76 ELECTRICAL ENGINEERING 



53. Resistance. The resistance of a conductor varies di- 

 rectly as its length and inversely as its sectional area; it also de- 

 pends on the material of which the conductor is made; 



= PJ, ....... (113) 



where I is the length of the conductor, 



A is the sectional area, 



p is the specific resistance or resistivity of the material. 

 Wires are usually specified by gauge numbers, their lengths are 

 given in feet and their sectional areas in circular mils. A circular 

 mil is the area of a circle one mil or one thousandth of an inch in 

 diameter. The specific resistance is then the resistance of a 

 wire one foot long and one circular mil in section. 



54. Conductance. The reciprocal of the resistance of a con- 

 ductor is called its conductance and is represented by the letter 

 G, where 



The reciprocal of the specific resistance or resistivity of a material 

 is called its conductivity and is represented by the Greek letter 

 7, where 



i- 1 . 



p 



Conductivity is expressed in per cent of Matthiessen's standard of 

 conductivity. Electrolytic copper sometimes reaches a value of 

 101 per cent of this standard, but commercial copper ranges from 

 97 to 99 per cent conductivity. Aluminum wire has a con- 

 ductivity of 60 or 61 per cent. 



55. Effect of Temperature on Resistance. The electric re- 

 sistance of materials varies with their temperature. The varia- 

 tion of the resistance of metal conductors can be expressed by the 

 following formula, 



R t = Ro(l + at), ...... (114) 



where R Q is the resistance at a chosen standard temperature, R t 

 is the resistance at a temperature t degrees higher and a is the 

 temperature coefficient of resistance. It is the increase in re- 

 sistance per degree rise in temperature expressed as a fraction of 

 the resistance at the standard temperature. If the centigrade 



