388 Cady and Arnold — Electric Arc. 



J<ti. The Drop in Potential at the Anode. — "Mrs. Ayrton* 

 found for the carbon arc, that of the change in voltage in 

 passing through the hissing point, about two-thirds occurs at 

 the anode, the rest in the gas. There are serious difficulties 

 in the way of carrying out this test with iron, for although 

 the iron arc when undisturbed burns steadily enough, still the 

 approach of an exploring pencil, especially when the current 

 is small, causes the arc to be repelled and to burn with great 

 irregularity. f Indeed, the sources of error in the use of 

 exploring electrodes mentioned by Pollak;}; in connection with 

 the mercury arc and by Child§ seem to be particularly danger- 

 ous with the metallic arc in free air. 



Carbon was first tried as an exploring electrode, but it was 

 soon abandoned for fear of error from the vapor which it 

 introduced into the arc. It did, indeed, change the arc volt- 

 age by about iive volts. 



Then a rod of iron was employed. A small diameter could 

 not be used, as the rod then melted too rapidly. Platinum 

 melted almost as readily as iron. When an iron pencil 

 2.5mm thick was introduced into the arc, a globule of oxide 

 formed on its extremity, which was so large in comparison 

 with the length of arc that any thought of accurate explora- 

 tion of the iield was out of the question. Still, it was possible 

 to show at least qualitatively by means of a quadrant electrom- 

 eter that the change in voltage when passing through the criti- 

 cal point was chiefly, if not wholly, on the anode side of the 

 exploring electrode. 



When the arc is formed between a cathode of carbon or 

 copper and an iron anode, the critical point occurs practically 

 the same as between iron terminals. But when iron is the 

 cathode and carbon the anode, no sudden change takes place 

 until the hissing point for carbon is reached. With an iron 

 cathode and copper anode no critical point occurs (unless it be 

 the critical point for copper, which is discussed farther on) 

 until a deposit of iron has formed on the copper; then the 

 normal change for iron takes place. 



The Iron Arc in Nitrogen. 



§7. Pursuing the analogy with the carbon arc, the question 

 arose whether oxidation plays a part in producing the observed 

 effect. Some preliminary tests in which oxygen had been 

 excluded from the arc as far as possible indicated that either a 

 very small trace of oxygen is still capable of precipitating the 



*The Electric Arc, p. 288. 



f Child observed the positive and negative drop in the case of the metallic 

 arc, but he used a current of six amperes. See Phvs. Eev., xii, 149, 1901. 

 {Ann. Phys., xix, 218, 1906. § Phys. Rev., xxiv, 498, 1907. 



