78 



ELECTRICAL ENGINEERING 



Carbon, unlike the metals, has a negative temperature co- 

 efficient and it is not constant. The resistance of insulating 

 materials decreases very rapidly with increase of temperature, 

 but the variation is irregular and cannot be expressed by a simple 



200 



180 

 160 

 140 



100 



30 40 50 60 70 80 90 100 

 Temperature Degrees Centigrade 



10 20 



FIG. 62. Variation of resistance of slot insulation with temperature. 



equation. Fig. 62 shows the variation of the resistance of an 

 insulating material with temperature. The resistance /is ex- 

 pressed in megohms or millions of ohms. 



56. Resistance of Conductors. The resistance of a circular 

 mil foot of pure copper wire at 25 C. is 10.55 ohms; therefore, 

 the resistance of a copper wire at 25 C. is, by formula 113, 



R = 10.55 



length in feet 



section in circular mils 



= 10.55 



V 



cir. mils 



(117) 



In the case of rectangular conductors, as bus bars, the section is 

 expressed in square mils and the value of p is then the resistance 

 of a square wire, one mil on each side and one foot long. 



The following table gives the specific resistance of copper at 

 different temperatures and the corresponding values of the tem- 

 perature coefficient. 



