ARC IN AIR BETWEEN OTHER SUBSTANCES 69 



ages for lengths greater than 0.2 inch are in general greater 

 than those for the magnetite arc, while for arcs shorter 

 than this they are less. 1 



A somewhat similar arc is described by Weedon 2 in 

 which titanium carbide is used for the cathode and copper 

 for the anode. The characteristic curves which he gives 

 for this arc are nearly identical with those given by Stein- 

 metz for the magnetite arc. 



Arc between Electrolytes. As has been stated it re- 

 quires a very high E.M.F. to produce an arc between a 

 metal and an electrolyte, and when produced it is very 

 unsteady. As far as I have been able to learn, no measure- 

 ments of current nor potential difference have been taken 

 with such arcs. About all that has been attempted is to 

 show that it is possible to have arcs of this kind. Thus 

 Grove 3 found that water could be made the anode of an 

 arc and somewhat less easily the cathode, platinum being 

 the other terminal. His source of current was 500 nitric- 

 acid cells. 4 



Arc between Unlike Electrodes. It was noticed by 

 Grove in 1840 ~ a that the current through an arc is larger, 

 if it passes from a metal which is easily oxidized to 

 platinum than if it passes in the opposite direction. Many 

 years later it was discovered by Sahulka that with an 

 alternating-current arc between cored carbon and iron 



1 See also Ladoff, Elec. World, 45, 757; 1905. 



2 Paper read at the meeting of the Amer. Chem. Soc., Oct., 1909. 



3 Phil. Trans., 142, 88; 1852. 



4 See also the following: 



Van der Willigan, Pogg. Ann., 93, 285; 1854. 

 Stark and Cassuto, Phys. ZS., 5, 264; 1904. 

 Athanasiadis, Lond. Elec., 61, 873; 1906. 

 6 Phil. Mag., 16, 478; 1840. 



