during the Absorption of Electrons by Platinum. 181 



were taken, first with the thermionic current on for the given 

 interval, then with the thermionic current off for the same 

 interval, and so on in succession. In this manner the effect 

 of the drift and casual fluctuations could be eliminated. 



In the first experiments the heating effect of the thermionic 

 current was compared with the heating- effect arising from 

 an increase in the current produced by the batteries C 1? (J 2 

 of the Wheatstone's bridge. This current could be increased 

 or diminished at will by altering the resistance in the box R 

 in the bridge arm of the circuit. The heating effect pro- 

 duced in this way in the strip S is equal, of course, to the 

 resistance of the strip multiplied by the difference of the 

 squares of the current passing through it, and could thus be 

 calculated, the currents through the bridge being measured 

 by a suitable milliammeter. It was found, however, that the 

 change of resistance thus produced did not vary with the 

 time in quite the same way as that due to the thermionic 

 current, but got up to its maximum value somewhat less 

 rapidly. There is an important difference in the mode of 

 liberation of heat in the two cases. That due to the ther- 

 mionic current is developed at the surface of the metal, 

 whereas the other is a volume effect. On these grounds we 

 should expect the final state of equilibrium to be reached in 

 different times in the two cases, so that there is an objection 

 to the measurement of the heating effect by comparison of 

 the effects produced in equal times in the two cases. 



There is, however, a simple method of deducing the heating 

 effect arising from the difference of potential energy of the 

 electrons inside and outside of the metal, which appears to be 

 free from this objection. We have seen that if a difference 

 of potential is applied so as to drive the electrons from the 

 filament F to the strip S, their kinetic energy will be in- 

 creased by a calculable amount, which is proportional to this 

 difference of potential. So that if we compare the effect 

 produced by the electrons when they fall through no voltage 

 with that produced when they fall through a voltage V, we 

 shall at once be able to determine the difference in the 

 potential energy in terms of the kinetic energy gained by an 

 electron when it falls through a potential difference of one 

 volt. When the difference of potential driving the electrons 

 is zero, the thermionic current is so small that it is impossible 

 to measure the heating effect to which it gives rise with 

 accuracy. But this difficulty can be avoided if we compare 

 the heating effect produced by the thermionic current with 

 two different voltages. On the view developed at the be- 

 ginning of this paper, the effect per unit current will be a 



