of Electricity from Hot Bodies. 311 



moment o£ observation the resistance of the platinum wire, 

 and so its temperature, had the required value. In this 

 way the relation between the P.D. and the leak for this 

 particular temperature was obtained. The platinum wire 

 was then heated to another temperature and the experiment 

 repeated. 



When the temperature was such that the current was pro- 

 perly saturated, the curve showing the relation between the 

 leak and P.D. was similar to that shown in diagram III. 

 No. IX., in which the current becomes saturated at about 

 100 volts, and on increasing the P.D. ionization by collisions 

 sets in between 200 and 300 volts. 



It was found that as the temperature rose the P.D. for 

 saturation became nearer the P.D. at which ionization by 

 collisions occurred, as shown in III., YIIL, and at a slightly 

 hio-her temperature no indication of saturation could be 

 observed. 



In diagram II. will be seen the curves obtained for lime- 

 coated platinum in air. Each line shows the relation between 

 the leak and the P.D. 



On plotting the logs of the leaks with the P.D. diagram III. 

 was obtained. Very similar results are obtained from lime- 

 coated platinum in hydrogen, as shown in diagram IV. 



From III. or IV. we can get the relation between tempe- 

 rature and leak for different P.D/s; and so we can plot the 

 relation between the reciprocal of the temperature and the 

 log of the leak, which according to the equation 



Q 



should give nearly a straight line since the variation in 6* is 

 small. 



Diagram V. gives the result obtained for lime-coated 

 platinum in hydrogen at potentials between 50 and 300 volts. 



It will be seen that the curves are precisely analogous to 

 those originally obtained. We get a series of parallel lines 

 for lower temperatures, and a maximum leak indicated at 

 higher temperatures. With increasing P.D. this maximum 

 occurs at higher temperatures, indicating that it is due to 

 non-saturation of the current. 



The curves obtained for greater P.D/s than 300 volts are 

 of little value, as ionization by collisions then affects the 

 results. 



If it is assumed that the leak in each case, if saturated, 

 would at all temperatures obey the law of variation indicated 

 by the straight parts of the curves in diagram L, then the 



