Discharge of Electricity from Hot Platinum. 407 



XL. The Discharge of Electricity from Hot Platinum in 

 Phosphorus Vapour. By 0. W. Richardson, M.A.,D.Sc, 

 Clerk Maxwell Student and Fellow of Trinity College, 

 Cambridge*. 



IN the Philosophical Magazine for July 1903 it was 

 shown that the positive leak from a fresh, clean 

 platinum wire, maintained at a constant temperature in vacuo, 

 fell off asymptotically with the time until it reached values 

 of the order of one-thonsandth of that it originally possessed. 

 More recently f the author has been investigating the con- 

 ditions under which a wire would recover its discharging 

 power, with a view to explaining, if possible, the abnormal 

 properties of new wires. Among other results, it has been 

 found that a wire does not show any appreciable recovery if 

 left in vacuo for a fortnight, or if exposed to the vapours 

 from tap-grease (a mixture of vaseline and solid paraffin) or 

 mercury. Further exposure to air at atmospheric pressure 

 for periods of the order of one day only produces very small 

 and rather uncertain effects. 



In one case, however, an apparatus which had been 

 showing no recovery when connected with one pump and 

 phosphorus-pentoxide bulb, was found to behave quite 

 differently when it was cut down and subsequently sealed on 

 to another pump. As the possible sources of contamination 

 appeared to be only tap-grease or phosphorus-pentoxide, and 

 the former had been previously found to be harmless, 

 experiments were tried to see whether the recovery was 

 caused by some peculiarity in the latter. 



To test this point, the pentoxide-bulb was heated by 

 surrounding it with an asbestos-lined brass cylinder, which 

 could be heated by a Bnnsen burner. At the commencement 

 of the experiments, the pressure inside the apparatus was 

 very low ; and even after the bulb had been heated for some 

 time it did not rise to more than about a hundredth of a 

 millimetre. Thus, any increase in the current could not be 

 due to ionization by collisions. The tube containing the 

 wire was separated by nearly a metre of glass tubing which 

 included a tap from the pentoxide-bulb, so that there was no 

 danger of any of the contents of the latter being convej^ed 

 mechanically to the former. The platinum wire was 

 maintained at a constant temperature, as measured by its 

 resistance ; -o that any alteration in the leak from it could 



• Communicated by the Author. 

 + See Phil. Mag. [6] vol. viii. p. 400. 



