THE MERCURY ARC 97 



for the exploring electrode, so that no potential difference 

 was introduced by using a different metal from that of the 

 cathode. They found the cathode drop to be 5.27 0.09 

 volts and stated that it is independent of the current. 



The Anode Drop of the Mercury Arc. The anode 

 drop has two distinct values; the first when the anode 

 is in the direct path of the cathode rays, that is, when it 

 is not covered by any glow, or "anode layer" as it has been 

 called by Stark; the second value occurs when there is this 

 glow. In the first case Stark, Retschinsky and Schapo- 

 schnikoff, using an iron anode, found the anode drop to 

 vary from 3.14 volts with a current of 3.6 amperes to 3.65 

 volts with one of 9.3 amperes. 



In the second case the anode drop would appear to be in 

 the neighborhood of 5 volts. Arons found it to be 7.4 volts. 

 Wills found it to be 6.5 volts with a current of 1.25 amperes, 

 decreasing to 4.6 volts with one of 3.25 amperes. These 

 measurements were made with an exploring wire i cm. 

 from an iron anode. Pollak 1 found the value 6 mm. from 

 the anode to be 5.7 volts when the current was 6.7 amperes. 

 Knipp measured the drop in potential between a platinum 

 anode and a platinum exploring wire when the current was 

 3 amperes and found the anode drop to vary from 3.6 volts 

 when the pressure of the gas in the apparatus connected 

 with the tube was 0.083 mm - to 5-4 volts when the pressure 

 was 3 mm. 2 



The Electric Force in the Mercury Arc. The electric 

 force required to send a current through the vapor of a 

 mercury arc is surprisingly small. Ordinarily the resist- 

 ance of a column of gas is very high, even when there is a 



1 Ann. d. Phys., 19, 217; 1906. 



2 See also Phys. Rev., 30, 319; 1910. 



