112 THE ELECTRIC ARC 



discharge between two hot bodies where there is no 

 ionization by impact at the surface of the electrode, 

 as, for example, the discharge from a hot piece of iron or 

 platinum. 



With this simpler form of discharge the current will be- 

 come smaller as the voltage becomes smaller. The two 

 do not necessarily approach zero at the same time, 

 there may be a small E.M.F. due to the difference in 

 perature between the two bodies and since it takes an 

 appreciable time for the ions to move across the gap. But 

 the characteristic curve for such a discharge would at least 

 go near the origin, as do the curves in Fig. 33. 1 



Fall of Potential in Alternating-current Arc. The fall 

 of potential in different parts of the alternating-current arc 



Pd. between e d 



Pd. between a & 

 Pd. between b 



FIG. 35. 



was examined by Duddell and Marchant. Their results 

 are shown in Fig. 35. Fig. 34 shows the position of the 

 exploring carbons. The left hand part of Fig. 35 shows the 

 curves of current and total potential difference, while the 

 part on the right shows the potential difference in different 

 parts of the arc. All the curves of potential difference show 

 the same general shape. The carbons used were "Apostle " 

 carbons 13 mm. in diameter. The arc was 6 mm. long 



1 A very complete study of dynamic characteristic curves with the arc 

 in various gases and in air at reduced pressures has been given by Gertrude 

 Lange (Ann. d. Phys., 32, 589; 1910). 



