NKi: \TIVK l.l.KCrUCITY FlmM HOT CALCIUM AND FROM LIME. 



151 



tube until the pressure as indicated on the MacLeod gauge remained constant. In 

 the first exj>eriments the gas employed was argon, while in the later experiment* 

 helium gas was used. Tin- helium wa-s purchased out of a grant obtained from the 

 Royal Society. 



The arrangement of apparatus used to measure the current from the surface of the 

 platinum i> indicated in fig. 2. 



To Pump 



Earth 



Fig. 2. 



A, A, platinum anodes ; C, platinum strip ; 

 H, H, leads of heating circuit; V, volt- 

 meter ; B, )>attery ; K, key ; G, galva- 

 nometer ; K, tin-foil fuse. 



F 

 Fig. 1. 



One end of the platinum strip C was connected to earth and also, through a fine 

 tin-foil fuse, R, to the negative pole of a battery of small storage cells, B. The 

 positive pole of this battery was connected to the anodes A, through the key K, and 

 the sensitive d'Arsonval galvanometer G, which was well insulated on paraffin 

 blocks and served to measure the current. The difference of potential between the 

 electrodes A and C was determined by means of the electrostatic voltmeter V. 

 H, H, are the leads for heating the platinum strip. 



The temperature of the cathode was determined by means of a thermocouple which 

 was welded on to the strip at its middle. The wires forming the thermocouple were 



