152 DR. FRANK HORTON ON THE DISCHARGE OF 



pure platinum, and platinum with 10 per cent, of rhodium. They were of very small 

 diameter (0'0025 centimetre), so that the temperature of the strip was not materially 

 lowered by the heat conducted along them. Each wire was sealed into a fine glass 

 tube, and these tubes were sealed into the bulb vertically above the platinum strip at 

 E (fig. 1). The other ends of the thermocouple wires were some 20 centims. away 

 and were soldered on to wires from a d'Arsonval galvanometer, the junctions being 

 enclosed in a water jacket, through which a stream of water, at a known constant 

 temperature, was kept circulating. The thermocouple was standardised before the 

 platinum strip was placed in the bulb. Very small grains of pure potassium sulphate 

 were placed on the strip near to the junction. The strip was then heated by an 

 electric current, which was increased until the grains of sulphate (observed through 

 a microscope) just melted. The galvanometer deflection corresponding to this 

 temperature" was noted. Use was then made of the curve given by CALLENDAR* 

 for transposing the galvanometer readings into degrees Centigrade. 



(2) Investigation of the Negative Leak from Platinum. 



Professor H. A. WILSON has shown that, in order to get constant values of the 

 negative leak from platinum, great care must be taken to remove all traces of 

 hydrogen, as this gas has a huge effect on the negative leak. For this purpose 

 Professor WILSON recommends boiling the platinum in nitric acid. This method was 

 adopted in the present research. The apparatus shown in fig. 1 consists of two parts, 

 the lower of which, holding the calcium wire and leads, slides into the upper part and 

 is held in position by a sealing-wax joint. When the apparatus had been made, the 

 upper part containing the platinum strip to be used as a cathode was inverted and 

 filled with strong nitric acid. This was boiled for about an hour, and then the acid 

 was replaced by a fresh supply, which was also boiled for some time. After several 

 boilings with strong nitric acid the apparatus was washed out with distilled water 

 and dried by sucking dry air, filtered by passing through cotton wool, through it by 

 means of a water pump. The calcium wire was carefully cleaned with fine emery 

 paper and quickly sealed in position. The whole apparatus was then fixed on to the 

 mercury pump, and the air pumped out until a good vacuum was obtained. The 

 platinum strip was raised to incandescence by means of the current from 10 E.P.S. 

 motor cells. After the platinum strip had been treated with nitric acid there was 

 found to be only a slight increase of gas pressure inside the apparatus when the strip 

 was left at a high temperature for a long time. The slight evolution of gas is 

 probably hydrogen, which had been occluded in the platinum and was not completely 

 eliminated by boiling with nitric acid. The negative leak from the platinum was 

 found to decrease slightly as the gas was evolved, but it soon became fairly constant. 



* CAU.ENDAR, 'Phil. Mag.,' vol. 48, p. 519. 



