127 
1906-7.] Photo-Electric Discharge from Platinum. 
(3) Air at a Pressure of 0 0035 mm. Mercury . 
A large number of series of observations were made in air at a pressure 
of about 0'0035 mm. of mercury. At this pressure the mean free path of 
an ion in air is about 10 cms., and is therefore much greater than the 
distance between the electrodes (1*2 cms.). A potential of 80 volts was 
ample to give saturation, and the photo-electric current varied very slowly 
with the pressure, even doubling or halving the pressure only increasing 
or decreasing the current by about 7 to 8 per cent., whilst doubling or 
halving the potential gradient had a scarcely greater effect. Hence we 
may assume that under these conditions we are measuring the variations 
in the actual photo-electric discharge, quite independent of any charges 
due to secondary ionisation in the gas. 
In all the experiments made under the above conditions, the sensibility 
of the electrometer was 5940 divisions per volt, and the capacity in 
parallel 0 - 05 mfd. A current, therefore, which gives to the electrometer 
a charge producing a deflection of 1 scale division per 10 seconds is equal 
to 8 x 10“ 13 amperes. 
When the apparatus had been standing exhausted to 3 Jq mm. pressure for 
nineteen days, the photo-electric current was 36 units (1 unit = 8 x 10 ~ 13 amp.). 
On applying a heating current of 10 amperes, giving a temperature of 
163° C., the photo-electric current rose at once to 129 units, and remained 
constant at that value for half an hour, readings being taken every few 
minutes. 
On breaking the heating circuit the current fell to — 
123 units in 4 minutes. 
103 „ 14 „ 
90 „ 40 
71 „ 5J hours. 
65 „ 24 „ 
On now applying a heating current of 5 amperes, giving a temperature 
of 50° C., the photo-electric current rose to 127 units in about fifteen 
minutes, and remained constant at that value for more than an hour. On 
increasing the heating current to 10 amperes, giving a temperature of 
163 c C., there was no appreciable change in the sensibility, the photo- 
electric current now having a value of 129 units. A further increase of 
temperature to 278° C. reduced it very slightly, to 122 units, whilst it 
returned to 129 units again on reducing the temperature to 163°. 
On breaking the heating circuit the sensibility fell in twenty-four hours 
to 66 units again, and it was repeatedly found that after the platinum had 
