so Mr. Richardson on the Positive Ionization produced 



form would prevent leakage, I evacuated a tube onto which 

 the stopcock was sealed, and, after allowing n couple of days 

 for air condensed on the glass to escape, sealed it off from 

 the pump at a stage at which the Crookes dark space just 

 extended to the end of the tube (20 cms.), and a bright green 

 fluorescence of the glass was obtained throughout. After a 

 week's exposure to leakage from the atmosphere through the 

 stopcock, 1 could detect only a trace of blue coloration near 

 the further end of the tube, indicating a diminution of two or 

 three centimetres only in the length of the dark space. It is 

 possible that some at any rate of the effect was due to further 

 escape of air from the glass (which had not been heated 

 during the evacuation), but in any case it is evident that the 

 leakage through the stopcock was extremely minute. AVhen 

 the mercury nii was removed, the vacuum fell off at the rate 

 of several cms. per day. 



XII. On the Positive lonizatiort produced hy Hot Platinum 

 in Air at Loiv Pressures. By 0. W. Richardson, B.A., 

 B.Sc, Fellow of Trinity College^ Cambridge *. 



i [Plate v.] 



THE experiments to be described were originally under- 

 taken in order to examine the connexion betw-een the 

 leak from a hot wire, when charged positively and surrounded 

 by air, at a low pressure, and the potential and temperature 

 of the wire respectively. 



The arrangement of apparatus at first used was similar to 

 that employed by the author in investigating the negative 

 leak from hot conductors t' The wire, w^hich was in the 

 form of a spiral, was supported along the axis of a cylindrical 

 electrode from which it was insulated. The spiral was of 

 pure i platinum wire '1 mm. thick, and was heated by an 

 electric current, its temperature being determined by means 

 of its resistance. It w^as found that the absolute temperature 

 could easily be kept constant to one part in a thousand, so 

 that it did not vary more than one degree at the highest 

 temperature investigated. The vacuum-tube containing the 

 spiral and electrode was connected with a Topler pump and 

 McLeod gauge, so that the gas could be kept at any desired 

 pressure. 



The leak w^as measured by means of a quadrant electro- 

 meter. The wire was charged to any desired positive 



* Communicated by tlie Physical Society : read June 12, 1903. 

 t Proc. Camb. Phil. See. xi. p. 286 ; Proc. Roy. Soc. Ixxi. p. 415. 



