54 THE ELECTRIC ARC 



measure the potential difference, unless the metals were 

 kept cool, because they melted away so rapidly. They 

 avoided this difficulty by running a current of water 

 through the inside of the electrode. With nickel it re- 

 quired from 500 to 600 volts in hydrogen while the corre- 

 sponding values in air were less than 100 volts. 



The only other gas with which experiments have been 

 made to any extent is nitrogen. The voltages required 

 with this do not greatly differ from those required with 

 the arc in air. The first data with the arc in nitrogen were 

 given by Arons. The comparison of his results with those 

 which had been found by von Lang for the arc between 

 metal in air is given in the following table. The data are 

 given for a distance between the electrodes of 1.4 mm. and 

 a current of 4 amperes. 



Arons found it impossible to maintain an arc in nitrogen 

 between silver terminals on a io5-volt circuit with 6 ohms 

 in the circuit. On the other hand Malcolm and Simon 

 found the voltage with silver terminals only about 20 volts 

 higher in nitrogen than in air. 



