392 Cady and Arnold — Electric Arc. 



Thus although more energ} 7 is expended on the first than on 

 the second stage, still the loss in weight of the positive globule 

 is seven times as great on the second as on the first. The loss 

 on the second stage, according to this theory, is due to vapor- 

 ization of the globule ; that on the first, in so far as it cannot 

 be accounted for by errors, is due in part perhaps to ionic 

 bombardment, and in part to local heating from the arc itself. 



Strictly speaking, the energy considered should have been that 

 expended at the surface of the anode, and not that for the whole 

 arc. Still, since the change in voltage occurs chiefly at the 

 anode, it follows that if the true anode drop were known and 

 multiplied by the value of the current, the ratio of the energies 

 for the first and second stage would be even greater than is 

 indicated in Table I, thus making the argument even more 

 conclusive. 



§12. Spectroscopic Evidence. — The arc was formed between 

 a carbon cathode and iron anode; its image was then projected 

 onto the slit of a spectroscope. On the first stage, the spectrum 

 of iron was faintly visible, owing to traces of iron in the 

 carbon and probably to a small amount of iron set free from 

 the anode, as indicated in the preceding paragraph. At the 

 instant when on decreasing the external resistance the critical 

 point was reached, the iron spectrum flashed out with great 

 brilliancy. The same is true of the arc between a carbon 

 cathode and silver anode (§24). This indicates that even if 

 vaporization is taking place on the first stage, it is exceed- 

 ingly small in comparison with that on the second. 



§13. Evidence from Temperature of Positive Globule. — 

 A series of tests was made with a thermo-electric junction 

 imbedded in the positive globule. A junction consisting of a 

 platinum wire 0*1 3 mm in diameter and a wire of platinum and 10 

 per cent rhodium 0*25 mm in diameter, held by an arm that could 

 be controlled by slow-motion screws, was thrust into the globule 

 after the arc had been formed. It was connected to a d'Arson- 

 val galvanometer, and scale deflections were noted as the arc 

 current gradually increased. According to the vaporization 

 theory, the positive globule on the first stage, with the excep- 

 tion perhaps of a small region immediately under the arc, is at 

 a temperature below the boiling point. At the critical point, 

 copious vaporization suddenly sets in, and from then on the 

 temperature of the globule would remain practically constant 

 if it were not for losses by conduction and radiation. Owing 

 to these losses, and to the' diminution of energy on passing 

 through the critical point, changes in the mean temperature of 

 the globule cannot be taken as a sure indication of its physical 

 state in the neighborhood of the arc. 



It was found, however, in spite of the great decrease in 



