THE MAGNETIC CIRCUIT ELECTROMAGNETS 35 



WIRE TABLE, STANDARD WIRE GAQE, COPPER 



i A variation in resistance up to 2 per cent, increase on the calculated values for pure 

 copper is generally allowed. 



The cross-section of a wire may be expressed in square inches 

 or in square mils (1 mil = 1/1000 in.) ; the metric system is rarely 

 used in English-speaking countries. 



Circular Mils. The cross-section of a wire or conductor may 

 also be expressed in "circular mils." This is the unit of area 

 commonly used in America when the cross-section of electrical 

 conductors is referred to. The confusion of ideas resulting from 

 the conception of the circular mil as a unit of area may be com- 

 pensated for by certain practical advantages, but these advantages 

 are not obvious. The circular mil is the area of a circle 1 mil 



