------- TURNS PERLINEAR INCHSINGLECOTTON25.7 28.3 31.0 34.4 36.9 38.0DOUBLE22.5 24.5 26.7 28.97 31.35 33.92 COTTONSINGLE SILK27.70 30.97 34.39 38.19 42.37 47.02DOUBLE SILK26.22 29.07 32.11 35.53 39.14 42.94 --------------------------------------------------------------------- TURNS PERSQUARE INCHSINGLECOTTON660.5 800.9 961.0 1183.0 1321.6 1444.0DOUBLECOTTON506.3 600.2 712.9 839.2 982.8 1150.8SINGLE SILK767.3 959.1 1182.7 1458.5 1795.2 2210.9DOUBLE SILK687.5 845.0 1031.0 1262.4 1532.0 1843.8 --------------------------------------------------------------------- OHMS PERCUBIC INCHSINGLECOTTON.646 .981 1.502 2.359 3.528 5.831DOUBLECOTTON.533 .795 1.188 1.772 2.595 3.802SINGLE SILK.801 1.261 1.956 3.049 4.739 7.489 ---------------------------------------------------------------------
A.W.G. B & S26 27 28 29 30 31 --------------------------------------------------------------------- DIAMETERMils15.940 14.195 12.641 11.257 10.025 8.928 --------------------------------------------------------------------- AREACircular Mils254.01 201.50 159.79 126.72 100.50 79.71 --------------------------------------------------------------------- DIAMETER OVERINSULATIONSINGLECOTTON21.840 20.095 18.541 17.157 15.925 14.828DOUBLECOTTON26.140 24.395 22.841 21.457 20.225 19.128SINGLE SILK18.240 16.495 14.941 13.557 12.325 11.228DOUBLE SILK20.140 18.395 16.841 15.457 14.225 13.128 --------------------------------------------------------------------- TURNS PERLINEAR INCHSINGLECOTTON42.0 48.0 53.0 56.5 59.66 64.125DOUBLECOTTON36.29 38.95 41.61 44.27 46.93 49.78SINGLE SILK52.06 57.67 63.36 70.11 77.14 84.64DOUBLE SILK46.81 51.59 56.43 61.56 66.79 72.39 --------------------------------------------------------------------- TURNS PERSQUARE INCHSINGLECOTTON1764.0 2304.0 2809.9 3192.3 3359.2 4112.2DOUBLECOTTON1317.0 1517.2 1731.0 1959.9 2202.5 2478.0SINGLE SILK2710.3 3326.0 4014.5 4915.5 5950.2 7164.0DOUBLE SILK2191.2 2661.6 3184.5 3789.8 4461.0 5240.0 --------------------------------------------------------------------- OHMS PERCUBIC INCHSINGLECOTTON6.941 10.814 17.617 25.500 34.800 48.5DOUBLECOTTON5.552 8.078 11.54 16.47 23.43 32.83SINGLE SILK9.031 13.92 26.86 41.29 62.98 95.70 ---------------------------------------------------------------------
A.W.G. B & S32 33 34 35 36 37 ---------------------------------------------------------------------- DIAMETERMils7.950 7.080 6.304 5.614 5.000 4.453 ---------------------------------------------------------------------- AREACircular Mils63.20 50.13 39.74 31.52 25.00 19.83 ---------------------------------------------------------------------- DIAMETER OVERINSULATIONSINGLECOTTON13.850 12.980 12.204 11.514 10.900 10.353DOUBLECOTTON18.150 17.280 16.504 15.814 15.200 14.653SINGLE SILK10.250 9.380 8.504 7.914 7.300 6.753DOUBLE SILK12.150 11.280 10.504 9.814 9.200 8.653 ---------------------------------------------------------------------- TURNS PERLINEAR INCHSINGLECOTTON68.600 73.050 77.900 82.600 87.100 91.870DOUBLECOTTON52.34 55.10 57.57 60.04 62.51 64.70SINGLE SILK92.72 101.65 112.11 119.7 130.15 140.6DOUBLE SILK78.19 84.17 90.44 96.90 103.55 110.20 ---------------------------------------------------------------------- TURNS PERSQUARE INCHSINGLE4692.5 5333.5 6068.5 6773.3 7586.5 8440.0 COTTONDOUBLECOTTON2739.5 3036.1 3314.2 3605.0 3907.5 4186.1SINGLE SILK8597.5 10332.0 12570.0 14327.0 16940.0 19770.0DOUBLE SILK6114.0 7085.0 8179.5 9389.5 10772.0 12145.0 --------------------------------------------------------------------- OHMS PERCUBIC INCHSINGLECOTTON73.8 104.5 151.4 202.0 298.8 418.0DOUBLECOTTON46.19 64.30 70.58 125.9 166.3 225.6SINGLE SILK144.70 217.8 342.1 489.0 721.1 1062.0 ---------------------------------------------------------------------
A.W.G. B & S38 39 40 -------------------------------------------- DIAMETERMils3.965 3.531 3.144 -------------------------------------------- AREACircular Mils15.72 12.47 9.89 -------------------------------------------- DIAMETER OVERINSULATIONSINGLECOTTON9.865 9.431 9.044DOUBLECOTTON14.165 13.731 13.344SINGLE SILK6.265 5.831 5.344DOUBLE SILK8.165 7.731 7.344 -------------------------------------------- TURNS PERLINEAR INCHSINGLECOTTON95.000 100.700 106.000DOUBLECOTTON66.80 68.80 71.20SINGLE SILK151.05 163.04 177.65DOUBLE SILK116.85 122.55 129.20 -------------------------------------------- TURNS PERSQUARE INCHSINGLECOTTON9025.0 10140.5 11236.0DOUBLE4462.2 4733.6 5069.8 COTTONSINGLE SILK22820.0 26700.0 31559.0DOUBLE SILK13655.0 15018.0 16692.0 -------------------------------------------- OHMS PERCUBIC INCHSINGLECOTTON567.0 811.0 1113.0DOUBLE305.5 409.8 545.5 COTTONSINGLE SILK1557.0 2266.0 3400.0 -------------------------------------------
If the length and the depth of the winding s.p.a.ce of the coil as well as the diameter of the core are known, it is not difficult to determine how much bare copper wire of a given size may be wound on it, but it is more difficult to know these facts concerning copper wire which has been covered with cotton or silk. Yet something may be done, and tables have been prepared for standard wire sizes with definite thicknesses of silk and cotton insulation. As a result of facts collected from a large number of actually wound coils, the number of turns per linear inch and per square inch of B.&S. gauge wires from No. 20 to No. 40 have been tabulated, and these, supplemented by a tabulation of the number of ohms per cubic inch of winding s.p.a.ce for wires of three different kinds of insulation, are given in Table IV.
Bearing in mind that the calculations of Table IV are all based upon the "diameter over insulation," which it states at the outset for each of four different kinds of covering, it is evident what is meant by "turns per linear inch." The columns referring to "turns per square inch" mean the number of turns, the ends of which would be exposed in one square inch if the wound coil were cut in a plane pa.s.sing through the axis of the core. Knowing the distance between the head, and the depth to which the coil is to be wound, it is easy to select a size of wire which will give the required number of turns in the provided s.p.a.ce. It is to be noted that the depth of winding s.p.a.ce is one-half of the difference between the core diameter and the complete diameter of the wound coil. The resistance of the entire volume of wound wire may be determined in advance by knowing the total cubic contents of the winding s.p.a.ce and multiplying this by the ohms per cubic inch of the selected wire; that is, one must multiply in inches the distance between the heads of the spool by the difference between the squares of the diameters of the core and the winding s.p.a.ce, and this in turn by .7854. This result, times the ohms per cubic inch, as given in the table, gives the resistance of the winding.
There is a considerable variation in the method of applying silk insulation to the finer wires, and it is in the finer sizes that the errors, if any, pile up most rapidly. Yet the table throughout is based on data taken from many samples of actual coil winding by the present process of winding small coils. It should be said further that the table does not take into account the placing of any layers of paper between the successive layers of the wires. This table has been compared with many examples and has been used in calculating windings in advance, and is found to be as close an approximation as is afforded by any of the formulas on the subject, and with the further advantage that it is not so c.u.mbersome to apply.
_Winding Calculations._ In experimental work, involving the winding of coils, it is frequently necessary to try one winding to determine its effect in a given circuit arrangement, and from the knowledge so gained to subst.i.tute another just fitted to the conditions. It is in such a subst.i.tution that the table is of most value. a.s.sume a case in which are required a spool and core of a given size with a winding of, say No. 25 single silk-covered wire, of a resistance of 50 ohms.
a.s.sume also that the circuit regulations required that this spool should be rewound so as to have a resistance of, say 1,000 ohms. What size single silk-covered wire shall be used? Manifestly, the winding s.p.a.ce remains the same, or nearly so. The resistance is to be increased from 50 to 1,000 ohms, or twenty times its first value.
Therefore, the wire to be used must show in the table twenty times as many ohms per cubic inch as are shown in No. 25, the known first size.
This amount would be twenty times 7.489, which is 149.8, but there is no size giving this exact resistance. No. 32, however, is very nearly of that resistance and if wound to exactly the same depth would give about 970 ohms. A few turns more would provide the additional thirty ohms.
Similarly, in a coil known to possess a certain number of turns, the table will give the size to be selected for rewinding to a greater or smaller number of turns. In this case, as in the case of subst.i.tuting a winding of different resistance, it is unnecessary to measure and calculate upon the dimensions of the spool and core. a.s.sume a spool wound with No. 30 double silk-covered wire, which requires to be wound with a size to double the number of turns. The exact size to do this would have 8922. turns per square inch and would be between No.
34 and No. 35. A choice of these two wires may be made, using an increased winding depth with the smaller wire and a shallower winding depth for the larger wire.
Impedance Coils. In telephony electromagnets frequently serve, as already stated, to perform other functions than the producing of motion by attracting or releasing their armatures. They are required to act as impedance coils to present a barrier to the pa.s.sage of alternating or other rapidly fluctuating currents, and at the same time to allow the comparatively free pa.s.sage of steady currents. Where it is desired that an electromagnet coil shall possess high impedance, it is usual to employ a laminated instead of a solid core. This is done by building up a core of suitable size by laying together thin sheets of soft iron, or by forming a bundle of soft iron wires. The use of laminated cores is for the purpose of preventing eddy currents, which, if allowed to flow, would not only be wasteful of energy but would also tend to defeat the desired high impedance. Sometimes in iron-clad impedance coils, the iron sh.e.l.l is slotted longitudinally to break up the flow of eddy currents in the sh.e.l.l.
Frequently electromagnetic coils have only the function of offering impedance, where no requirements exist for converting any part of the electric energy into mechanical work. Where this is the case, such coils are termed _impedance_, or _r.e.t.a.r.dation_, or _choke coils_, since they are employed to impede or to r.e.t.a.r.d or to choke back the flow of rapidly varying current. The distinction, therefore, between an impedance coil and the coil of an ordinary electromagnet is one of function, since structurally they may be the same, and the same principles of design and construction apply largely to each.
_Number of Turns_. It should be remembered that an impedance coil obstructs the pa.s.sage of fluctuating current, not so much by ohmic resistance as by offering an opposing or counter-electromotive force.
Other things being equal, the counter-electromotive force of self-induction increases directly as the number of turns on a coil and directly as the number of lines of force threading the coil, and this latter factor depends also on the reluctance of the magnetic circuit.