188 Profs. Richardson and Cooke on the Heat developed 



voltages used were 15, 8, 11*1, 5, 24' 2, in the order named. 

 The corresponding values of the heating effect per unit 

 thermionic current in scale-divisions are respectively 4*08, 

 3-04, 3*03, 2*07, and 10*04. If these are plotted against the 

 voltage applied to the negative end of the filament, it will be 

 seen that they are all practically on a straight line with the 

 exception of the one with five volts, and in this case, since 

 the effect to be measured is small on account of the smallness 

 of the thermionic current and the voltage through which the 

 electrons fall, the deviation from the line is probably no 

 greater than the experimental error. It will be seen that 

 the line cuts the voltage axis at a point on the negative side 

 of the origin corresponding to — 3*0 volts. If we confine 

 ourselves to the points corresponding to the voltage for which 

 the current was practically saturated (which in the present 

 case comprises all the points which fall accurately on the line) 

 it is clear that the mean potential difference through which 

 the electrons fall will correspond to that at the middle point 

 of the filament. Our origin of voltage should, therefore^ be 

 taken at a point to the right of that in the diagram by an 

 amount equal to ^ of the drop along the filament, or 1*81 

 volt. The voltage, therefore, which is equivalent to that 

 through which the electrons would have to fall in order to 

 give rise to a heating effect equal to that part of the effect 

 which is independent of the Aoltage will be 3*0 -f i X 3*62 = 

 4-81 volts. 



Each determination of the value of the constant <f> involves 

 the determination of a number of points on the diagram in 

 the manner indicated. In most cases measurements were 

 taken for a number of different voltages, and a line drawn 

 through the series of points. In a few cases it was considered 

 sufficient to obtain two concordant readings at each of two 

 points sufficiently far apart, say 8 and 24 volts, and to draw 

 a straight line through them, but generally speaking this 

 was checked up by determining the point midway between 

 them, and if all the points did not lie on a straight line the 

 matter was investigated further. 



In figure 3 the results of another series of observations 

 are shown diagram matically. The abscissas represent voltages 

 at the negative end of the filament. The thermionic current 

 is shown thus O, and the heating effect per unit thermionic 

 current thus © . In this case again the points are seen to 

 be on a straight line, which cuts the axis of abscissas at about 

 -3 volts. 



