22 THE ELECTRIC ARC 



the current accompany the hissing, has been shown by 

 Firth and Rogers, 1 and by Duddell and Marchant. 2 



Fall of Potential in Different Parts of the Arc. The 

 relation between the potential difference and the current 

 through the arc is so different from that given by Ohm's 

 law that it is a matter of interest to examine the potential 

 in different parts of the arc and to find whether all parts 

 show the same peculiarity. To do this some point in the 

 arc must be connected to a measuring instrument by a 

 conducting substance, such as a metallic wire. But any 

 wire placed in the arc is immediately vaporized. Even a 

 carbon filament will not answer, since it burns away 

 rapidly. The only conducting substance which can be 

 used is a carbon pencil. But a carbon pencil distorts the 

 form of the arc and changes the conditions which existed 

 before its introduction. 3 



However, approximate results can be found by this 

 method, and these always show a considerable difference in 

 potential between the anode and a pencil placed as close to 

 it as possible. This is called the anode drop. There is a 

 similar difference in potential in the neighborhood of the 

 cathode, which is called the cathode drop. Between these 

 there is a gradual change in potential as in a wire. This 

 is illustrated by Fig. 9 taken from some of Mrs. Ayrton's 

 data, which will be given in the following paragraph. 



Measurements of these quantities have usually been 

 made with either a voltmeter or a galvanometer. This in- 

 volves a passage of electricity from the gas to the explor- 

 ing electrode. As is well known, there is usually a large 



1 Phil. Mag., (5), 42, 407; 1896. 



2 Journ. Inst. Elec. Eng., 28, 78; 1899. 

 8 Mrs. Ayrton's " Electric Arc," p. 210. 



