86 THE ELECTRIC ARC 



With the same electrodes in hydrogen and with a form 

 of discharge which required only 0.35 ampere and a poten- 

 tial difference of about 300 volts he found that the cathode 

 lost 0.004 g- in i^ hr. and the anode gained an equal amount, 

 but it is a question whether this was really an arc or not, 

 so that no definite conclusion can be drawn. But the two 

 experiments would at least show that nothing as simple 

 as Faraday's laws will apply to the loss of material of the 

 electrodes of the arc. 



The only measurements which have been published con- 

 cerning the potential in different parts of the arc in other 

 gases than air were made by myself when working with 

 the arc in hydrogen. It was found that with graphite 

 terminals the larger drop was at the cathode. When the 

 pressure of the gas was 700 mm., the anode drop was 

 57 volts and the cathode drop 65 volts. On the other hand 

 with the carbon arc in hydrogen by far the greater drop 

 was at the anode. With 700 mm. of pressure the anode 

 drop was 85 volts and the cathode drop 15 volts. 



With lower pressures the behavior of the arc is much the 

 same as it is with low pressures of air. The principal differ- 

 ences are found when using low pressures of hydrogen. 

 Striations are much more prominent than they are when 

 the residual gas is air and these appear at much higher 

 pressures. With pressures of approximately 20 mm. I 

 found four distinct forms of the arc, three of these showing 

 Striations. 



