406 Profs. Richardson and Cooke on the Heat liberated 



found to be no good with the Gaede pump used, as, in order 

 to obtain a pressure appreciably higher than that given by 

 the pump when working at its most efficient speed, it was 

 necessary to work it so slowly that the changes in the tempe- 

 rature of the apparatus caused by the periodic variations 

 of pressure entirely precluded any attempt at measuring the 

 effect. 



Several methods were tried, and that finally adopted con- 

 sisted in connecting the apparatus to a side tube provided 

 with an indiarubber joint which leaked slightly. The 

 connexion was made through a good ground-^lass cock. In 

 this way two different pressures were available, namely, the 

 limiting values obtained when the side tube was, and was not, 

 connected with the apparatus. These pressures were about 

 2 X 10~ 3 and 5 x 10 ~ 3 mm. respectively, and were quite free 

 from the slow periodic variations which had previously been 

 found so objectionable. 



With this arrangement several experiments were made, of 

 which the following are typical examples. With 8 volts 

 applied potential-difference, and the side tube shut off, the 

 pressure both before and after one set of observations was 

 20 X 10 -4 mm. The effect in scale-divisions per unit thermionic 

 current under these conditions was found to be 1*680. After 

 connecting the side tube to the apparatus the pressure at the 

 beginning of an experiment with 8 volts applied potential- 

 difference was 56 x 10~ 4 mm., and at the end 46 x 10~ 4 mm. 

 The effect in this case, in the same units as at the lower 

 pressure, was 1*683. Thus changing the pressure from 

 20 x 10~ 4 mm. to 51 x 10 -4 mm. produces no change in the 

 effect per unit thermionic current, 



The same was true at other voltages. Thus with 18 volts 

 the pressure was 20 x 10~ 4 mm. at the beginning of an expe- 

 riment with the side tube shut off and 19 x 10~ 4 mm. at the 

 end. The effect per unit thermionic current was 3'01 scale- 

 divisions. After connecting, the pressure was 46 x 10~ 4 mm. 

 at the beginning and 42 X 10~ 4 mm. at the end of an expe- 

 riment under the same potential-difference. In this case the 

 effect was found to be 2*98 in the same units as before. This 

 is identical with 3*01 within the limits of observational error. 



These experiments prove conclusively that the measure- 

 ments are quite unaffected by small fluctuations in the 

 pressure of the surrounding gas ; so that such irregularities 

 as have been found must be attributed to other causes. 



Experiments with the Different Metals. 



A general review of the results which have been obtained 

 .shows that they are much more inconsistent than those yielded 



