82 ELECTRICAL ENGINEERING 



is 30 ohms at a room temperature of 25 C. and after running 

 under load is found to be 31.5 ohms, determine the average tem- 

 perature of the winding. 



If t is the average temperature of the winding when hot, formula 

 115 gives 



Rt = #25 1 1 + 0.00386 (t - 25) j, 

 or substituting 



31.5 = 30 Jl + 0.00386 (t - 25) j ; 



the rise in temperature is 



31.5 l 



i 9*; 1 *} P 



" 0.00386 " 



and the average temperature of the winding is 

 t = 25 + 13 = 38 C. 



This method is used in measuring the temperature rise in the 

 field and armature windings of electrical machines to replace the 

 inaccurate indications given by the use of thermometers. 



60. Kirchoff's Laws. Two laws enunciated by Kirchoff are 

 of great value in solving problems dealing with continuous cur- 

 rent circuits. 



-i vwv I D 



AA/WV 



T* 



FIG. 64. Kirchoff s first law. 



First law. The sum of all the electromotive forces around a 

 closed circuit, taken in their proper direction, is equal to zero; 

 here the e.m.f. consumed by resistance is considered as a counter 

 e.m.f. opposing the current. 



Second law. The algebraic sum of all currents flowing toward 

 a distributing point is equal to zero. 



The first law is illustrated by the circuit A BCD in Fig. 64. Be- 

 tween the points A and B an e.m.f. E is applied which drives a 

 current I around the circuit in the clockwise direction against the 



