9 6 



THE ELECTRIC ARC 



7 



CURRENT IN AMPERES 

 FlG. 28. 



from 4.8 amperes to 6.1 amperes. As the pressure of the 

 gas increased the current gradually decreased and the 

 voltage increased, until finally the point b was reached. 



The dotted line in this 

 figure indicates that the 

 voltage is very nearly con- 

 stant when there is no 

 change in the density of 

 the vapor, even when large 

 changes are made in the 

 current. This probably 

 comes very near to the true 

 relation between the voltage 

 of the mercury arc and the 

 current with constant pres- 

 sure. 



The very great increase in the voltage required with 

 increased vapor pressure of the mercury arc in quartz tubes 

 will be shown in a following paragraph. 



The Cathode Drop of the Mercury Arc. In a similar 

 way we find it difficult to determine just what the drop in 

 potential in different parts of the arc really is. Thus Arons 

 found the cathode drop to be 5.4 volts. Wills found it to 

 be approximately 5 volts and Pollak gives it as slightly 

 more than this. Knipp 1 measured the drop in potential 

 between a platinum exploring wire and a mercury cathode 

 when the current was 3 amperes and found the cathode 

 drop to vary from 6 volts when the pressure of the gas in 

 the pump connected to the lamp was 0.083 mm - to 6-8 volts 

 when the pressure was 1.9 mm. 



Stark, Retschinsky and Schaposchnikoff 2 used mercury 



1 Phys. Rev., 31, 106; 1910. 2 Ann. d. Phys., 18, 243; 1905. 



