282 
DR. FRANK HORTON ON THE ORIGIN OF THE 
I. A Comparison of the Electronic Emissions from a Nernst Filament when Heated 
by Continuous and by Alternating Currents. 
It was originally intended to perform this experiment with the filament in a 
vacuum, but it was soon found that, although with an alternating current the 
filament continued to glow steadily in a vacuum, when a continuous current was used 
one end of the filament became much hotter than the other end. At the same time 
the gas pressure in the apparatus increased, so that evidently some electrolysis of the 
material of the filament was proceeding. It was always the positive end of the 
filament which became the hotter, probably because some metallic constituent of the 
filament is set free by electrolysis at the negative end and lessens the resistance in 
that part of the circuit. In order to maintain the whole of the glowing portion of 
the filament at a uniform temperature when a continuous heating current was used, 
it was found to be necessary to have oxygen present in the discharge tube. This no 
doubt oxidises the metal liberated by electrolysis and so maintains the uniformity of 
the filament. The comparison was therefore performed with air in the apparatus at 
atmospheric pressure, and the observations were taken in the following manner :— 
The leads to the filament were connected to a switch so that either an alternating 
current from the transformer or a continuous current from a battery of storage cells 
could be used. In addition to the adjustable resistances in the transformer circuit 
there was another set of adjustable resistances in the battery circuit, so that the 
E.M.F. applied from the storage cells could be made equal to that being supplied at 
any instant by the transformer before switching the filament on to the continuous 
current circuit. In this way observations at about the same temperature could be 
made alternately, with the filament connected to the transformer or to the storage 
cells. A filament was* first of all heated on the alternating circuit and a series of 
observations of the negative thermionic current at different temperatures was taken, 
the potential difference applied across the discharge tube, in addition to that due to 
the heating circuit, being 210 volts. The negative emission at atmospheric pressure 
from a new Nernst filament generally increased during the first day’s heating, after 
which it remained fairly constant. When the steady state had been reached a 
comparison of the emissions from the filament heated on the “ alternating ” circuit 
and on the “continuous” circuit was made. The temperature of the filament was 
adjusted to be as low as practicable, and readings of the thermionic current and of 
the pyrometer were made; the filament leads were then switched on to the 
continuous current circuit, and the pyrometer and galvanometer readings were again 
taken. The filament was immediately switched back on to the alternating circuit, so 
that it never remained for more than a minute or two with a continuous current 
flowing through it. The direction in which the continuous current was to flow 
through the filament was then reversed by means of a reversing key connected with 
the storage cells, and the observations of the emission and temperature, with alterna- 
