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462 NOTES ON COPPER WIRE
Notes on Copper Wire.
Table No. 85 gives the weight and electrical resistance of
copper wire, the following facts are well to remember in
connection with the use of the table:
The gage number is according to B. & S. gage.
The second column gives the diameter of the copper wire
in mils (one mil is 0.001 inch).
The third column is the area of the copper wire in circular
mils, which is the same as the square of the values in the second
column. For instance, the diameter of No. 10 wire is 101.89
mils, and the area is 101.89 X 101.89 = 10382 circular mils.
The resistance given is at 6o°F., and may be considered to
vary 0.22 per cent, for each degree Fahr. between 32 and 212
degrees. The resistance increases with the temperature.
Each consecutive number in the Brown & Sharpe gage fol-
lows each other in geometrical progression. No. 1 is 0.2893 inch.
No. 40 is 0.003144 inch.
The size from one number to the next is obtained by mul-
tiplying by the constant number 1.1 23.
The area of one size or number of wire to the next is ob-
tained by multiplying by the constant number 1.261.
In approximate calculation this is of great practical value,
as for instance, the area of copper wire drawn to the Brown &
Sharpe gage is about one and one-quarter times larger for each
lower number on the gage.
Three numbers lower gives wire of about twice the area,
and of course of about twice the carrying capacity.
For instance, the area of No. 10 wire is about twice the
area of No. 13 wire.
When the current in a coil is constant, as in a series wound
dynamo, the number of ampere turns depends on the number
of turns of wire in the coil, but the heating depends on the
diameter of the wire.
When the voltage is constant between the terminals of the
coil, as in a shunt wound dynamo, the number of ampere turns
is independent of the number of turns; but the heating depends
on the 1 ength of the wire, because the shorter the wire the fewer
the turns, and the more amperes will flow and the ampere turns
will be constant, but if there is not wire enough in the coil the
heating will be excessive. Under these conditions a coil of a
given diameter and a given size of wire can only give a certain
number of ampere turns, which may be calculated by the formula:
EXL
N -rxf
N = Number of ampere turns.
E = Volts. L = Length of wire in feet per pound.
F = Middle length of wire in feet in one turn in the coil.
R = Resistance in Ohms per pound.
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