390 Cady and Arnold — Electric Arc. 



duced into it. The curvature of the rod was such that it 

 could just be slipped in through the neck of the bulb. T was a 

 side outlet leading to the manometer, Geryk pump and nitro- 

 gen generating and purifying apparatus. 



This construction was chosen in order to exclude from the 

 arc all substances that might introduce traces of oxygen, and 

 at the same time to permit as much freedom as possible in manip- 

 ulating the electrodes. Great precautions were taken in puri- 

 fying and drying the nitrogen, which was generated chemically. 

 The bulb was carefully heated to drive off occluded gases. 

 Several sets of observations were made, in each case the appa- 

 ratus having been exhausted and filled with nitrogen several 

 times. In some cases a trace of oxidation was noticed at the 

 start, which soon ceased, leaving the tips of the electrodes bright 

 and silvery in appearance. 



In agreement with an observation of Arons* we found that 

 the black metallic dust from the arc became deposited more 

 densely on the negative than on the positive electrode. This 

 would seem to indicate that the dust particles on leaving the 

 arc carry a positive charge. It is not what one would expect 

 in view of the fact that it is chiefly the negative electrode that 

 disintegrates. 



§8. The following results were noted. As long as a trace of 

 black oxide remained on the .electrodes both stages could be 

 obtained, in nitrogen as well as in air. But when the discharge 

 took place between terminals of metallic iron, it presented the 

 appearance of the second rather than the first stage, even when 

 the current was reduced until the arc went out. Only in the 

 last instant before the arc ceased, when it was flickering irreg- 

 ularly, was there any indication of the appearance of the first 

 stage. This alone is enough to show that oxidation is not the 

 cause of the second stage in the iron arc. 



§9. Observations in a Partial Vacuum. — We shall show in 

 §23 that in the case of certain metals the critical point can easily 

 be obtained at reduced gas pressures, although it does not appear 

 in free air. With unoxiclized iron and platinum it is almost 

 impossible to get the first stage at any pressure. With a sup- 

 ply of 475 volts, and iron terminals in nitrogen, using the 

 apparatus of fig. 2, a glow discharge could be obtained at any 

 pressure, but when the current was sufficiently increased the 

 glow persisted in changing to the second stage of the arc, pro- 

 vided the pressure was high enough to make the formation of 

 an arc possible. 



A careful examination was made at various pressures, but no 

 stable first stage could be found. At a pressure of 60 cm and 

 with very short arc there was what seemed to be an exceed- 

 * Ann. Phys., i, 700, 1900. 



