62 THE ELECTRIC ARC 



were a few facts clearly shown in the early part of the last 

 century. For example, it was shown by Davy 1 that car- 

 bon terminals give out more light than any of the metals. 

 It was shown by Metteucci 2 that the conductivity of the 

 arc between copper terminals was more than that between 

 some of the other metals. 



Somewhat later it was shown by Edlund 3 that there was 

 no measurable conductivity between silver terminals 1/80 

 sec. after the arc had been extinguished, although with 

 carbon terminals there was a very appreciable amount of 

 conductivity. It has since been shown that all arcs be- 

 tween metals lose their conductivity very rapidly after the 

 removal of the impressed E.M.F. 4 



It is only in recent years that measurements have been 

 made on the relation between potential difference, current 

 and length of metal arcs. These measurements show that 

 the metal arcs require a much smaller voltage than the 

 carbon arcs for the same current and length of arc and 

 that the characteristic curves are quite similar to those for 

 the carbon arc, but the fact which they most clearly show 

 is that different experimenters working under different 

 conditions arrive at very different results. This is seen 

 from the following formulas which have been given for this 

 relation for the arc between copper terminals. In all of 

 these E represents the potential difference in volts, / the 

 current in amperes and I the length of the arc in milli- 

 meters. 



1 Journ. Roy. Inst., i, 166; 1802. 



2 C. R., 30, 201; 1850. 



3 Pogg. Ann., 134, 250; 1868. 



4 See also the following : 



De la Rive, Phil. Trans., part 2, 31; 1847. 

 Arons, Wied. Ann., 57, 197; 1896. 



