DISCHARGE OF ELECTRICITY FROM HOT PLATINUM. 
265 
The following table gives some numbers showing the effect on the negative leak at 
a constant temperature of about 1350° C., due to letting in liydrogen :— 
Pressure in millims. 
of mercury. 
Current with 120 volts. 
divisions. 
0-0006 
10 
0-0015 
20 
0-0033 
40 
0-0053 
. 50 
0-0080 
75 
0-014 
110 
This increase of the leak with the pressure cannot be ascribed to ionization by 
collisions, for at the highest pressure used, one negative ion can only produce 0'2 fresh 
negative ion in going 1 centim. The leak in air at 1350° C. from a clean wire would 
have given about division in the above experiment. Thus the leak in hydrogen 
at 0'014 millim. in this experiment was more than 25,000 times the leak from a very 
well cleaned wire at the same temperature in air. 
In the experiment just described, the hydrogen was let in at intervals through the 
platinum spiral, so that the pressure increased during the experiment. Another way 
is to let in hydrogen to the highest pressure desired and then pump out during the 
experiment. It was found that the results were not the same in the two cases. On 
pumping down, the leak falls, but not so quickly as it rises on letting in the gas. It 
was soon discovered that the leak is not always a single-valued function of the 
pressure, but depends also on the previous treatment of the wire. The results can 
be explained on the hypotheses— 
(1.) That the wire absorbs an amount of hydrogen depending on the pressure 
and temperature, and that when these are changed it takes a considerate 
time for equilibrium between the gas in the wire and that outside to be 
established; and 
(2.) That the leak at any temperature depends on the amount of hydrogen in the 
wiie, and not directly on the pressure outside. 
Consequently, if the equilibrium is established at a particular temperature and 
pressure, then if the temperature is raised or the pressure lowered the leak should 
not be steady at first under the new conditions, but should gradually fall ofi, in 
consequence of gas leaving the wire, until equilibrium is again established. On the 
other hand, if the temperature is lowered or the pressure increased, the leak should 
gradually rise until equilibrium is again established. Experiments showed that these 
effects occurred exactly in the way expected. 
The following tables give the results of some measurements of the negative leak in 
hydrogen, showing the effects of suddenly changing the temperature or pressure :— 
VOL. CCIT.—A. 2 M 
