410 Cady and Arnold — Electric Arc. 



was most pronounced in the unstable state just before the arc 

 went out (§8). 



§39. Oscillations in the Silver and Copper Arcs. — The cop- 

 per arc was formed in air, the silver in nitrogen. The appara- 

 tus shown in fig. 6 was used, with 460 volt supply. In the 

 case of' copper, the galvanometer deflections at currents over 

 1*5 amperes "were small, but they increased with decreasing 

 current, and at a little below one ampere the light disappeared 

 from the scale. This is similar to the effect found when the 

 bolometer circuit was in parallel with the arc, and indicates 

 that we have to do with a peculiarity of the arc itself, and not 

 with oscillations imposed upon it from without. No special 

 effect was noted on passing from the second to the first stage 

 of the copper arc. 



In the case of silver, the effect was even more pronounced, 

 especially at low pressures. 



A silver anode and carbon cathode were used. At two 

 amperes and over the deflections were very small, but as the 

 current was decreased, the deflections grew larger, becoming 

 very pronounced on the first stage. With small currents at 

 T cm pressure, the deflection was far too great to be observed. 

 The tests with silver indicate that the cause of the effect lies 

 chiefly at the anode. There are reasons for thinking that these 

 oscillations may be due to an exceedingly rapid changing back 

 and forth between the two stages of the arc, or perhaps 

 between arc and glow : this point is to be investigated further. 



It is possible that the sensitive condition of the arc in the 

 neighborhood of its critical point may be such that this region 

 will prove to be a particularly good one to select for the pro- 

 duction of high frequency oscillations. 



Summary. 



§40. I. There exist between iron terminals in free air two 

 types of arc, distinct from each other in appearance and in 

 characteristic curves. To these types the terms first stage and 

 second stage have been applied for the sake of brevity. The 

 transition from one stage to the other is somewhat similar to 

 the hissing point of the carbon arc, but the cause is not the 

 same. Evidence from loss in mass of the electrodes, spectrum 

 of the arc, and temperature of the anode, shows that the first 

 stage is characterized by the absence of vaporization of the 

 positive terminal. 



II. Experiments are described on the change from the glow 

 discharge to the arc in free air between terminals of iron, 

 platinum and copper. Attempts were made at identifying 

 the two stages for arcs between several different metals in air. 



