123 
1906-7.] Photo-Electric Discharge from Platinum. 
vapour. The apparatus was also connected through suitable stop-cocks 
with a Geryk vacuum pump, which enabled the exhaustion to be carried 
out with great rapidity and ease down to a pressure of 0*05 mms. mercury, 
below which, of course, the Topler pump had to be employed. Arrange- 
ments were also made for filling the apparatus with other pure gases when 
required. 
Experiments in Air. 
(1) Air at Atmospheric Pressure. 
Series of observations were made in air at atmospheric pressure in order 
to repeat Zeleny’s determinations under the very different conditions under 
which we were working. The results obtained by us may perhaps be best 
seen from the following table, which gives the mean values of the photo- 
electric currents at different temperatures for one typical series of observa- 
tions, in which the platinum was charged to — 200 volts : — 
Photo-electric 
current in 
arbitrary units. 
Heating 
current in 
amperes. 
Temperature 
in °C. 
Ill 
0 
14° 
113 
10 
78° 
74 
15 
178° 
24 
20 
319° 
64 
25 
555° 
The photo-electric currents were in all cases measured as the electrometer 
scale deflections obtained for ten seconds illumination. 
In the above series the sensibility of the electrometer was such that 
5720 scale divisions represented a potential difference of one volt between 
the quadrants. A capacity of 0*002 microfarad was placed in parallel with 
the electrometer, making a total capacity of 0*0024 microfarad, so that one 
scale division per 10 seconds represented a current of 4*2 x 10 - 14 amperes. 
In the above experiments the final values of the photo-electric currents 
were not attained immediately after increasing the temperature. At 178° C. 
the value 7 4 was not reached until the heating current had been on for over 
ten minutes, although the platinum took up its final temperature within a 
few seconds. At 319° C., too, the current fell to 38 units one minute after 
applying the 20 amperes heating current, and only ten minutes later had 
reached the steady value 24. At 555° C. the current rose from 50 units, one 
minute after increasing the heating current, to 64 units twenty minutes 
