38 THE ELECTRIC ARC 



Granqvist, concluded that there was no E.M.F. 1/600 

 second after the arc was interrupted as great as 0.16 volt. 

 He used both solid and cored carbons, long and short arcs, 

 changed the time between the disconnection of the arc 

 from the main circuit and the connection to the galva- 

 nometer circuit, and in other ways varied the conditions 

 as far as possible. 



On the other hand Hotchkiss, 1 using an oscillograph hav- 

 ing a period of 0.0002 second, found indications of an 

 E.M.F. This was less than 0.66 volt, but was still clearly 

 discernible. Mitkiewicz 2 found a residual E.M.F. of from 

 1.5 to 2 volts. Becknell 3 found an appreciable E.M.F. 

 existing for several seconds after the impressed E.M.F. 

 was removed. He determined the amount by balancing it 

 against a known potential difference by means of a sen- 

 sitive galvanometer. The values which he found are 

 shown in Fig. 12. When the arc was first extinguished the 

 resistance was small, but increased very rapidly with the 

 time. He showed that the residual E.M.F. was not due to 

 the thermal junctions outside the arc. 



It is very difficult to reconcile the work of Blondel with 

 that of the others. Furthermore, it is hard to see how it 

 could be otherwise than that there is such a residual E.M.F. 

 It was shown by Dubs 4 and Olivette 5 that the current flows 

 from a cooler carbon to one very hot. This may amount 

 to several volts. All the phenomena observed by Becknell 

 are such as would be expected from the difference in tem- 



1 Trans. Amer. Phys. Soc., 2; 1901. 



2 Science Abs., 7, 360; 1903. Beib., 27, 465; 1903. 



3 Phys. Rev., 21, 181; 1905. 



4 Centralbl. f. Elektrot., 10, 749; 1888. 

 8 Lond. Elec. Rev., 31, 728; 1892. 



