IMPAETED TO A VACUUM BY HOT CONDUCTORS. 
509 
§ 2. Variability of the Current. 
As some unsteadiness had been observed in the galvanometer readings for the leak 
duiing the earlier observations, a series of experiments was made in order to examine 
if the current from the wire varied when the conditions were kept as steady as 
possible. ^ When the current was passed through the wire the tube became hot and 
gas was given off from the walls and from the hot wire, so that it was impossible to 
keep the pressure absolutely constant. However, by continuously pumping out the 
gas the pressure was kept practically constant, the limits of variation being very 
small. A constant current was run through the wire so that its temperature and 
resistance were invariable except in so far as they depended on tiie pressure of the 
gas in the apparatus. 
Since the rate of escape of heat from the wire is determined largely by the gas 
pressuie, the temperature of the wire is a function of the pressure. In fact, the 
galvanometer spot was a far more sensitive indicator of the pressure than the McLeod 
gauge. By carefully watching the galvanometer and jDumping accordingly, the 
variations of both pressure and temperature were kejit very small indeed. 
Undei these conditions it was hoped that the rate of leak from the wire Avitli a 
constant voltage Avoiild remain aiDproximately constant. It was found, however, that 
it varied in the most haphazard manner, oscillating irregularly lietween the limits of 
10 and 10 * ampere. The current did not become any steadier with continuous 
heating. Readings taken every three or four minutes for tlie space of three hours 
showed the same continuous irregular periodicity. The irregularities Avere quite 
independent of the potential tliat Avas or had lieen applied to the Avire, and also 
seemed to have no relation to the rate at Avhich gas Avas given ofb There was no 
measurable falling off Avith time. 
It ought, perhaps, to be mentioned that the tube used for tliis experiment seemed 
far more varialile than those used for the temperature experiments, though they AA^ere 
never examined systematically. The platinum wire used for this tube was the purest 
olitainable. 
These results are taken to indicate that the negative ionisation depends to a great 
extent on small clianges in the condition of tlie surface of the hot Avire. We should 
expect this to lie the case on the view that the phenomena are due to tlie escape of 
corpuscles from the metal, since an alteration of 14 per cent, in the work done by an 
ion in going through the surface would multiply the current by 100. 
Fuithei exj^eriinents shoAved that fairly steady readings Avere obtained if the 
heating current was stopped and the tube allowed, as it were, to recover itself 
between each observation. The initial value of the current was almost constant, it 
then began to decrease slowly and afterAvards varied in fhe irregular manner 
described above. Tlie folloAving readings taken with constant voltage^ temperature, 
and piessuie at the times stated, indicate the sort of agreement AA'hich is 
obserA^ed ;—^ 
