Ay Hot Platinum in Air at Low Pressures. 



87 



iu anticipation that in general this procedure was found to 

 restore to the first spiral the power it originally possessed of* 

 discharging positive electrification when raised to the tem- 

 perature 6. 



The first experiments were made at pressures of about 

 •001 mm. At pressures so low as this the temperature of 

 a wire heated by a current depends only on the strength of 

 the current, and is independent of the pressure of the gas. 

 In order therefore to make sure of having the wire at a 

 constant temperature it was only necessary to observe the 

 current through it and not to measure its resistance. 



In a series of observations made on July 29th the heating- 

 current was kept constant and = -241 ampere. The follow- 

 ing values for the current were obtained at the times indicated. 

 The current is in scale-divisions of the electrometer per 

 minute. The pressures are also given (in mms.). 



Current. 



Thue. 



Pre:5sure. 





hours. 



minutes. 





46 



12 



34 



•0008 



43 



12 



40 



■0008 



30 



12 



47 



•0012 



22 



12 



.39 



•0009 



21 



1 



7 



•ocos 



17 



1 



15 



•0009 



10 



1 



23 



•0005 



15 



1 



31 



-0008 



13 



1 



40 



•0007 



The apparatus was now^ left till next day to see if the 

 leaking power of the wire would be revived. The following 

 observations were taken : — 



Current. 



1 





T3 



lue. 





Pressure. 





Lours. 





1 



niinutes. 





12 







1 



36 



•0005 



5 









41 



•0(J05 













50 



•0008 



1 1 







1 



58 



•0006 



4 



12 





1 



40 



•C005 



It is evident from these numbers that the wire is not 

 revived to any great extent by being left exposed to air at a 

 pressure of '001 mm. 



The wire was now allowed to cool and charged to —200 



