244 PROCEEDINGS OP THE AMERICAN ACADEMY 



These lines are all less inclined than those for the same current 

 with whistling arc under normal pressure ; showing that as the arc 

 is lengthened the resistance proper increases less rapidly when the 

 pressure is reduced. The total equivalent resistance is, of course, 

 greatly diminished by reduction of pressure, but this reduction seems 

 to be wholly due to diminished conductive resistance, the inverse 

 electromotive force for the arcs used, all of which were whistling ones, 

 being even slightly greater than at normal pressure, so far as can 

 be judged from the data at present obtained. 



The scanty data at hand would not justify us, however, in conclud- 

 ing this apparent increase to be real. If such proves to be the case, 

 the action is probably of the same nature as that supposed by Edlund 

 to exist in the discharge of electricity in ordinary vacuum tubes ; that 

 is, an increase in the opposition offered to the passage of electricity 

 from the solid electrodes, as the pressure of the intervening gaseous 

 medium is diminished. Further experiment, extending to higher 

 vacua, and also with increased density, is necessary before any definite 

 conclusions can be drawn as to this point. 



In addition to those facts which are commonly recognized, the 

 following conclusions seem to be justified by our experiments. 



1. There is a definite inverse electromotive force for the whistling 

 arc, whose value is approximately 15 volts. ^ 



2. The inverse electromotive force for both the silent and whistling 

 arcs diminishes slowly as the current increases. 



3. The inverse electromotive force, at least for the whistling arc, 

 is less for the inverted than for the upright arc. 



4. The great change in equivalent resistance which occurs when 

 volatile salts are introduced into the arc is chiefly due to a large fall 

 in the inverse electromotive force, although there is at the same time 

 a marked diminution in the conductive resistance. 



5. The diminished total resistance of the arc in rarefied air is 

 due solely to a diminution in the conductive resistance. 



6. There is some evidence to show that, with considerable reduc- 

 tion of pressure, there is a slight increase in the inverse electromotive 

 force. 



Rogers Laboratory of Physics, 

 June, 1886. 



