Electricity from a Nernst Filament. 231 
mentions incidentally in a paper in the Annalen der Physik 
that he obtained a current of 10—-* ampere between a Nernst 
filament glowing in a vacuum and a metal electrode 10 cms. 
distant. 
It may be mentioned that the Nernst filament is composed 
of certain oxides of metals (so-called “ rare earths”’) and has 
the property of being able to stand an exceedingly high tem- 
perature. 
In all the experiments the lamps used were of the B type, 
adapted to carry half an ampere when connected to a 100- 
volt alternating supply. The filaments were 0°75 mm. in 
diameter, the length of the glowing portion being 8 mms. 
The iron wire resistance (20 ohms) supplied with the lamp 
was always kept in series with the filament. 
The experimental difficulties in this investigation were 
somewhat greater than in investigations on leaks from hot 
wires, insomuch as the Nernst filament has to be made red- 
hot by some external source of heat before an electric current 
can be sent through it, the substance of which the filament 
is made being a non-conductor at temperatures below a red- 
heat. This difficulty was, however, got over by making the 
electrodes by which the electricity discharged from the fila- 
ment was collected serve the further purpose of acting as the 
“heater.” The preliminary apparatus in which this plan 
was put into execution may be briefly described, as it affords 
a convenient way of showing the leak from a Nernst fila- 
ment. The filament fixed to its porcelain base was erected 
inside a lamp chimney. The original heater surrounding it 
had been removed, and in its place a spiral of thick platinum 
wire was fixed in such a way as to be insulated from the fila- 
ment. .Owing to the danger of breaking the fragile filament 
in endeavouring to coil a spiral of thick platinum around it, 
the spiral was made in the following way:—A flat grating 
was first made. This was then bent round a pencil so as to 
be nearly closed. The gap left was just wide enough to 
allow the filament to pass through. The spiral was attached 
to its copper leads in such a way that the filament lay along 
its axis. A current of about 10 amperes was sufficient to 
raise the spiral to a bright yellow heat. In a few seconds 
the temperature of the filament was raised high enough for it 
to conduct. When this took place the current through the 
spiral was cut off, and the latter connected to earth through 
a sensitive galvanometer. The currents for heating both the 
spiral and the filament were got from the secondaries of a 
transformer, the primary of which was connected to the 
100-volt alternating town mains. The primary had 166 
