Electricity from a Nernst Filament. 245 
of the gas. The rapid increase of the current with the 
potential at pressures lying between 1 mm. and 200 milli- 
metres is explained as being due to this cause. At a pressure 
of 141 mm. (diagram 4) ionisation by collision only began 
to any extent when the filament was charged to 350 volts. 
The secondary ionisation produced by collisions will not, 
however, increase indefinitely with diminishing pressure 
owing to a compensating effect coming into play. As the 
pressure is diminished the chance of a collision taking place 
is also lessened, and so less secondary ionisation is produced. 
This explains why the curves below a certain pressure in 
diagrams 3 and 5 gradually descend with diminishing pres- 
sure. Ina vacuum, since no ionisation by collision occurs, 
perfect saturation curves are there obtained. 
The mean free path at any pressure increases with the 
temperature of the gas. Hence we should expect the maxi- 
mum effect of ionisation by collision to occur at greater 
pressures the higher the temperature of the filament. That 
this was found to be the case may be seen from the curves in 
diagram 6. Curves similar to those given in diagrams 3 
and 5 have been obtained by Dr. H. A. Wilson* in the case 
of the negative discharge from platinum, and by Dr. W. 
Mansergh Varley in his experiments on the photo-electric 
discharge from metallic surfaces. 
It will be seen from Table III. that the current with no 
potential on the filament increases continuously with dimi- 
nishing pressure until a pressure of about 2 mm. is reached. 
The current now remains nearly constant as the exhaustion 
is continued right down to a low vacuum. A similar result 
was obtained at 1605° C. with filament II., as may be seen 
from the following table. The readings are given in the 
order in which they were taken. 
ABT EY LVg A. 
Filament Il. March 14. Temperature = 1605° C. 
pine pate reneemmnaniiss So ee Get wo 
Pressure, | ‘ é 
’> | Current with no Current with no 
apes potential on filament. Ee potential on filament. 
756 | 2 positive eer a 4750 negative 
190 1A ee hy a4 600 __,, 
| 141 ris ae 16 4500s, 
90 121 negative | 0-42 4500 _ i, 
| 40 DOO 5; 0-14 A500) |; 
29 (Uae te 0:06 4500 i, 
| 13 2800". 5 5, 0-032 4150 ,, 
* Phil. Trans. A. vol. ecii. pp. 243-275 (1903). 
T Ibid. pp. 489-458 (1903). 
