310 Mr. Gr. H. Marty n on the Discharge 



resulting curves are straight lines except at the highest tem- 

 peratures in each case. It will be noticed that these lines 

 are roughly parallel. 



The diagram shows that the effect of an atmosphere 

 of hydrogen is identical with raising the temperature of 

 the wire about 320° C. ; the effect of coating it with lime is 

 identical with raising the temperature about 340° C, and the 

 effect of using a lime-coated wire in hydrogen is identical 

 with raising the temperature about 670° C. ; and thus that 

 the effects produced by hydrogen and by coating the wire with 

 lime are in this sense additive. 



It will be observed that the lines obtained cease to be 

 straight for higher temperatures, the negative leak seeming 

 to reach a maximum value. It is to be noted that though the 

 lime has almost exactly the same effect as the hydrogen on 

 the temperature at which a certain small leak is produced, the 

 maximum leak obtained with hydrogen is very much greater 

 than that obtained with lime. 



The occurrence of a maximum leak in this way is probably 

 due to the ions at high temperatures becoming attached to 

 minute particles of platinum or lime which are lost by the 

 wire. The speed of these ions will thus be reduced, and 

 the potential-difference between the wire and the cylinder 

 will become insufficient to saturate the current. If this is 

 the explanation, it will be seen that the ions become so 

 weighted much more easily when lime is frequent than in its 

 absence. 



In order to test whether the current was no longer saturated 

 at the temperature at which the curve obtained ceased to be 

 straight, the apparatus was modified as follows. 



The platinum wire and the brass cylinder were connected 

 to the two poles of a secondary battery so that any difference 

 of potential could be introduced up to 1000 volts, this 

 difference being measured by an electrostatic voltmeter. 



In order to measure the temperature of the platinum, this 

 and a standard resistance formed the two arms of a Wheatstone 

 bridge arrangement, the other two arms being formed by a 

 resistance-box, so that the ratio of the platinum- wire resistance 

 to that of the standard could be measured, and hence the 

 platinum temperature calculated. The apparatus was con- 

 nected as shown in the figure. 



The experiments were conducted as follows : — The platinum 

 wire was heated until the ratio of its resistance to that of 

 the standard was of some fixed value. The leak was then mea- 

 sured with a certain P.D. The P.D. was then increased and 

 the corresponding leak noted, care being taken that at the 



