Gady and Arnold — Electric Arc. 411 



Both stages could be maintained steadily only in the cases of 

 copper and of iron. In nitrogen at reduced pressures the glow 

 and both stages of the arc were observed between a carbon 

 cathode and anodes of lead, silver, and carbon. There is 

 reason to believe that similar results can be obtained with 

 most other metals. Platinum, and iron free from traces of 

 oxide, did not show a stable first stage under any conditions. 



The diminution in potential difference between the arc ter- 

 minals in passing through the critical point from the first to the 

 second stage is attributed partly to a decrease in the thermal 

 e.m.f. at the anode, partly to increased conductivity of the arc 

 vapor. 



There is some evidence of a connection between the critical 

 point and the melting temperature of the material of the 

 anode, but it is not well marked, and is likely to be masked 

 by effects due to the physical condition of the surface of the 

 anode. 



III. The characteristic curves of the iron arc are discussed, 

 together with the probable form of the equation connecting 

 voltage, current, and length. 



IV. A series of experiments was carried out on electrical 

 oscillations in arcs with iron, copper, and silver anodes. 



At a current a little below two amperes, on the second stage, 

 the iron arc rotates and emits a whistling sound. This is 

 often attended by spasmodic changes back to the first stage. 



The application of Lecher's test for continuity led to observa- 

 tions on the "singing" iron and copper arcs, the energy 

 expended in the oscillations on both stages of the arc being 

 observed by means of a bolometer. To test the continuity of 

 arcs between various metals by a method more free from error, 

 a new manner of connecting the bolometer was devised. A 

 general agreement with Lecher's results for large currents was 

 found. But the iron arc became continuous on the first stage, 

 while in the case of arcs with silver or copper anodes, marked 

 oscillations were detected as the current decreased. 



Scott Laboratory of Physics, Wesleyan University, 

 Midclletown, Conn., July, 1907. 



