676 



Prof. 0. W. Richardson on the 





current between 10~ 13 ampere and 1 ampere can be readily 

 measured by the same instrument. 



The temperatures were obtained by measuring the resistance 

 of the strip at the temperature of the room and at the melting- 

 point of sodium sulphate, of potassium sulphate, and of 

 platinum, and constructing a calibration curve from the data 

 thus obtained. 



A number of experiments were first made with tribasic 

 sodium phosphate (Na 3 P0 4 , c.p., by Eimer and Amend). 

 The results were similar to those obtained by Horton with 

 sodium phosphate. In air there was a maximum positive 

 emission at 2*3 millimetres pressure when the temperature 

 was 730° C. At very low pressures the emission was quite 

 small, being only one-fiftieth of the maximum at O00002 mm. 

 pressure. The value of the maximum current at this tem- 

 perature was 3*3 x 10" 9 amp. 



A more detailed examination was made in the case of 

 sodium sulphate (Na 2 S0 4 , by Kahlbaum). The salt was 

 placed on the strip and melted in the air of the room before 

 being tested. It emitted white fumes during the process. 

 The general character of the thermionic behaviour of this 

 substance was found to be similar to that of the sodium 

 phosphate. When the salt was heated at a constant tempe- 

 rature the emission increased rapidly during the first five 

 minutes and then fell in the manner which is characteristic 

 of the positive ionization from hot bodies generally. The 

 following observations, taken at 1005° C, are typical: — 



Time heated, minutes 



Thermionic Current 



o 



6-0 



5 



25 



6 



26 



9 

 23 



14 

 22 



20 

 15 



32 

 13 



60 

 95 



150 



4-6 



(1 = 1-25 XlCT 7 amp.) 





Time heated 



190 

 3-3 



215 

 0-86 



242 

 0-26 



295 

 017 



330 

 0-19 



990 

 0-02.5 



1140 

 0-02O r 



1290 

 10167 



C urrent 















After about 24 hours' heating at this temperature the rate 

 of decay of the emission becomes quite small, and might be 

 due to loss of salt by evaporation. In these experiments 

 sudden but rather temporary increases of the emission are 

 apt to occur, particularly during the period between the 

 lapse of half an hour and an hour and a half from com- 

 mencing to heat the salt. Such increases have been omitted 

 from the table as far as possible. 



As a preliminary to further experiments all the specimens 



