22 ELEMENTS OF ELECTRICAL ENGINEERING. 



the same value, namely, 0.0037, that is, the resistance of a pure 

 metal is very nearly proportional to the absolute temperature as 

 measured by an air thermometer. The value of /3 for high grade 

 commercial copper is about 0.004 per centigrade degree. 



Solutions of acids and salts, graphitic carbon, and nearly all 

 so-called insulating substances such as glass and rubber, decrease 

 in resistance with rise of temperature ; in other words their tem- 

 perature coefficient of resistance is negative. 



15. Power required to maintain a current in a circuit expressed 

 in terms of resistance and current. When the work which is 

 spent in maintaining a current all appears in the circuit as heat, 

 then the rate at which heat appears in the circuit is equal to the 

 rate at which work is done in the maintenance of the current. 

 Therefore, the power P required to maintain a current" /through 

 a wire having a resistance R is equal to RP if no portion of the 

 power is used for producing mechanical or chemical effects. 

 That is : 



P=RP (14) 



in which P is the power in watts (joules per second) required to 

 maintain a current of / amperes through a wire having R ohms 

 of resistance. Of course equation (14) holds true when all quan- 

 tities are expressed in c.g.s. units. (P in ergs per second, / in 

 abamperes, and R in abohms.) 



16. Electromotive force. It may seem from equation (14) 

 that the power delivered by a generator should be proportional 

 to the square of the current, but the current delivered by a given 

 generator under fixed conditions can be increased only by reduc- 

 ing the resistance of the circuit to which the generator delivers 

 current. In fact the power delivered by a given generator under 

 fixed conditions of running is very nearly proportional to the cur- 

 rent so that the ratio P/I is nearly constant. This ratio P/7, 

 whether it is constant or not, is called the electromotive force of a 

 generator. That is : 



P-EI (15) 



