during the Absorption of Electrons by Platinum. 187 



set so that when the rocking switch A was turned the 

 reading on the voltmeter V changed from to 8 volts. The 

 balance was then tested by the method previously described, 

 and if it was out by more than two or three per cent, of the 

 expected deflexion it was adjusted so as to be as near right 

 as possible. But if the proportion was not greater than that 

 indicated it was measured and subsequently allowed for. In 

 the present instance the balance test uave 2"4 divisions " with 

 the effect/' that is to say, it tended to produce a spurious 

 effect which would increase the true effect by that amount. 

 This was recorded and no further adjustment was made. 

 The thermionic current was next measured and found to be 

 equal to 33*8 divisions, the micro-ammeter being shunted with 

 ten ohms. The potential drop along the filament was then 

 observed on the voltmeter V 2 , and found to be equal to 

 3*62 volts. The resistance of the grid was observed as 

 15*28 ohms. The deflexions of the galvanometer G were 

 now recorded as the thermionic current was turned on and 

 off consecutively at the end of every GO seconds. The 

 following readings were taken : — 



Off 77'5 80-5 80 79 S2 82 83 83-5 83 



On 183 185 182-5 184 188 191 192-5 1915 



The thermionic current was again read and found to be 

 34*4 divisions. It had thus increased *6 of a division during 

 the experiment, so that the mean value 34*1 was taken as 

 the correct one. The balance was again tested and found to 

 be 1*4 division " with," or one division less than at the 

 'beginning. The mean correction on account of the lack of 

 balance is, therefore, 1*9 divisions to be subtracted. When 

 the seventeen readings given in the table are treated by the 

 rule described on page 184, the mean heating effect is found 

 to be 105"84 divisions, from which we have to subtract 1*9 on 

 account of the balance not being exact, leaving 10394. 

 This is then divided by the value of the thermionic current, 

 34'1 divisions, giving 3*043 as the magnitude of the effect 

 with eight volts. As a rule, and particularly if the ther- 

 mionic current was increasing at all rapidly with the time, 

 its value was observed after each four successive readings of 

 the galvanometer, and by so doing a more exact estimate of 

 the average value of the thermionic current could be obtained. 

 Experiments in which there was a big jump in the thermionic 

 current or a big change in the balance during the course of 

 an experiment were invariably rejected. 



A set of measurements similar to the above was made for 

 each of the voltages tested. In the present experiment the 



