Volume I Part 14 (1/2)
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.
+-------+----------+----------+-----------------------+--------------------+-----------------------+
RESISTANCE
LENGTH
WEIGHT
A.W.G.
DIAMETER
AREA +-----------------------+--------------------+-----------------------+
B.&S.
MILS
CIRCULAR
OHMS PER
OHMS PER
FEET PER
FEET PER
POUNDS PER
POUNDS PER
MILS
POUND
FOOT
POUND
OHM
FOOT
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