THEORY OF THE ELECTRIC ARC 177 



factory. The first of these was one used by myself where 

 the positive ions were drawn from an arc by a charged 

 cylinder placed about the arc. In this experiment the 

 positive ions were found to have a greater velocity than 

 the negative ones. 1 However, the positive ions probably 

 came from the hot positive electrodes and showed nothing 

 concerning the ions in the arc itself. 2 



That this is the correct interpretation appears from the 

 work of Merritt and Stewart who showed that when ions 

 are blown from an arc by a draft of air and their velocities 

 tested a few centimeters from the arc the negative ions 

 have the greater velocities. 3 The same thing has since been 

 shown by myself 4 and also by McClelland, 5 who be- 

 lieved that it proved that the negative ions had the greater 

 velocity not only outside of the arc, but also within it. 



A third method of attacking this problem was given by 

 Swinton. 6 In his experiment one of the carbons was hollow 

 and an insulated Faraday cylinder of brass was placed 

 within this. It was found that the cylinder became 

 charged positively as compared with the electrode in which 

 it was placed when this was the cathode, and negatively 

 when it was the anode, and that the negative charge was 

 acquired more rapidly than the positive. From this 

 Swinton concluded that the negative ions moved the more 

 rapidly. However, this effect might easily be due to the 

 fact that the difference of potential between the vapor and 

 the anode is greater than that between the vapor and the 

 cathode. 7 



1 Phys. Rev., 12, 137; 1901. 5 Proc. Camb. Phil. Soc., 10, 241; 1899. 



2 Phys. Rev., 15, 345; 1902. 6 Proc. Roy. Soc., 76 A, 553; 1905. 

 8 Phys. Rev., 7, 129; 1898. 7 Phys. Rev., 24, 506; 1907. 



4 Phys. Rev., 12, 147; 1901. 



