616 Mr. N. Campbell on the Effect of Change of 



accurate : the " hot " leak is about 5 per cent, smaller than 

 the " cold " leak. Since the pressure of the gas in the 

 vessel is kept constant, the difference may be attributed to 

 diminution of its density when heated.) 



5. Such measurements as are possible indicate that the 

 effect of heating is to cause a definite quantity of electricity 

 of the same sign as the charge on the walls to cross from the 

 walls to the electrode : and that the effect of cooling is to 

 cause an equal quantify of electricity to pass in the opposite 

 direction. The evidence for this statement will now be 

 given. 



6. The leak was measured by observing the time t required 

 for the insulated electrode to reach a known potential V : 

 t is about three minutes and V about '26 volt. When the 

 vessel was cold individual readings varied about 8 per cent., 

 and the mean of a long series was constant to 2 per cent. : 

 when the vessel was hot these variations were doubled. When 

 the effect of heating is to be observed, the electrode is 

 insulated as usual and the flame immediately placed below 

 the vessel : the time t' required for the electrode to reach 

 the potential V is less than t. Similarly during the first 

 cooling the time £"is greater than t. Let V be the capacity* 

 of the electrode system, Q the extra quantity of electricity 

 which passes during the initial heating, and Q' that which 

 passes during the initial cooling. Then if t ± is the time 

 required for the leak when the vessel is hot, q the steady 

 current when the vessel is cold, q 1 when the vessel is hot, 



CY=qt = qt' + Q and CV=q 1 t l = q i t" + Q' 

 ,. g=V / (1) and ^ = y\ (2). 



7. The following table gives the values of Q and Q' for 

 several series of experiments performed on the same vessel 

 at different times. The series have been specially selected, 

 for the inconsistency between different observations is such 

 that it is useless to draw quantitative conclusions except 

 from those experiments which were thought from the general 

 behaviour of the apparatus to be especially reliable. It will 

 be seen that Q= — Q' approximately and is independent 

 of V : accordingly the statement in the last paragraph is 

 substantiated. 



* Direct observations showed that the alteration of the eapacitj- with 

 the heating was not appreciable. 



