74 THE ELECTRIC ARC 



between the terminals of the arc with different pressures 

 of the gas, the current being maintained at 6 amperes. 



Pressures Less than One Atmosphere. While the arc 

 in a vacuum has no great commercial value, it gives us 

 much information which is useful in forming a theory of 

 the arc, for with very low pressures there are no com- 

 plications due to chemical actions, the temperature is 

 sufficiently low so that small exploring wires may be used 

 without appreciably distorting the field, and the existence 

 of striations and cathode rays shows the similarity between 

 this form of discharge and those having much smaller 

 currents. 



The earliest experiments on an arc in a vacuum were per- 

 formed before the distinction between arc and spark was 

 made and with many of these experiments it is impossible 

 to determine which form of discharge was really used. 



Thus, an experiment was performed by Davy in 1809* 

 to ascertain whether any heat sensible to a thermometer 

 is produced by the electric flame in a vacuum, but it can 

 not be definitely stated whether this was an arc or a glow 

 discharge. The same statement must be made of the work 

 by Van Breda. 2 



1 Mrs. Ayrton's " Electric Arc," p. 25. 



2 C. R., 23,462; 1846. 



Note. The work of De la Rive (Phil. Trans., 1847, part i) appears to 

 have been with an arc, while that of De la Rue and Muller (Phil. Trans., 

 171, A, 65; 1880; Proc. Roy. Soc., 29, 286; 1879) was apparently on a 

 continuous glow discharge, since the current which they used was very 

 small and the voltage several thousand volts. Stenger, in 1885 (Wied. 

 Ann., 25, 31; 1 885), gives four differences between the arc and the glow 

 discharge in a vacuum, but only one of these, namely, that the gaseous 

 portion has less resistance in the arc than in the glow discharge, can be 

 said to be correct, while some of the more important differences were not 

 noted. 



