Cady and Vinal — Electric Arc. 91 



springs in passing from glow to arc, cb' the interval traversed 

 in the reverse direction." The slope of the line he' is deter- 

 mined by E , the supply electromotive force. 



Let the resistance initially be relatively small, so that the 

 arc is stable at d. As the resistance is now increased, there 

 comes a point where the energy at the cathode is too small to 

 maintain vaporization, and the discharge changes suddenly 

 along cb' to a glow. The anode may or may not be in a state 

 of vaporization. Generally the anode ceases to vaporize before 

 the cathode ("first stage" of our former paper). The oscilla- 

 tions dealt with here are a purely cathode phenomenon, and 

 the material of the anode is of no consequence. 



If the line cb' is steep (high impressed electromotive force) 

 then it may be that the energy expended at the cathode at b' is 

 greater than that at c. If conditions are such that the heat is 

 largely confined to the neighborhood of the cathode, the tem- 

 perature of the negative terminal rises rapidly, so that the part 

 b'b of the characteristic, possibly somewhat modified on account 

 of high temperature, is automatically traversed, until evapora- 

 tion sets. in, an arc forms along be', and the cycle repeats itself 

 again. The period of this cycle is exceedingly variable, 

 depending upon a large number of factors, of which the chief 

 are nature and form of cathode, means of disposing of heat, 

 distance between electrodes, nature and pressure of gas, 

 electromotive force and resistance of the circuit, and, as will 

 be seen, the self-inductance and capacity of the circuit con- 

 sidered as a system capable of performing electrical oscilla- 

 tions. 



§ 3. For example, with a supply of 460 volts, using copper 

 electrodes, free from oxide, one to two mm. in diameter, in free 

 air, the discharge can be made to pulsate irregularly between 

 arc and glow with a period of from one-half second to several 

 seconds. On the glow phase the negative mantle covers the 

 end of the electrode, which becomes incandescent for a con- 

 siderable distance from its tip before vaporization sets in. 



If now the gap is shortened and the current diminished, the 

 glow will take place in gas left ionized by the arc, and con- 

 fined in the narrow space between the electrodes. The nega- 

 tive mantle will occupy approximately the area of the negative 

 base of the arc, and the temperature of this small area will rise 

 very rapidly. This hastens vaporization and shortens the glow 

 phase. Since the energy available for the maintenance of 

 vaporization on the arc phase is now very small, the duration 

 of the arc is also much reduced. The discharge then pulsates 

 with a frequency which may reach into the thousands between 

 the two phases, the temperature of the cathode surface varying 



*Cf. Kaufmann, Ann. Phys., ii, p. 162, 1900 ; also our first paper, p. 395. 



