6SG 



Prof. 0. W. Richardson on the 



quite different, both from the curves given by sodium phos- 

 phate by the strip method and from all the curves given by 

 sodium sulphate. Curve No. 2 on fig. 2 shows the mean 

 curve with +40 volts. The mean curve with +160 volts 



Fiar. 



of 15° 

 (fig. 2) 

 800° C 



2 4 



/°/?ESSUf?£ fMlLLI meters) 



was practically identical with this except for a difference in 

 the absolute values of the currents. The mean temperature 

 during this experiment was 775° C. with a steady drop 

 C. from the beginning to the end. Curve No. 1 

 is taken from Hor ton's paper and was obtained at 

 by the strip method. His currents seem a little 

 larger than mine after allowing for the different temperatures 

 and size of apparatus, but he does not say which of the phos- 

 phates of sodium he used. The lowest pressure in the tube 

 experiments was *005 mm. and the emission was greatest at 

 this pressure. It fell rapidly to a minimum at 1*5 mm. 

 and after that rose very slightly. In the strip experiments 

 the emission is least at the lowest pressure, undergoes a rapid 

 increase to a maximum at 2*3 mm. and then falls off slowly. 

 The behaviour is almost the exact opposite of that given by 

 the experiments with the tube. 



On taking down the apparatus after this experiment, the 

 inner electrode was found to be covered with a thin layer of 

 salt which had a fishy smell and deliquesced rapidly. 



Aluminium Phosphate. — The salt used was supplied by 

 Kahlbaum. A sample from the same stock was used by 

 Mr. Davisson in his measurements of e\m for the positive ions 



