IMPAETED TO A VACUUM BY HOT CONDUCTORS. 
585 
to the surrounding earthed tube ABDO. By altering the capacity of the 
condenser a suitable rate of movement of the spot of the electrometer could be 
obtained each time. 
In using the galvanometer the arrangement was practically the same as that 
employed before. One end of the battery was put to earth while the other was 
connected through the galvanometer to A^Bj. The battery was capaljle of supplying 
any number of volts up to 420, the potential lieing measured by tlie Weston volt¬ 
meter used previously. A D’Arsonval galvanometer giving 1 millim. deflection for 
2x10 ® ampere, and having a resistance of 500 ohms, was used. 
With the exception of tlie change m the materials of the couple the thermoelectric 
circuit was exactly the same as that employed in the experiments on carbon. To 
reduce the galvanometer readings to temperatures use was made of the recent 
observations of Mr. E. P. Harelson.^" Only one fixed point was determined, viz., 
that of the boiling-point of sulphur. The electromotive force at that temperature 
was found to correspond to 227 microvolts per degree over the whole range, a result 
which agrees very accurately with that given by Mr. Harrison. The relation 
between electromotive force and temperature was not assumed to Ije linear, but 
corresponding values for intermediate points were calculated from Mr. Harrison’s 
curves. From these figures a curve was plotted which gave temperatures in terms 
of galvanometer readings directly. The galvanometer employed gave 1 millim. 
deflection for 1'39 X 10 ^ ampere, and the total resistance of galvanometer and 
thermoelectric circuit with no resistance out of the box was 197 ohms. 
In the various forms of glass apparatus jireviously tried it was found that 
considerable currents were obtained at ordinary temperatures when the sodium was 
charged negatively. This was ultimately found to be due to the photoelectric effect 
produced by the light present in the room, since it disappeared wlien the ex])eriments 
were made in the dark. The steel tube finally used in the experiments had the 
great advantage that it could easily be made absolutely light-tight. In order to 
make sure tliat no light reached the sodium, the glass tubes through which the 
wires were let in at each end of the steel tube were painted over with black enamel. 
The leak wms then tested and found to be small and the same in both directions; so 
that it was all due to imperfections in the insulation. 
§ 3. The Relation between the Current and the Electromotive Force. 
After testing the insulation and pumping down the apparatus exjieriments were 
first made to see how the current varied with the direction of the electromotive force. 
The first measurements showed that, at a temperature of about 300° C., the current 
when the wire was at a potential of -f- 40 volts was 3500 times its value when the 
wire was charged to — 40 volts. The value of the current when the wire was 
positive was 1‘5 X 10 ampere. Later experiments showed, however, tliat the 
* ‘Phil. Mag.,’ (6), vol. 3, p. 177. 
