THE 



AMERICANJOURNALOFSCIENCE 



[FOURTH SERIES.] 



Art. XXXV. — On the Electric Arc "between Metallic Elec- 

 trodes;* by W. G. Cadt and II. D. Arnold. 



First Paper. — Introduction, 



§1. Up to the present time most investigations on the electric 

 discharge between metals have been confined either to the 

 glow discharge, — chiefly at low gas pressures, — or else to the 

 arc discharge at relatively high current densities, where a pro- 

 nounced volatilization of both electrodes takes place. No 

 systematic examination of the transition from one of these 

 forms of discharge to the other, for various metals, seems to 

 have been carried out. The present paper has to do with this 

 transitional region, having regard particularly to the phenomena 

 observed with the electric arc at relatively small currents. f 



The starting-point of the investigation was the observation 

 made by one of the writers, that the iron arc at a certain crit- 

 ical value of current undergoes an abrupt change somewhat 

 similar to the well-known " hissing point " of the carbon arc. 

 The similarity was so strong that in our preliminary reports we 

 used the terms " quiet state, " " hissing state, " and " hissing 

 point " to denote the phenomena observed. More recent obser- 

 vations have shown that the effect is not to be compared to the 

 hissing point of the carbon arc, but that it is a different phenom- 

 enon, casual reference to which has been made by various 

 observers in the past. Thus Maisel^: notes that the iron arc 



* This investigation is being carried on with the aid of a grant of $200 from 

 the Elizabeth Thompson Science Fund, grateful acknowledgment of which 

 the writers desire to make here. 



f Brief reports on these experiments have appeared in Nature, lxxiv, 443, 

 1906 ; Electrician, lviii, 816, 1907 ; Phys. Rev., xxiv, 381 and 446, 1907. 



tPhys. Zeitschr., v, 550, 1904. 



Am. Jour. Sci. — Fourth Series, Vol. XXIV, No. 143. — November, 1907. 



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