THE MERCURY ARC 



89 



tube 2 cm. in diameter and 12 cm. long. Platinum ter- 

 minals were sealed into the two legs and these were nearly 

 filled with mercury. The arc was started by tipping the 

 tube until the mercury in the two legs flowed together and 

 then tipping it back to its original position. The following 

 table gives the relation between current and potential 

 difference which he found when the pressure of the residual 

 gas was only a fraction of a millimeter. 



With currents greater than 1.4 amperes the tube was 

 filled with light. At the cathode there was a spot of in- 

 tense light which was in constant motion. With small 

 currents stratifications occasionally appeared. 



The presence of small amounts of air or hydrogen did 

 not greatly affect the relation between the current and the 

 potential difference. For example, when the pressure of 

 the remaining gas was 5 mm., there was a current of 

 8.5 amperes with a potential difference of 18 volts, which 

 is but little different from that given above for the same 

 current. In that case, however, the light did not fill all 

 of the tube, but appeared as a band in the central part, 

 and the glass became heated very quickly. 



A few years later Arons found that the mercury vapor 

 loses its conductivity very quickly after the arc has been 

 extinguished and that it is not possible to have an alter- 

 nating-current arc with such a lamp. 1 



The commercial form of this arc as developed by Cooper 

 Hewitt is shown in Fig. 26. In this lamp both terminals 

 are mercury. It is started by tipping the tube so that the 



1 Wied. Ann., 57, 192; ^1896, and 58, 73; 1896. 



