ELECTRON EMISSION FROM GLOWING SOLIDS. 
281 
(3) The emission from a Nernst filament heated in the ordinary manner has been 
compared with that given by the same material when powdered and heated upon 
platinum. This experiment is similar to that just mentioned. 
The discharge tube used in these experiments is similar to that described in 
a former paper,* the only difference being that in the present apparatus the two 
parallel platinum plates which form the anode are 1'5 cm. apart. The cathode, when 
in position, is parallel to these plates and mid-way between them. It can easily be 
removed from, or replaced in, the discharge tube. Experiments were made with 
twenty different Nernst filaments of the B type and meant for use on a 100-volt 
alternating supply. They were all of about the same dimensions, the length of the 
glowing portion being about 9‘5 mm. and the diameter about 078 mm. When 
a filament was to be heated by passing an electric current through it, the platinum 
wires attached to it were welded on to the platinum leads of the discharge tube. 
When an alternating current was used, this was obtained from the secondary of 
a transformer, the primary coil of which was connected to the alternating town 
supply, and the current through the filament could be varied by changing the 
resistance in both the primary and the secondary circuits. One of the fine iron wire 
resistances supplied with Nernst lamps was always kept in series with the filament. 
As this resistance has a large positive temperature coefficient, it tends to steady the 
current through the circuit. In order to start the filament glowing it had to be 
taken from the discharge tube and heated by holding it above, and near to, a glowing 
“ heater ” of the kind supplied with an ordinary Nernst lamp. It was then replaced 
in the apparatus which could be rapidly exhausted, when required, by means of 
a water-pump, a mercury pump, and a charcoal tube cooled in liquid air. In this 
way the gas pressure in the apparatus could be reduced to '0001 mm. within twenty 
minutes from the time that the filament was started glowing. 
The temperature of the glowing filament was determined by means of a Fery 
optical pyrometer for the use of which I am indebted to Prof. T. Mather, of the 
City and Guilds College, London. This instrument was standardised by observations 
of the readings corresponding to different temperatures of a platinum tube of about 
the same diameter as a Nernst filament and having a standardised thermocouple of 
fine wires of platinum and platinum-rhodium welded to it. In order that the surface 
of the platinum tube should be exactly similar to that of the filament, a filament was 
finely powdered and mixed with water, and the platinum tube was then covered with 
a thin layer of Nernst filament material by evaporating this mixture upon it. This 
platinum tube was fitted up in the place of the filament in the discharge tube, and 
observations of the thermo-electromotive force and readings of the optical pyrometer 
were taken at several temperatures between 900° C. and 1600° C. From these 
observations, the temperature of a Nernst filament corresponding to any reading of the 
pyrometer between these limits can be ascertained with fair accuracy. 
* ‘Roy. Soc. Proc.,’ A, vol. 88, p. 117, 1913. 
VOL. CCXIV.—A. 2 O 
