394 



Cacly and Arnold — Electric Arc. 



II. Observations with Various Metals. 



§15. Change from Glow to Arc with Iron and Platinum 

 Terminals. — Mention lias already been made (§9) of the glow 

 discharge in nitrogen between iron terminals with a supply 

 e.m.f. of 475 volts. This has been observed in free air with 

 terminals of iron, copper, and platinum, but only when the 

 electrodes were nearly free from oxidation. In each case, 

 almost the whole supply e.m.f. was used up in the arc. The 

 effect could only be obtained with an arc less than l-5 mra long. 



VOLT 



DULI 























400 























V 



V^A 





















\ 



\ 



V 



\ 



\ 

















J 00 





\ 



\ 



w 



«, 















200 





\ 



\ 



W 



















\ 



\ 



\ 



\ 













100 







\ 

 \ 





\ 



\ 

















B' 







\ 























C U B 









I 2 AMP. 



Fig. 3. Change from glow to arc in free air. Heavy lines, iron. 

 Light lines, platinum. 



The space between the electrodes looked entirely dark, but the 

 discharge remained in this state only a few seconds, for the 

 cathode rapidly became incandescent, causing the glow to 

 change to an arc, with the accompanying drop in voltage and 

 increase in current. 



With iron electrodes on which there were only small parti- 

 cles of oxide, the arc at small currents burned irregularly, and 

 at the positive end it sprang wildly from one particle to 

 another, while its negative end was concentrated on one parti- 

 cle. If the wandering of the arc caused the negative end to 

 leave its minute globule of oxide, the discharge instantly 

 changed to a glow. Then as soon as the whole cathode had 

 become incandescent, the arc formed again, concentrating itself 



