98 THE ELECTRIC ARC 



large amount of ionization. Even with a carbon arc in 

 air the resistance of the gaseous part with a current of six 

 amperes is approximately 5.5 ohms per centimeter. In an 

 experiment by Knipp when the tube containing the mer- 

 cury arc was 22.4 cm. long and 3.4 cm. in diameter and 

 the current was 3 amperes, the resistance was as low as 

 0.13 ohm per centimeter. With larger currents it is even 

 less. 



The electric force in the mercury arc depends on the 

 pressure of the vapor, the diameter of the tube, the amount 

 of current and the temperature. As the pressure is in- 

 creased there is a decided increase in the electric force 

 required. When working with the tube mentioned above 

 Knipp found the electric force to vary from 0.39 volt per 

 centimeter, when the pressure of the gas in the pump con- 

 nected with the tube was 0.083 mm - to 0.78 volt per centi- 

 meter when the pressure was 3.0 mm. The work of Wills 

 is quite in agreement with these data. 



Wills found that the larger the tube the smaller the 

 electric force required. For example, the electric force 

 with a current of 2 amperes and a pressure of i mm. varied 

 from 0.98 volt per centimeter when the diameter of the 

 tube was i cm. to 0.37 volt per centimeter when the 

 diameter was 8 cm. Arons also found the force to be 

 greater with smaller tubes. 



The electric force decreases as the current is increased, 

 if the pressure of the mercury vapor remains constant. In 

 some data which I hope soon to publish, it is shown that 

 doubling the current causes the electric force to decrease 

 about 10 per cent. 



Pollak 1 measured the potential through an arc when the 



1 Ann. d. Phys., 19, 239; 1906. 



