245 
DISCHARGE OF ELECTIHCITY EROiM 
HOT PLATINUM. 
with its oiigiual value. The ciirreut between platinum electrodes in air with a 
constant P.D. increases rajjidly with rise ol temperature. II is the current (due 
to a small P.D.) at the absolute temperature 6^, and a-o that at 6»o, and Q the energy 
recpiired to produce a gramme molecular weight of positive and negative ions, then it 
is shown that 
and Q is found to he nearly independent of the temperature, so that we may put 
X = where A. is a constant quantity. This formula is deduced on the 
assumption tliat the air close to the surface of the platinum dissociates into ions.^^ 
The value found for Q is 60,000 small calories. This gives 2-6 volts for the P.D., 
through which the ionic charge must fall to olitain enough energy to ionize one 
molecule of air in contact with hot platinum. [When a large P.D. is used, so that the 
current is the maximum, the ap})ro})riate formula is x = Ad"e p. 250 wfra.'] 
It is also shown in this paper that tlie maximum current between hot platinum 
electrodes immersed in a stream of an alkali salt vapour is ecpial to the current required 
to electrolyse in a solution the amount of salt passing between the electrodes.! It 
is therefore clear that the leak is due to the ionization of the salt in contact with the 
platinum. 
Professor McClelland, in 1002, pulilislied the results of some experiments on the 
leak from hot platinum in . air at comparatively low pressures (‘Proc. Camli. Phil. 
Soc.,’ vol. 11, Part IV., p. 206). A fine platinum wire surrounded liy a cylindrical 
electrode could be heated by passing a current through it. The current from the 
wire to the electrode was measured with various differences of potential lietween 
tliem. It was found that, at pressures lielow 1 millim., the current is much greater 
when the wire is negatively charged than when it is positively charged. The current 
with the wire positively charged is nearly independent of the P.D. between the 
limits 40 and 200 volts, but above 200 volts it rises rapidly with the P.D. When 
the wire is negatively charged, the current at f millim. always rises rapidly with 
the P.D., but at millim. the rate of rise diminishes as the P.D. increases. These 
results can be explained by supposing that the ions coming from the wire produce 
others liy colliding with the gas molecules when the electric intensity is large enough. 
This effect was discovered by Professor Townsend^ for the ions produced m air Ijy 
POntgen rays, and it was shown by him that the negative ions produce others by 
collisions when tlie electric intensity is comparatively small. The intensity recpiired 
to make the positive ions produce others by collisions is probably nearly that 
required to start an ordinary discharge through the gas, liut in AIcGlelland s 
* No assumption need be ma<le as to the nature of the ions; they may be molecules, atoms or 
corpuscles carrying the ionic charge e = 3 x 10“^® E.S. unit. 
t See also “ The Laws of Electrolysis of Alkali Salt Vapours,’ ‘ Phil, Mag., August, 190^. 
t ‘Phil. Mag.,’ February, 190E 
