Positive Emission Currents from Hot Platinum. 503 



curve 2 in fig. 4 at 344° C. The heating current was then 

 cut off, and air at atmospheric pressure allowed to remain in 

 the tube for 22 hours. It was then pumped out to a pressure 

 of 0*0002 mm. on the McLeod gauge and the wire again heated 



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160 200 240 280 320 



Potent ial (Volts) 



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at 342° C. It then gave curve 1, fig. 4. Our experiments 

 show that this freshening action produced by admitting air 

 diminished with successive repetitions of the process, but we 

 did not succeed in reaching a stage at which the air ceased 

 to produce an effect. The effect occurred both when the 

 inner surface of the brass tube was left untouched and when 

 it was thoroughly scraped with emery-paper. The increased 

 sensitiveness could not be removed by mere pumping. In 

 one case, the tube, after exposure to air, was pumped out for 

 15 hours at a pressure below 0*0002 mm. and still found to 

 give a curve practically identical with fig. 4, curve 1. 



Similar effects were observed when the wire was raised for 

 a time to a higher temperature than that at which the obser- 

 vations of the positive current were made. Thus, after taking- 

 curve 6, fig. 3, the hot wire, under zero potential, was kept 

 at 600° 0. for 2 hours. The temperature was then reduced 

 to the previous value (378° 0.) and another curve taken. 

 The new curve was found to be similar to curve 1 of fig. 3. 

 The current at 400 volts had been increased in the ratio 

 of about 4 to 1 by this treatment. Results of the same 

 character were obtained with the glass-tube apparatus. 



The changes were more pronounced when the hot wire 

 was negatively charged and discharging electrons to the cold 

 electrode during the temporary heating. In a particular 



