Electricity from a Nernst Filament. 235 
investigating the variation of the leak with the temperature 
if the same couple be used throughout the set of observations. 
The currents for the filament and the heating electrodes 
were obtained from the secondaries of a transformer as 
described above. 
The temperature of the filament could be varied by placing 
different resistances in series with the filament. Leaks 
greater than 10-° amperes were measured by means of a 
D’Arsonval galvanometer, which yave 1 division for 7°5 x 10-8 
amperes. Smaller currents were measured by means of a 
quadrant-electrometer which gave 1 division in 1 minute for 
2°6 x 10-4 amperes when connected to 01 microfarad. Care 
was taken to see that there was no leak when the filament 
charged toa high potential was cold. One end of the fila- 
ment was always connected to one of the terminals of 
Weston voltmeter, the other terminal of which was connected 
to earth. To charge up the filament the same end as above 
was connected to one pole of a battery of small storage-cells, 
the other pole being earthed. Owing to the high resistance 
of the filament there was a dr op of potential of some 70 volts 
between its ends when fully glowing. One end of the 
filament was therefore either at zero potential or at some 
definite potential, V say. The potential of the other end had 
the alternating values V+70 volts. In the Tables which 
follow what is meant by no potential on the filament is that 
one end of the filament is earthed ; a potential of + 200 volts 
on the filament means that the same end is at a potential of 
+200 volts. 
The paper is divided into the following sections :— 
(1) Discharge of electricity from the filament at atmospheric 
pressure. Variation of the leak with the potential. 
(2) Variation of the negative leak with the potential a 
different pressures ; ionisation by collision. 
(3) Variation of the positive leak with the potential at differ- 
ent pressures. 
(4) Variation of the negative leak with the temperature in a 
vacuum. 
(5) (a) Variation of the positive leak with the temperature in 
a vacuum. 
(6) Relative magnitudes of the positive and negative 
leaks at different temperatures in a vacuum. 
(6) Variation of the leaks with the time at atmospheric 
pressure and in a vacuum. 
(7) The nature of the carriers of the electricity discharged 
from the filament at very low pressures. 
