121 
1906-7.] Photo-Electric Discharge from Platinum. 
one-fiftieth of a millimetre of mercury, satisfactory results could not be 
obtained by using wire or gauze electrodes, owing to the ions, shot off from 
the sensitive electrode normally to the illuminated surface, passing in part 
through the meshes of the gauze, just as would be the case with a stream of 
cathode rays. We decided, therefore, to use only plane electrodes, the 
ultra-violet light being admitted into the leak apparatus, as we may name 
it, through a quartz window Q, and falling obliquely on to the central 
portion of the platinum foil Pt. 
The foil, about 5 cms. long by 2*5 cms. wide, was held in position by 
being riveted to copper strips at the ends, whilst these were screwed on 
to two copper rods, W W, which latter passed through glass tubes, sealing- 
waxed into brass tubes attached to the main apparatus. These and all 
Rh 
other joints were made air-tight by sealing wax. The rods, W W, served 
as conductors for the heating current, whilst the copper strips served to 
ensure the equal distribution of this current throughout the breadth of the 
foil. Only the central portion of the foil, about 1*5 sq. cms. in area, was 
illuminated by the ultra-violet light, so that the field between this and the 
copper electrode, Cu, in front of it would be fairly uniform. 
The temperature of the foil at the centre was determined by means of 
a platinum-rhodoplatinum junction welded on to the foil, the two wires 
being led out of the apparatus through two brass tubes at the back, 
narrower glass tubing around the wires serving to insulate them. The 
connections of these wires with copper leads were made in a beaker con- 
taining an insulating oil, the temperature of which could be read by a 
suitable thermometer. The thermo-electric E.M.F. generated when the foil 
was heated was read by means of a Sullivan galvanometer, G (fig. 3), 
