ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 241 



127. Forced oscillations of a transmission line. The alternator 

 in Figs. 179 and 180 is assumed to have a frequency equal to the 

 frequency of free oscillation of the line, and the oscillations which 

 are represented in Figs. 179 and 180 are called free oscillations. 

 When the frequency of the alternator is not the same as the 

 frequency of free oscillation of the line, then we have what is 

 called a forced oscillation of the line, and the alternator is, in this 

 case, not at a voltage node. 



If the frequency of the alternator coincides with the frequency 

 of one of the simple modes of oscillation of the transmission line, 

 the violence of the oscillations will become very great if the line 

 losses are small, in fact the violence of the ultimate oscillations 

 which are built up in this case is limited only by the line losses. 



Fig. 181. 

 Voltage distribution over oscillating transmission line open at far end. 



When the frequency of the electromotive force of the alternator 

 does not coincide with one of the simple modes of oscillation of 

 the line, the ultimate steady state of oscillation of the line is 

 determined as follows (line losses being ignored). Figures 181 

 and 182 represent voltage distributions over a transmission 

 line which is connected to an alternator, the distant end of the 



to an alternator of the proper frequency at the other end. The alternator in 

 Figs. 179 and 180 is assumed to have a very low voltage because after the line is once 

 oscillating (its resistance and leakage being assumed to be negligible) no percep- 

 tible generator voltage is required to maintain the oscillation, 



17 



