Cady and Vinal — Electric Arc. 101 



maintained throughout this operation. Then the pressure in 

 the tube was varied and the observations repeated. Each 

 variation of gas pressure of course changed the minimum 

 sparking potential, so that after a series of such observations it 

 was possible to set a sufficiently close upper limit for the 

 maximum potential drop across the arc. 



The lowest discharge potential observed in this way from 

 the calibrating circuit was 340 volts, the gas pressure in the 

 spark tube being about 7 mm . With a 460 volt supply e.m.f. 

 for the arc, the maximum potential drop across the arc when 

 oscillations were present was always found to be at least 340, 

 usually about 385, but never higher than 397 volts. 



These values are hardly greater than what would be 

 expected for the drop across the terminals of a short glow dis- 

 charge, where the greater part of the total drop is that at the 

 cathode. Hence, in so far as the spark-gap method is permis- 

 sible in the case of high-frequency oscillations, it seems proven 

 that the discharge is not intermittent and discontinuous, but 

 consists of an exceedingly rapid change back and forth between 

 arc and glow. 



The use of auxiliary ionizing electrodes is in every way to 

 be recommended for work of this sort. By their aid, the 

 response of the tube to an applied voltage above the minimum 

 is instantaneous, while the minimum discharge potential itself 

 is sharply denned and well reproducible. The presence of the 

 auxiliary ionizing discharge did not seem to make the critical 

 discharge potential lower than it was after the usual lag with 

 no artificial ionization. 



Summary. 



I. The theory of a type of pulsating discharge, called for 

 convenience the "glow-arc" discharge, is explained. This is a 

 spontaneous and rapid change back and forth between arc and 

 glow, whose essential feature is that the rate of expenditure of 

 energy at the cathode on the glow phase is greater than that 

 on the arc phase. Under suitable conditions the frequency of 

 these oscillations is so great that they take place in synchronism 

 with the natural period of the neighboring portion of the dis- 

 charge circuit, as determined by its resistance, self-inductance, 

 and capacity, distributed or concentrated. An • oscillating 

 current is thus generated, whose intensity may be greater than 

 that of the supply current. It is shown that higher harmonics 

 must be prominent in the current wave. 



II. The conditions for best oscillations were investigated. 

 The copper arc in air gives pulsations slow enough to be 

 recorded on a photographic plate, but the oscillations are most 



