58 



Mr Richardson, Note on the Positive Leak 



Note on the Positive Leak from Hot Platinum in Air at Low 

 Pressures. By 0. W. Richardson, M. A., Trinity College. 



[Head 30 January 1905.] 



In the Philosophical Magazine [6], Vol. vi. p. 80, the author 

 showed that the current from a fresh positively charged hot 

 platinum wire in a good vacuum fell off asymptotically with the 

 time the wire was heated at a constant temperature. Recent 

 experiments have shown that if fresh air be admitted into the 

 apparatus from time to time and the wire reheated in this 

 atmosphere the falling off reaches a limit, or at any rate the rate 

 of decay of the current becomes inappreciable. With the wire in 

 this state the author has recently made experiments to see if the 

 magnitude of the current is affected by the surrounding gas, with 

 the following results. 



In commencing an experiment the apparatus was pumped out 

 until the McLeod gauge registered a pressure of about "0001 mm. 

 and the wire heated until the positive leak did not change by 

 10 °/o °f its value in half-an-hour. Air was then admitted to a 

 pressure of about 2 cms. and pumped out again to a pressure of 

 •2 mms. The temperature of the hot wire, as determined hy its 

 resistance, was then brought to its initial value and the positive 

 leak measured. After letting in fresh air the leak was found to 

 fall off at constant temperature in a similar way to that from a 

 new wire. This effect may be due to a number of causes and 

 has not been fully investigated. After a time the rate of decay 

 became small, and when the current did not fall off more than 

 10°/o m half-an-hour, readings were taken at different pressures. 

 The following table represents a series of results with 3 "5 volts 

 mean potential on the filament : — 



Pressure 



Current 



67 



33 



16 



8 



•5 



29-1 



20 



16-8 



14-5 



13-3 



