94 



Cady and Vinal — Electric Arc. 



a large family of harmonics, may confidently be predicted. 

 Owing to the lack of symmetry between arc and glow phases, 

 both odd and even harmonics may occur (§ 25). 



A further complication arises from the fact that since the 

 oscillations adapt their frequency to the natural period of that 

 part of the circuit which they are able to penetrate, a kind of 

 resonance sets in, with the result that the effective value of the 

 alternating current may he greater than that of the direct 

 current supplied by the generator. 



This shows that f of a certain time during each cycle the 

 direction of the current is reversed. It is evident that in order 



Fig. 2. 



\j 



\ i 



V 



for this to take place the capacity of the arc circuit must play 

 an important role. In this respect the oscillations have a 

 slight similarity with the third type of oscillation studied by 

 Blondel in the singing arc* 



The form of the current wave must be somewhat as repre- 

 sented in fig. 2. Here the full lines represent the normal 

 current pulsating between arc and glow where the effects of 

 resonance are neglected. The broken lines show the form of 

 the current curve when the pulsations take place in synchro- 

 nism with the natural period of the circuit. The current on 

 the arc phase rises much higher than before, while the glow 

 phase with the current in the positive direction practically 

 disajypears, a brief discharge in the opposite direction taking 

 its place. 



If the frequency were lower, the potential difference would 

 have to rise very high in order to re-start the discharge at each 

 reversal, and the result would be practically a succession of 

 sparks. But in view of the high frequency (on the order of 

 a quarter of a million) and of the short distance between the 

 * Blondel, l'Ecl. Elect., xliv, p. 81, 1905 



