Ai)' at different Tempevaturcs and. Pvessures. i^33 



placed ill position. This iirrangemeiit prevented the hot 

 air beino- driven aoainst the insulation. In order that the 

 cylinder mioht be heated as nnifornily as |)Ossible and pro- 

 tected from the flames a second cylinder H surrounded it. 

 The tube C was brought through the bottom o£ this cylinder 

 in the pipe G, which extended below the burners and so pro- 

 tected from the flames. The legs of the tripod were brought 

 through in a similar manner. The cylinder and tripod were 

 thus screened from the flames so that they held a charge for 

 some time. Around the cylinder H was another K, so that 

 the hot gas from the flames passed up between H and K and 

 out at the top. 



The temperature of the cylinder was measured by two C^u- 

 Ni couples, one at M the other at L, both being on the out- 

 side of the cylinder and held firmly against it. The cold ends 

 of the wires dipped into mercury cups placed in a water-bath 

 at a known temperature. The two cups for each couple 

 were connected through a galvanometer. 100 ohms resistance 

 was added to the galvanometer circuit, so that the increase 

 of resistance due to heating could be neglected. The couples 

 were calibrated by observing the galvanometer deflexions for 

 known temperatures. 



The cylinder A was connected to one pole of a battery of 

 small storage-cells, the other pole being earthed : C was 

 earthed and D was permanently connected to one pair of 

 quadrants of a quadrant electrometer, the other pair being to 

 earth. The current between the walls of the cylinder and the 

 electrode D was measured. By this arrangement there could 

 be no leakage from the charged cylinder to the insulated 

 electrode. Any leakage due to imperfect insulation would 

 make the current through the gas greater than that which 

 was measured. 



As very small potential-differences had to be measured it 

 was necessary to guard very carefully against external 

 electrostatic disturbances. All connecting wires, keys, and 

 electrometer were inclosed in conductors connected to earth. 

 The quadrants were opened by a mercury key which could 

 be worked froni a distance by means of a cord. At first 

 mercury cups in paraffin wax were used, but it was found 

 impossible to keep the paraffin from charging up, and conse- 

 quently it was discarded and steel thimbles mounted on 

 sealing-wax were used instead. These were very satisfactory. 



A Dolezalek electrometer was used. The needle of silver- 

 paper was suspended by a very fine quartz fibre and charged 

 to about 80 volts by touching with a fine wire connected to 

 the battery. The needle held its charge very well, and only 



