131 
1906-7.] Photo-Electric Discharge from Platinum. 
platinum had been frequently raised to temperatures over 300° C., was 
exhausted to a very low pressure, 8 divisions on the M‘Leod gauge (0‘003 
mm. of mercury), the sensibility steadily diminished with the time. The 
following table shows one set of observations taken : — 
Time. 
Photo-electric current. 
1 2 noon. 
Apparatus pumped out. 
12.7 p.m. 
98 
12.12 
90 
12.15 
88 
12.18 
85 
12.21 
82 
12.24 
78 
12.30 
73 
12.34 
74 
12.50 
69 
The pressure during this series rose to 12 divisions on the gauge (which 
would tend to increase the current slightly), and continued to rise steadily, 
being 21 divisions seven hours later and 28 divisions twenty-seven hours 
later, and practically constant. The current was now 50 units, and 
the pressure upon reducing to 7 divisions (the original pressure after 
exhaustion), fell to 45 units. This rise in pressure could only be accounted 
for by the liberation of hydrogen occluded in the platinum foil. Calculations 
showed that at the higher pressures the platinum would easily absorb 
hydrogen in sufficient quantity to cause this increase in pressure (in all 
about 0 - 006 mm. mercury) when liberated. Application of heat to the foil 
produced a very slight liberation of hydrogen, 3 or 4 divisions on the gauge, 
after which the pressure remained perfectly constant even when the foil 
was raised to 370° C. It thus seems certain that the absorbed hydrogen 
increases the photo-electric sensibility of a platinum surface, and this may 
easily account for the difference between the effect of temperature on the 
photo-electric currents in this gas at high pressure and those in air and 
carbon dioxide. 
Discussion of Results. 
The variations with temperature of the photo-electric currents in air, 
carbon dioxide, and hydrogen at atmospheric pressure are shown graphically 
in fig. 5, the results obtained by Zeleny in air being also plotted for 
purposes of comparison. The actual photo-electric currents observed for 
carbon dioxide have been multiplied by 1'22 in order to compare the values 
better with those for air, the values at 14° C. now coinciding. 
