Discharge of Electricity through Gases. 687 



above its centre of gravity) served to measure the total 

 potential-difference between the electrodes, and at very low 

 pressures (Table XIV.) also for the cathode fall. Its scale 

 was carefully calibrated by direct comparison with the 

 Kelvin multicellular voltmeter, which was used for measuring- 

 potential- differences between the other electrodes ranging 

 from 250 to 1000 volts. The German instrument was a little 

 sluggish in its movements, and its indications were used 

 mainly as a check on those of the other instruments, except 

 when special precautions were taken to tap it before each 

 reading. For potentials between 100 and 300 volts an 

 Ayrton and Mather direct-reading voltmeter (with vertical 

 cylindrical quadrants) was employed. 



The temperature of the hot electrode (KK) was measured 

 by means of a platinum platinum-rhodium thermo-couple. 

 The wires (0*1 mm. in diameter) were attached to adjacent 

 bars of the grating. They were fused through the end of 

 a small glass tube at the top of the discharge-tube, and passed 

 over into two glass tubes immersed alongside a mercurv 

 thermometer in a bottle of water. These glass tubes were 

 partly filled with mercury, which served to make good con- 

 tact vvith the copper wires leading to the galvanometer. This 

 was also a d'Arsonval of the Ayrton and Mather type. To 

 reduce the deflexions of the galvanometer to decrees centi- 

 grade use was made of Messrs. Heycock and Neville's deter- 

 mination of the melting-point of potassium sulphate*. The 

 thermal junction was attached to a strip of platinum-foil 

 which was heated up as in the course of the experiments. 

 The deflexion of the spot of light on the scale was then read 

 off just when the K 2 S0 4 began to melt. The observation 

 was repeated with very slow increments of current through 

 the foil. From the final value thus obtained a curve of 

 temperatures against deflexions was plotted in the manner 

 described by Callendar f , and verified by a determination of 

 the melting-point of sodium sulphate. The deflexions of the 

 spot at all distances of the galvanometer from the scale were 

 readily reduced to this standard distance. 



There is a little doubt attaching to some of the highest 

 temperatures recorded owing to a sagging of the wire-grating 

 which left the portion to which the thermo-couple wires were 

 attached slightly out of the plane of the grating, and so made 

 them rather cooler than the rest. 



With this apparatus, where there were only glass joints 

 and there was therefore practically no leak until very high 



* Chem. Soc. Journal, lxvii. p. 160 (1895). 

 f Phil. Mag. [5] xlviii. p. 519 (1899). 



