102 THE ELECTRIC ARC 



Hall effect in the mercury arc. He states that (fa ki) p 

 is 1.42 X io 7 cm. per second per volt, where ki is the 

 velocity of the positive ions, k z that of the negative ions 

 and p the pressure of the gas measured in "millimeters. 



Modifications of the Arc to Produce White Light. The 

 absence of some of the colors in the mercury arc is an ob- 

 jection to its use and many attempts have been made to 

 supply the missing colors by introducing other substances 

 with the mercury. For this purpose amalgams of mercury 

 have been tried, but none of them have proven altogether 

 successful. With the currents and temperatures com- 

 monly used the substance amalgamated with the mercury 

 takes little or no part in the passage of the current. This 

 probably is due to the mercury alone being vaporized. 



Several amalgams were tried by Arons who found that 

 the potential difference was even less with sodium and 

 potassium amalgams than with pure mercury. With a 

 potassium amalgam only io volts were needed to maintain 

 a current varying from 3 to 6 amperes. But unfortunately 

 these lamps give less light than the common mercury vapor 

 lamp. 



When the tube was cooled by being placed in the water 

 bath no potassium lines appeared in the spectrum of the 

 arc, even though twenty amperes of current were used. 

 When the lamp was not cooled the potassium lines were 

 apparent with five or more amperes. 



With silver amalgams the potential difference was some- 

 what larger than with pure mercury. The spectrum of 

 the silver appeared only at the cathode and then only one 

 line, except when the lamp was run for a long time. In 

 that case this line disappeared and two others of the silver 

 spectrum appeared in its place. There was excessive heat- 



