92 Cady and Vinal — Electric Arc. 



between the boiling point of the material and one very 

 slightly below it. This local heating is now largely dependent 

 on the condition of the cathode surface, and the period will 

 fluctuate as the discharge wanders from point to point over 

 this surface. The oscillations which we recorded photograph- 

 ically, using copper electrodes in air, are to be regarded as an 

 instance of this sort (■§ 9). 



§ 4. In order further to increase the rapidity of the pulsa- 

 tions, attention must be paid to the surrounding gas. As will 

 be seen (§ 10), when a suitable gas is chosen, the current 

 reduced to the right extent, and the distance between elec- 

 trodes made very small, the period of the pulsations is deter- 

 mined chiefly by the electrical constants of the circuit. In 

 other words, high-frequency electrical oscillations take place, 

 governed by the resistance, self-inductance, and capacity, con- 

 centrated or distributed, of the arc circuit, which react upon 

 the discharge, and control its periodicity. The discharge then 

 imparts energy to the oscillations as the escapement of a clock 

 imparts energy to the pendulum. The material of the cathode 

 may affect the intensity and permanency of the oscillations, 

 but not their frequency. 



In this respect the glow-arc discharge is analogous to the 

 spark-gap, except that it seems to consist of undamped oscilla- 

 tions (§ 29). It is also analogous to the oscillating arc 

 of the Thomson-Duddell type, whose period of oscillation 

 depends essentially on the capacity and self-inductance in 

 parallel with it. The distributed self -inductance and capacity 

 of the glow-arc discharge circuit itself correspond to the self- 

 inductance and capacity in parallel with the Duddell arc, and 

 the steep slope of the line be' (fig. 1) to the "falling" charac- 

 teristic of Duddell. 



High-frequency oscillations of the Duddell type, in which 

 the only self-inductance and capacity were those of the elec- 

 trodes and adjacent parts of the circuit, have been described 

 by Stschodro* and Simonf, but in each of these cases a stable 

 arc of the ordinary sort seems to have been used. 



§ 5. During the passage of the oscillations the discharge 

 circuit itself behaves like an open oscillator, radiating energy. 

 Thus it is possible to obtain energy from the discharge in either 

 one of two ways, viz.: by coupling a coil in the circuit induc- 

 tively with a secondary coil outside, or by making use of the 

 electromagnetic waves radiated from the various parts of the 

 circuit. The former method was used in our frequency 

 determinations. The radiations will be considered in §32. 



*Stschodro, Ami. Phys., xxvii, p. 225, 1908. 

 f Simon, Phys. Zeitschr., ix, p. 872, 1908. 



