26 



ALTERNATING CURRENTS 



It will be observed that with resistance only, the alternating- 

 current circuit follows the same laws as the direct-current circuit, 

 in regard to the relation existing among voltage, current, 

 resistance and power. 



14. Circuit Containing Inductance Only. It was shown in 

 Vol. I, Chap. VIII, that inductance always opposes any change 

 in the current flowing in a circuit. For example, when the cur- 

 rent starts to increase in an inductive circuit, the electromotive 

 force of self-induction opposes this increase. This is illustrated 

 in Fig. 24 (a), which shows the rise of current in a direct-current 

 circuit containing resistance and inductance, when a steady volt- 

 age is impressed. The current rises slowly to its ultimate value. 



Time 



(a) (4) 



FIG. 24. Increase and decrease of current in an inductive circuit. 



On the other hand, when the current attempts to decrease in 

 the circuit, the inductance tends to prevent this decrease, as is 

 shown in Fig. 24 (6). In other words, if inductance is present in 

 a circuit, it always opposes any change in the current. With a 

 steady direct current, however, the inductance has no effect. 



If in Fig. 24 (a) the voltage across the inductance be lowered 

 when the current reaches point a, the current will not reach its 

 Ohm's law value. This same effect occurs in alternating-cur- 

 rent circuits. With inductance in the circuit, the current does 

 not have time to reach its Ohm's law value before the voltage be- 

 gins to decrease either positively or negatively. The current change 

 is opposed by the electromotive force of self-induction, which at 



di 

 any instant is equal to L-r.> where L is the inductance inhonrys 



and -j- is the rate in amperes per second at which the current is 



changing at that instant. The minus sign signifies that this 

 voltage is opposing the change in the current. 



