74 



ELEMENTS OF ELECTRICAL ENGINEERING. 



() When the circuit has resistance and inductance and contains a condenser. In 

 this case the current which starts to flow is the maintained current m plus a decay- 

 ing oscillatory current, and after a very short time the decaying oscillatory current 

 disappears and the maintained current m alone exists. 



34. Electric resonance, A harmonic electromotive force of 

 given effective value will produce the greatest possible value of 

 alternating current in a given circuit (given resistance, given in' 

 ductance, and given capacity arranged in series as shown in Fig. 

 57), if the frequency of the electromotive force is such as to 



make 



I 



G)L 



a>C 



(0 



This is at once evident from equation (8) of Art. 27. This 

 production of a maximum current in a given circuit by an electro- 

 motive force of a given effective value at a certain critical fre- 

 quency is called resonance. The critical frequency in radians per 

 second (ca) is determined by equation (i), namely : 



\LC 



or since the frequency, /, in cycles per second is equal to &>/27r, 

 we have : 



/-= 



27T \LC 



(iii) 



