400 Cady and Arnold — Electric Arc. 



these forms of discharge, referring to them merely as " arcs. " 

 He also noted still another modification when cored carbons 

 were used. 



Theory of the Change in Voltage at the Critical Point. 



§27. Among the agents that influence the anode drop may 

 be mentioned (1) material and temperature of the anode, (2) 

 nature, degree of ionization, and temperature of gas in the 

 neighborhood of the anode, (3) ultra-violet light from the 

 cathode. 



In passing from first to second stage, these factors are 

 affected as follows : 



(1) The change in temperature of the anode may be 

 neglected, since, according to the theory that has been advanced, 

 the critical point is characterized by the arrival of the material 

 of the anode at its boiling point. 



(2) On the first stage the gas at the anode is chiefly that of 

 the surrounding space, with some vapor from the cathode. On 

 the second stage the current passes mainly through vapor sup- 

 plied by the anode. The contraction of the arc at the anode 

 on the second stage indicates a considerable rise in temperature 

 of this vapor over that of the gas on the first stage. 



(3) Since the change does not affect the cathode, the ultra- 

 violet light is unchanged, unless itbecomes partly absorbed by 

 the vapor before reaching the anode. On the other hand, 

 ultra-violet light from the anode itself on the second stage may 

 compensate for this. 



It is probably. that a thermal e.m.f. exists at the anode on the 

 first stage in a direction opposed to the current, on account of 

 the high temperature of the anode, and, with iron and copper 

 in free air, perhaps also the fact that the oxides of metals emit 

 negative ions with comparative ease. When vaporization of 

 the anode begins, the increased temperature and density of the 

 vapor in contact with the anode, and its identity with the 

 anode in substance, conspire to reduce this thermal e.m.f. 

 and thereby to lower the anode drop.* 



Great experimental difficulties would stand in the way of 

 measuring the counter electromotive-force at the anode by the 

 method used by Duddell for the carbon arc,f which so far is 

 the only method capable of yielding reliable results. It seems 

 most probable to the writers that the observed decrease in volt- 

 age at the critical point is due partly to this change in thermal 

 electromotive-force, partly to the increased conductivity of 

 the vapor in the arc. The theory would require modification 



*Cf. Stark, Ketschinsky and Scnaposchnikoff, Ann. Phys. , xviii, 213, 1905. 

 f Electrician, p. 918, 1901. 



