ARC IN AIR BETWEEN CARBON ELECTRODES 25 



made. A few measurements were made by myself 1 in this 

 manner, and the electric force was found to be 2.34 volts 

 per millimeter for a current of 10 amperes, but no great 

 accuracy can be claimed for these measurements. 



It has generally been assumed that the electric force 

 could be found by observing the increase in voltage when 

 the length of the arc was increased and that it is equal to 

 the ratio of the increase in voltage to the increase of length,, 

 the current remaining constant. This would be correct,, 

 if changing the length of the arc did not change either the 

 anode or cathode drop, but Mrs. Ayrton found that such 

 a change did exist in the anode drop. However, we can 

 arrive at a fair approximation by following this method! 

 and making a correction for this change. If, therefore,, 

 we assume the equation for E, given on page 14, and that 

 for the anode drop given on page 24, we find the value for 

 the electric force to be 2.07 + 7.44/7. This will give a 

 value of 2.81 volts per millimeter when the current is 

 10 amperes, which is not far from that found experimentally 

 by myself. 



We thus see that the peculiar relation which exists in 

 the arc between voltage and current must be ascribed to 

 all of the three parts. The voltage of each part decreases 

 when the current increases. The decrease in the anode 

 drop is greater than that in the cathode drop, while the 

 decrease in the drop of potential through the gaseous part 

 of the arc depends, of course, on the length of the arc. 

 However, with arcs more than 6 mm. long more than one- 

 half of the total decrease occurs in the gaseous part. 



Counter E.M.F. of the Arc. As has been stated Edlund 

 found that the "apparent resistance" of the arc equals 



1 Phys. Rev., 19, 122; 1904. 



