IMPAETEI) TO A VACUUM BY HOT CONDUCTORS. 
523 
(1) The resistance of the filaments ; 
(2) The currents used to heat the filaments; and 
(3) The temperature of the carbon surface, respectively. 
In what follows a section will be devoted to each of the above headings, 
§ 3. The Relation between the Saturation Current and the Resistance of the Filament. 
This was investigated in the same manner and with the same apparatus (fig. 2) as 
in the case of j^latinum. In all cases the apparatus which has already been described 
and is shown in fig. 6 was employed. The thick German silver resistance II (fig. 2), 
which served as an intermediate standard, had now a resistance of 1-62 ohms. The 
smaller currents were measured with the sensitive Thomson galvanometer. For the 
larger currents a D’Arsonval galvanometer, which gave a deflection of 1 millim. for a 
current of 3'46 X 10 ^ ampere, was employed, ovhng to its greater convenience. As 
the resistance of the filament decreased slightly during the observations, a reading 
was taken both before and after each observation of the leak, the mean of the two 
readings being taken as the value of the resistance which corresponded to the reading 
for the current. Resistance readings were taken over a range of saturation current 
extending from 2-8 X 10“^ to 6 X 10“^ ampere per sq. centimetre of surface. The 
corresponding range of the value of the ratio of the resistance of tlie filament to its 
resistance at 11° C. at the commencement of the experiment was from -610 to -567. 
In other words, while the resistance of the filament only alters in the ratio of 610 to 
567, the negative leak has become twenty thousand times as big as it was at first. 
It is evident, therefore, that, as in the case of platinum, the number of negative ions 
produced at the surface increases with enormous rapidity as the temperature rises. 
It will be shown later that, over a much greater range of temjDerature than this, 
there is no percejotible falling off in the rate at which the current increases. 
The corresponding values which were obtained for the saturation current and the 
resistance of the carbon filament are given in the following table. The resistances 
are expressed as fractions of the resistance which the filament possessed at 10° C. 
before it was heated. 
Saturation current in 
amperes. 
Resistance as a fraction 
of initial resistance. 
3-9 xlO-8 
-609 
9-43x10-8 
-604 
3-25 X10-7 
-600 
8-55 X10-7 
-594 
19-04 X10-7 
-588 
4-32 X10-« 
-581 
13-3 xlO-G 
-571 ‘ 
3-53 X10-5 
-560 ! 
7 - 80X10-5 
-547 ! 
2-47 X10-^ 
-528 
3-95 X10-4 
-509 
9-05 X10-4 
-48 
3x2 
