500 Profs. 0. W.Richardson and 0. Sheard : Variation of 



Before these experiments were made, the tube, which con- 

 tained nothing but glass and platinum, was carefully boiled 

 out with pure nitric acid and distilled water. The tube was 

 dried in a current of warm air before connecting to the 

 Gaede pump and pentoxide bulb. It was exhausted at a 

 pressure of less than 0*0001 mm. on the McLeod gauge 

 before the wire was heated. The diameter of the wire was 

 0*15 mm. and its length 5 cm. 



The results described above were obtained when the wire 

 was continuously maintained at the temperature at which 

 the emission was measured. After heating the wire, which 

 was positively charged, to a relatively high temperature 

 (718° C), and subsequently observing at the original 

 temperature (452° C), the resulting current-E.M.F. curves 

 were very peculiar. They were characterized by a very 

 sharp maximum at about 240 volts, which was more marked 

 with descending than with rising potentials. It seemed to us 

 that these peculiarities might be due to the glass insulation 

 becoming charged to a high potential under the influence of 

 the copious emission of positive ions which had occurred at 

 the higher temperature. The brass-tube apparatus shown 

 in fig. 2 was therefore constructed in order to eliminate this, 

 possibility. 



Fig. 2. 



To HIGH 

 ^POTENTIAL 



To HIGH 



POTENTIAL 



^BRIDGE 



EARTH 



The platinum wire W was stretched along the axis of the 

 brass tube T which, suitably supported and properly shielded,, 

 formed the receiving electrode. The wire W was welded to 

 two stout platinum end-pieces, which in turn were soldered 

 respectively to the brass rod B/ and the tube R. This was 

 done to prevent the solder from coming into contact with 

 the hot part of the platinum and causing it to rot. R and 

 W were supported by glass tubes surrounded by earthed 

 brass tubes, E and E', provided with flanges, F and F'. 

 E and E / were supported by rubber stoppers, A and A'. 

 The only insulation in this arrangement which the ions 



