ARC IN AIR BETWEEN OTHER SUBSTANCES 6l 



was found that the drop between the anode and carbon 

 pencil i mm. in diameter placed as close to the anode as 

 possible without touching it was 12 volts. The drop be- 

 tween such a pencil and the cathode was 8.5 volts. These 

 observations were made with open arcs i cm. long and 

 having a current of 10 amperes. The values were found 

 to be nearly independent of the length of arc or the current 

 and were the same for several different makes of flaming- 

 arc carbons. The anode drop with arcs of the same 

 length and current between solid carbons is 35 volts (p. 14) 

 which is approximately three times that for the flaming 

 arcs, while the cathode drop is nearly the same for both 

 kinds of carbons. 



The electric force through the arc can be found ap- 

 proximately by getting the rate of change of the potential 

 difference between the terminals of the arc with respect 

 to the length. The data on the preceding page show 

 that this rate of change was approximately 1.65 volts per 

 millimeter for the open flaming arc when the current 

 was 10 amperes, while with the arc between solid car- 

 bons the value of this quantity is 3.12 volts per milli- 

 meter. 



Arc between Metals. With the exception of the arc 

 between mercury terminals in a vacuum none of the arcs 

 between metals have proven to be of commercial value, 

 for the metals are consumed much more rapidly than 

 carbon and the arcs between them sputter continually and 

 give much less light than carbon arcs. Many investiga- 

 tions have, however, been made for the purpose of learn- 

 ing how such arcs behave. The earlier of these were of 

 less value because one can not be sure whether the experi- 

 menter was using an arc or only a spark. However, there 



