FUNDAMENTAIL PRINCIPLES. 5 
I, and A are in convenient units. In English-speaking 
countries it is usual to take I, in feet and A in circular 
mils, Z.e., circles tgi7y of an inch in diameter. The con- 
stant connecting I, in feet and A in circular mils with the 
resistance in ohms, for copper wire of ordinary quality at 
ordinary temperatures, is 11. ‘Thisis approximately the 
resistance in ohms of a commercial copper wire one foot 
long and 1455 of an inch in diameter. The exact figure is 
a trifle less, but the ordinary contingencies of temperature, 
joints, etc., make it desirable to take 11. 
Substituting now this value of R in Ohm’s law it be- 
comes, reckoning the area in circular mils, 
¢ & 
I 
fp =t 
or, transposing, 
  
  
c.7. 
; 1L C 1) 
Cr) ¢ M 
This is the fundamental equation of electrical distribu- 
tion. It is like the original form of Ohm’s law, strictly a 
linear equation, so that all the quantities are connected by 
simple proportions. Doubling E, for example, halvesc.7., 
while doubling I, doubles c.7z. A convenient transposed 
form 1is 
e 
e 
which determines the current which a particular line will 
carry without exceeding a given loss, and another, 
e 
c.m. 
is convenient in figuring the actual fall of voltage. 
Throughout these equations ¥, represents the fall in volts 
through the conductor under conmsideration, and I, is al- 
ways the total length of the wire, 7.c., double the length of 
the circuit, assuming a uniform return wire. For grounded 
circuits the equations give correct results for so much of the 
circuit as is exclusively copper—the grounded portion in- 
volves a different constant and must be taken up as a sep- 
arate problem.