when an Electron escapes from Surface of a Hot Body. 201 



for osmium *. According to the theory given above, this 

 should have been equal to the heating effects already mea- 

 sured. They found for the cooling effect at the osmium 

 surface, 4*7 equivalent volts. The values in eq. volts 

 for the series of metals whose heating effects were deter- 

 mined (omitting iron) were 7*26. 5*3, 7*1, 5*82, 5*6, 5*15, 

 and 7*4, which gives a mean of 6*23. No one of the 

 measurements gave a value as low as that obtained for the 

 cooling effect of osmium, and the difference from the mean 

 was 1*5 volts. It seems, therefore, that the experimental 

 evidence does not verify the relation expressed in (1). 

 Further reference to this discussion will be made when 

 contact potentials are considered. It will be shown that 

 the sign of V c in equation (1) probably should be changed. 



II. Measurements of $ from the Cooling Effect. 



In 1913 Cooke and Richardson published experiments 

 that showed conclusively the existence of the cooling effect, 

 and described a method of measurement that is capable 

 of giving quite accurate results. This method has been 

 employed in the present investigation and will be described 

 in some detail. Their rather lengthy mathematical deduc- 

 tions are omitted. For a complete discussion of the method 

 the reader is referred to the original paper. 



The expression for the loss of thermal energy due to the 

 escape of electrons is the same as that for the gain of 

 thermal energy in the heating effect, viz. 



ifa+'^-eS). 



The symbols have the same significance as before. The 

 pooling effect, as in the former case, was measured by 

 noting the change of resistance caused by the change in 

 temperature (in the cathode in this case), and by comparing 

 this change of resistance with that produced by a known 

 change of electrical energy. The experimental arrangement 

 is shown in fig. 1. 



Battery B 2 supplied energy to a bridge network in one 

 arm of which the filament was inserted. The other arms 

 consisted of a 10-ohm Wolff standard, a 1000-ohm ratio-coil 

 •of a Leeds & Northrup bridge, and the variable resistance 

 from the same bridge. The bridge current could be reversed 

 •by the commutator K x , and could be increased by a very 



* Phil. Mag. April 1913. 



