398 Gady and Arnold — Electric Arc. 



which formed merely a thin coating over the surface of the 

 anode. 



At 30 cm the change from second to first stage was easily 

 observed. This is shown in tig. 4, points G and H. To the 

 left of G the arc is stable on the first stage; to the right of IT, 

 on the second stage. 



At 55 cm .and over only the second stage could be maintained. 



When on the first stage, there is a soft diffused positive glow 

 over the anode and the arc is very dark except close to the 

 cathode. On the second stage the usual bright spot appears 

 at the anode and the whole arc becomes luminous. 



§24. Silver. — In free air with a carbon cathode the silver 

 anode always melted, though there was no troublesome oxida- 

 tion. Only the second stage could be observed. 



Observations in nitrogen were begun at a pressure of about 

 a millimeter, using the apparatus described in §22. An e.m.f. 

 of 460 volts was applied. The chief observations are shown 

 in fig. 4. A and B are on the glow discharge, G and D on 

 the first stage of the arc, and E and F on the second stage. 

 The length of arc was 3 mm except for the point F, where it 

 was somewhat greater. The change from B to G took place 

 spontaneously when the pressure had increased to 4'5 cm , and 

 from D to E when it had increased to 31 om . It was not found 

 possible to get both changes at the same pressure. The lines 

 AB, CD and EF are what would result if the external resist- 

 ance were gradually diminished and at the same time the gas 

 pressure increased. 



The appearance in the neighborhood of the anode on the 

 two stages of the arc was similar to that in the case of lead, 

 but a decided tendency to the formation of stria tions was 

 manifest. 



There can be but little doubt that most metals would yield 

 curves similar to those in fig. 4. In other words, between a 

 carbon cathode and metallic anode three distinct forms of dis- 

 charge are possible, for each of which a separate characteristic 

 curve must be drawn ; the glow, the arc with non-volatilizing 

 anode, and the arc with an anode that is giving off vapor. 

 With electrodes of iron or copper, all three stages may be 

 seen in free air by merely decreasing the external resistance 

 (%. 3). 



§25. The Striated Arc. — When the nitrogen pressure was 

 in the neighborhood of 6 cm , the arc with non-volatilizing silver 

 anode (first stage) showed several exceedingly distinct stria- 

 tions, sometimes as many as six or seven. This occurred with 

 a length of arc from 10 to 15 mm and with current ranging 

 from - 3 to 1*5 amp. The discharge began at an incandescent 

 spot on the carbon cathode, from which a luminous column 2 



