DISCHARGE OF NEGATIVE ELECTRICITY FROM HOT PLATINUM. 
265 
5. The Effect of Hydrogen on the Resistance of the Platinum. 
It is well known that when platinum is saturated with hydrogen its resistance is 
increased by a small amount. It was thought that this effect might be employed as 
a test of the presence of hydrogen in the wire. A wire which had been heated for 
some time in hydrogen and which gave a large leak independent of the pressure 
was used. It was heated for some hours in a very good vacuum and still gave the 
same current as in hydrogen at several millimetres pressure. Its resistance at 0° C. 
was then carefully determined and it was then heated to 1400° C. for a few seconds 
in air. The leak immediately fell to the air value. The resistance at 0° C. was 
again determined and was about one part in a thousand less than before. On again 
heating in hydrogen the resistance rose again. 
This experiment shows that the increase in the resistance due to hydrogen is 
probably a purely surface effect, for oxygen does not diffuse into platinum. If we 
regard the increase in the resistance as due to the formation of the compound 
discussed in Section 2, then it is necessary to suppose that this compound is only 
formed in the surface layer of the platinum. It is well known that finely divided 
platinum absorbs much more hydrogen than platinum in large pieces, which also 
suggests that the compound formed is confined to the surface. The effect of the 
hydrogen on the negative leak is, of course, a surface effect and so depends only on 
the state of the surface layer. 
6. Effects due to Passing a Self-Luminous Discharge between the Wire and 
surrounding Electrode. 
After heating an old wire in air it gives a very small leak in hydrogen, at any rate 
for a time. If, while it is giving a very small leak, a luminous discharge is passed 
between the wire and the surrounding electrode by means of a large battery of cells, 
the leak on stopping the discharge is enormously increased. If the discharge is only 
passed for a few seconds, a moderate increase can be obtained, and then the leak has 
a maximum value at a certain temperature. After passing the discharge for some 
minutes the leak obtained always rises with the temperature. 
These effects, it will be seen, are very similar to the effects observed during the 
increase of the leak when an old wire which had been heated in air was heated to a 
moderate temperature in hydrogen. The discharge appears to accelerate the recovery 
of the leak. The maximum value of the leak was obtained both with rising and 
falling temperatures. The current was nearly independent of the potential difference 
in all these experiments. This was tested specially at every temperature by taking 
two measurements with different potentials. The maximum value is, therefore, not 
due to the currents at the higher temperatures not being saturated. I hope to make 
further experiments on these effects. 
YOL. CCYIII.-A. 
2 M 
