464 Prof. F. Horton on the Application of Thermionic 



end of the wire at thus serves as a terminal for making 

 electrical contact with the plate A. The glass tube in which 

 A is suspended is 3'5 cm. in diameter and about 20 cm. long. 

 It has a connexion at the top through a drying tube to a 

 mercury pump and McLeod gauge. The phosphorus pent- 

 oxide used in the drying tube was purified from traces of 

 phosphorus by heating for several hours at 250° C. in 

 a stream of ozonized oxygen, a method recommended by 

 Mr. J. J. Manley. 



The lower plate, B, is of aluminium *5 mm. thick and fits 

 into a depression turned in the end of the thin brass tube C, 

 the outside diameter of which is slightly larger than that 

 of B. Fixed to the lower end of C are three brass spring- 

 clips which hold it in position on the upper end of the inner 

 glass tube of the ground joint. The spring clips are not 

 shown in the diagram. The plate B with the cylinder C can 

 thus be taken out of the upper glass tube. When placed in 

 position in that tube, C presses gently against a spring S 

 made of a platinum wire, the other end of which is sealed 

 through the glass tube at P, and serves as a terminal for 

 making electric connexion with the plate B. In the centre 

 of the plate B there is a small square hole, each side of 

 which is 1*3 mm. long. About 1*5 mm. below the plate B 

 a platinum strip, which can be heated electrically, is held 

 in a horizontal position by two leads of stout platinum wire, 

 H and K, to which it is welded and which for the greater 

 part of their length are covered with glass to insulate them 

 and to make them more rigid. A short distance below the 

 platinum strip the two leads are connected together by a 

 glass cross-piece so that there can be no relative motion of 

 their ends. The platinum strip is about 1 cm. long ; it is 

 1 mm. wide and '02 mm. thick. Its central part is bent 

 upwards through the square hole in the middle of the 

 plate B, and arranged so that the upper surface of the strip 

 inside the hole is in the same plane as the upper surface of 

 the plate. The part of the platinum strip which is exposed 

 fills the small square hole except for a space about *15 mm. 

 wide all round it. The placing of a new strip in position 

 was a matter requiring considerable patience. The central 

 part of the strip was first bent into the shape required by 

 means of a small press, so that the central raised part was 

 made exactly the right size — l'O mm. square. The ends of 

 the strip were then welded on to the platinum leads, and 

 these had usually to be bent slightly to get the square 

 in the centre of the end of the brass tube C and in the 

 same plane as that end. The aluminium plate B was then 



