58 ELEMENTS OF ELECTRICAL ENGINEERING. 



acts on the circuit is used to cause the current to change. The 

 portion of the electromotive force so used is equal to the product 

 of the inductance of the circuit and the rate of change of the cur- 

 rent.* Therefore a discussion of the relation between current and 



gfB 



o 



o 

 o 



Fig. 49. 



Fig. 493. 



voltage in an alternating-current circuit depends upon a knowledge 

 of the rate of change of the current. 



The rate of change, dijdt y of a harmonic current i is a har- 



<... . t 





Fig. 50. 



monically varying quantity, that is, dijdt is proportional at each 

 instant to the sine or cosine of a uniformly increasing angle wt, the 



* The student who is not familiar with the phenomena of inductance should read 

 Appendix A. 



