IM;n|.rrKI> 



I'l.ATINTM IN WFFKKENT 



peutoxide. As thus prepared, the gas was liable to contain traces ot hydrogen ami 

 hydrocarbons, but that these were not in sufficient amount to affect the results was 

 proved by the fact that the oxygen prepared in this way gave the same results as 

 that obtained from potassium permanganate, which must have been free from 

 these impurities. 



The resistance readings were reduced to platinum temperatures by the method 

 described by CALLENDAR.* The wire was standardised by determining the melting 

 point of potassium sulphate by the method previously described, t For the parabolic 

 correction, which is small at all the temperatures considered, WiLSON'sJ value of the 

 constant (A = 151) was assumed. This assumption seems justifiable as the value 

 referred to the same kind of platinum wire. 



It has been pointed out above that it is necessary to get rid of the initial positive 

 leak from hot platinum before experiments can be made on the part of the leak which 

 is due to oxygen. The following figures give the actual magnitude of the two leaks 

 for the wire which was employed in this investigation. The wire was cleaned by 

 boiling with nitric acid and distilled water before commencing and the apparatus was 

 pumped out to a pressure of 0'00005 inillim. The initial positive leak under these 

 conditions at a temperature of 804 C. was found to be equal to 1*62 x 1 0~ 8 ampere. 

 With the wire at a constant temperature this fell to half in about 10 minutes and, 

 the rate of decay falling off with time, reached about one-tenth its original value 

 after an hour's heating. Even after heating the wire for several hours a day for 

 nearly a fortnight the part of the leak which was independent of the pressure could 

 not be neglected in comparison with that which depended on the pressure, as the 

 following numbers, which were obtained at a temperature of 721 C., testify : 



Pressure. 



millim. 



0-045 



0-03 



0-016 



0-004 



0-0003 



Current. 



ampire 

 1-8 xlO-" 

 1-52x10-" 

 1-4 xlO-'- 

 9-8 xlO- w 

 9-6 xlO-" 



These numbers show that increasing the pressure from to 0'045 millim. increases 

 the leak from 9'6 x 10~ 1S to 1'8 x 10~". The part of the leak due to the gas, for very 

 small pressures like the above, is very nearly proportional to the gas pressure. The 

 residual initial leak (9'6 x 10~ 13 ampere) exhibited by these results was about equal to 

 the leak produced in oxygen at a pressure of 0'05 millim. ; it was not permanent, 

 however, but fell away till it could no longer be detected on the electrometer at this 



* 'Phil. Mug.' [5], vol. 48, p. 519. 

 t ' Phil. Trans.,' A, vol. 201, p. 497. 

 J 'Phil. Trans.,' A, vol. 202, p. 243. 



