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50
RICHARDSON'S BLUEBOOK OF PROJECTION
(23) As a practical example, suppose a new line is to be run to 8 loud speaker units of the type requiring i ampere at 7 volts field excitation, located 100 feet distant and connected in parallel. Use of No. 18 solid wire, with a cold resistance of 1.2 ohms per 200 feet, will produce a voltage drop in that wire (when cold) of 8 amperes x 1.2 ohms, or 9.6 volts. Under those circumstances it may be hastily assumed that a source located in the projection room, which is capable of delivering 8 amperes at 16.6 volts will fully supply the needs of the speakers. It will not ! The current, 8 amperes, will overload a No. 18 wire enough to raise its temperature appreciably; therefore its resistance will rise above the assumed value of 1.2 ohms and the voltage drop will rise above 9.6 volts. This will occur even with a current of somewhat less than 8 amperes. The speakers will not receive correct excitation. Additionally, the temperature increase will have a very unfavorable effect on the life of the wires and their insulation, as well as, possibly, on nearby apparatus. Still further, such overheating represents a perpetual waste of electricity, the power being converted into heat which serves no useful purpose, yet must be paid for.
TABLE NO. 1
Varnished
Diameter of
Area in
Rubber
Cambric
No.
Solid Wires
Circular
Insulation,
Insulation,
Gauge
in Mils
Mils
Amperes
Amperes
18
40.3
1,624
3
16
50.8
2,583
6
14
64.1
4,107
15
18
12
80.8
6,530
20
25
10
101.9
10,380
25
30
8
128.5
16,510
35
40
6
162.0
26,250
50
60
5
181.9
33,100
55
65
4
204.3
41,740
70
85
3
229.4
52,630
80
95
2
257.6
66,370
90
110
1
289.3
83,690
100
120
0
325.0
105,500
125
150
00
364.8
133,100
150
180
000
409.6
167,800
175
210
200,000
200
240
0000
460.0
211,600
225
270
(24) Table No. 1 may be used to find the correct wire gauge, or number, for any required current. Look up the current to be carried in the right-hand column or first