82 



THE ELECTRIC ARC 



maintain the arc. The following is a table of cathode drop 

 with the metals which could be used in a vacuum. 



This difficulty does not exist with oxides of metals. An 

 arc can easily be maintained in a vacuum with oxides for 

 the cathode. This is especially true of calcium oxide. 

 On the other hand calcium sulphide seems to have no 

 effect, nor the super-oxides of lead, manganese or zinc. 1 



An attempt was made by Arons to form an arc between 

 terminals of Wood's metal kept liquid by being placed in 

 a water bath near the boiling point, but in every case the 

 tube broke almost as soon as the lamp was started. In 

 only one case was it possible to take a reading of the current 

 and potential difference. At that time the potential dif- 

 ference was between 55 and 65 volts and the current about 

 2 amperes. 



The results with an alloy of sodium and potassium were 

 somewhat better. Some of the lamps which contained 

 this also broke on first starting them; others lasted suffi- 

 ciently long to make observations. He found it possible 

 to have currents from 2 to 1 5 amperes with a potential dif- 

 ference between 7 and 8 volts. Similar lamps which lasted 

 several hours were made by Weintraub. 2 



1 Stark, Phys. ZS., 5, 81; 1904. 



2 Phil. Mag., (6), 7, 114; i94- 



