7 6 



ELEMENTS OF ELECTRICAL ENGINEERING. 



of coL is very great, and the current is limited by the inductance 

 reaction. It is interesting to note that at the critical frequency 

 of 60 cycles per second, with a current of 100 amperes, the 

 electromotive force across the condenser terminals is 13,270 

 volts effective (= I/aC) although the total electromotive force 

 acting on the circuit is only 200 volts. 



Mechanical resonance, analog of electric resonance. A weight 

 M, Fig. 67, is attached to the end of a flat spring which is 

 clamped in a vise, and the weight M is caused to oscillate back 



and forth in a vessel 'of water 

 by an alternating force / of 

 definite frequency. In general 

 the force / has to do three 

 things, namely : (a) Overcome 

 the resistance with which the 

 water opposes the to-and-fro 

 motion of M\ (&) overcome the 



vise 



f 



Spring 



f 



Fig. 67. 



inertia of the weight M as it 

 gains and loses velocity ; and 

 (<r) overcome the elastic reaction 

 of the spring. These three parts 

 of / are related to each other 

 and to the changing velocity v 

 of M in exactly the same way 

 that the three parts, Ri, L dijdt, 



and q\C (see Art. 27), of an impressed electromotive force are 

 related to each other and to the changing current i in the circuit 

 shown in Fig. 57. The weight J/when mounted as shown in Fig. 

 67 has a definite frequency of free oscillation, and at this frequency 

 the elasticity of the spring takes care of the inertia reaction of 

 M, and the inertia reaction of M provides the force necessary to 

 bend the spring. Therefore, if the frequency of the force/, Fig. 

 67, is the same as the frequency of the free oscillations of M y 

 then, after the oscillatory motion of M.is fully established, the 

 force /is used only to overcome resistance. 



