Currents to the Study of Ionization by Collision. 465 



carefully placed in position in the circular groove cut to 

 fit it. 



The temperature of the heated platinum strip was measured 

 by means of a thermocouple of wires of platinum and platinum 

 with 10 per cent, of rhodium. These wires are *025 mm. 

 thick and the junction is very small. It was welded on to 

 the under side of the platinum strip — as nearly as possible 

 at the middle of the raised central portion. The welding- 

 was accomplished by raising the strip to a bright red heat 

 and pressing the thermojunction against it. If the strip was 

 hot enough the junction stuck to it, but if this did not occur 

 the strip was raised in temperature very cautiously until 

 welding took place. That the thermojunction wires do not 

 tap off any difference of potential from the heating circuit 

 was shown by leaving the platinum strip heating until a 

 steady temperature was obtained and then suddenly reversing 

 the direction of the heating current, when the same reading 

 was obtained on the d'Arsonval galvanometer used to measure 

 the thermoelectric current. The temperature of the central 

 portion of the strip was obtained from the deflexions of this 

 galvanometer which were standardised by observations of 

 the melting-point of pure potassium sulphate, making use 

 of the curve given by Callendur* for this purpose. The 

 thermocouple wires were insulated by fine glass tubes and 

 parsed out of the apparatus at L and M. The platinum 

 strip was heated by a current from a battery of three large 

 accumulator cells, the current being regulated by wire 

 resistances. 



When the platinum strip was tested as a source of 

 ionization, it was found that it had to be raised to a very 

 high temperature in order to obtain currents measurable on 

 a galvanometer. This caused the strip to sag, so that the 

 raised central part was no longer in the same plane as 

 the upper surface of the plate B. For several reasons it is 

 desirable to work with the strip at as low a temperature 

 as possible, and it was therefore decided to make use of 

 the much greater thermionic emission which is obtained by 

 covering the platinum with lime. A small piece of sealing- 

 wax and about an equal quantity of lime were finery powdered 

 and intimately mixed in a mortar. A little water was added 

 and a small drop of the mixture was placed in the middle of 

 the central raised square of the platinum strip. This was 

 gradually heated to a bright red heat and the combustible 

 material of the sealing-wax burnt away, leaving a very thin 

 deposit of lime, which adhered firmly to the platinum surface. 

 * H. L. Callendar, Phil. Mag. [5] xlviii. p. 519 (1899). 



