ARC IN AIR BETWEEN CARBON ELECTRODES 



II 



the shape which the ends of the carbons have when the 

 arc is started, but even when their shape at the beginning 

 is that which they finally take, it is many minutes before 

 the arc reaches a constant condition. 



The variation in voltage during an hour's run is shown 

 in Fig. 3. The current during this time was kept at 10 



25 



20 



30 40 50 60 



TIME IN MINUTES 



FIG. 3. 



amperes, and the length of the arc 3 mm. The negative 

 carbon in each case was a solid carbon 1 5 mm. in diameter. 

 This had been shaped by being previously used for a long 

 time with an arc which was also 3 mm. long having a cur- 

 rent of 10 amperes. In curve A the positive carbon was 

 solid with the end filed flat before the arc was started. 

 In B the positive carbon was cored with the end filed 

 flat. In C the positive carbon was cored with a crater 

 mechanically formed before the arc was started. With 

 these carbons the arc had hardly reached a constant con- 

 dition at the end of an hour. 



