The " Back Electromotive Forcer 1 5 1 



we know from investigations made inside the vacuum tube 

 that all these conclusions would be incorrect, and the 

 consideration of this case brings out the weak spot in the 

 argument which has been applied to the voltaic arc. It is 

 obviously impossible to separate a fall of potential due to 

 an electromotive force from one due to other causes, as long 

 as the fall is independent of the current intensity. Now we 

 know that in vacuum tubes the fall of potential in the glow, 

 as long as it does not cover the electrode, is constant, 

 and also that the fall in the positive part of the discharge 

 is, to a great extent, independent of the current. Hence, 

 as long as the glow has room to expand on the kathode, 

 the difference of potential between the electrodes does not 

 rise, although the current may increase considerably. This 

 was the case in the first three experiments quoted, and here 

 we have the explanation why the whole difference of 

 potential appears as " back electromotive force " and the 

 " true " resistance seems to vanish. 



Our familiarity with Ohm's law renders it difficult for 

 us to imagine that the gradient of potential may remain 

 the same though the current intensity varies, but experi- 

 ments show that it is often the case in a gas. If we imagine 

 the current to be due to a diffusion of ions it is easily seen 

 that if more ions are set free at the electrodes, the 

 same fall of potential must mean a greater current strength. 



We should be cautious, therefore, in the interpretation 

 of the experimental results obtained by Von Lang, Arons, 

 and Mr. Frith. 



Mr. Firth is correct in stating, at the end of his paper, 

 that the constancy of the results obtained shews that it is 

 due to some process which is independent of the current 

 strength, but that process need only be a dissipation of 

 energy, according to Joule's law, such as takes part in the 

 positive part of the discharge in a vacuum tube, and does 

 not necessarily imply any chemical or physical work done. 



