182 Profs. Richardson and Cooke on the Beat developed 



linear function of the voltage, and from the position of the 

 point at which the heating effect produced by unit thermionic 

 current cuts the voltage axis, on the diagram in which this 

 effect is plotted against the voltage, we can at once deduce 

 the difference of potential energy in terms of the work done 

 on the electron when it falls through one volt. All that is 

 necessary, then, is to measure simultaneously both the ther- 

 mionic current and the change it produces in the resistance 

 of the strip S for a series of different voltages. This method 

 has the advantage that in every case the heating* effect is 

 produced at the surface of the strip, so that the conditions 

 are more comparable than when the heating effect of the 

 thermionic current is compared with the heat production due 

 to an increase of current in the AVheatstone's bridge. 



It is also simpler to work with, because the sensitiveness 

 of the galvanometer depends on the current actuating the 

 Wheatstone's bridge ; so that in measuring the heating 

 effect of an increase in this current, it is necessary to deter- 

 mine the sensitiveness of the bridge for each current used, 

 The experiments made with this method of standardizing the 

 effect agreed as to order of magnitude with those which 

 depend on a direct comparison of the effects of the different 

 voltages. There was, however, a definite difference in the 

 magnitude given by the two methods, which we believe to be 

 due to the fact that the mode of liberation of the heat is 

 different in the two cases. 



It was stated above that when there is no voltage driving 

 the thermionic current the effect is too small to measure 

 with accuracy. It can, however, be detected and measured, 

 and it is found to agree with the value determined by the 

 less direct experiments within the order of accuracy of its 

 measurement. 



Another point which was tested in the preliminary ex- 

 periments was whether the change of resistance in the strip 

 produced in a given time was proportional to the energy 

 supplied to it. Measurements of this were made by varying 

 the current in the Wheatstone's bridge circuit. The measure- 

 ments gave the following numbers : — 



(1) Change of resistance in 15 seconds 



(Scale-divisions.) 



... 30 



66 



103 



(2) Watts X 10 4 



i 

 ... 4-55 



99 



147 







(D-K2) 



... 6-60 



6-66 



700 



