﻿the Magnesium High-frequency Arc. 



95 



type as previously * used in an investigation of the Ponlsen 

 arc. The arc was struck between horizontal electrodes of 

 copper and carbon in an atmosphere of coal-gas and in the 

 field of a powerful electromagnet. 



The supply current of about 4 amps, was taken from the 

 public supply at 240 volts. An oscillatory circuit consisting 

 of a variable inductance wound with thick copper rod (^ in.) 

 and a variable leyden-jar battery formed a shunt to the arc, 

 large capacity block condensers being inserted to cut off 

 the supply volts from this circuit. When large choking-coils 

 were included in the arc supply circuit and the copper anode 

 was water-cooled no difficulty was experienced in maintaining 

 continuous oscillations for as long as was desired. 



If a piece of metal or carbon held in the hand or earth- 

 connected is brought into contact with the terminal of the 

 inductance which is joined to the capacity, an arc of 1 or 

 2 cm. in length can be drawn out and maintained. Obser- 

 vations with a wave-meter showed that this discharge had 



Fiff. 1. 



TO £#tfTH 



practically the frequency of the undisturbed oscillatory 

 circuit. We have, therefore, an arc which consists of a 

 variable high-frequency discharge of undamped oscillations 

 which can be produced between any selected electrodes. 

 A preliminary examination of an arc produced in this way 

 was the subject of the present investigation. Fig. 1 shows 

 diagrammatically the disposition of the apparatus. 



* E. O'Connor, Phys. Zeit. vol. xii. p. 196 (1911). 



