Positive Ionization from Hot Salts. 



677 



of salt tested were positively charged and heated for 24 

 hours or more in a good vacuum until they got into a fairly 

 steady state. Measurements of the positive emission were 

 then made with both increasing and diminishing pressures 

 of air up to about 10 mm. The results do not seem to be 

 absolutely definite, as the curves connecting the ionization 

 and the pressure often differed considerably under what were 

 apparently the same conditions. The salt behaved as though 

 its nature was changing considerably during the course of 

 the experiments. The rough outlines of the phenomena 

 were, however, definite enough. The ionization was always 

 small at a very low pressure, and increased rapidly with the 

 pressure at low pressures. In many cases a maximum value 

 of the emission was observed at a certain pressure, although 

 this did not always happen. When there was a pressure of 

 maximum emission it always occurred at a higher pressure 

 the lower the temperature. This point is illustrated by the 

 following table, which shows (column 2) the pressure at 

 which the emission was a maximum at a series of tempera- 

 tures (column 1), the value of the maximum positive emission 

 (column 3), and the value (column 4) of the small emission 

 given at the lowest pressure (column 5) which could be 

 attained by working the Graede pump continuously: 



1 



1 

 Temperature. 



Pressure for 

 maxim uai 

 emission. 



Maximum 

 Emission. 



Least 

 Emission. 



Least 

 Pressure. 



730° C. 

 1005° C. 

 1160° C. 



20 mm. 



4 mm. 



02 mm. 



1-67x10" amp. 



35X10" 8 amp. 

 7'8 X 10 amp. 



—13 



8X10 amp. 



2xl0~ 9 amp 



1-3 Xl0~ 7 amp. 



0-00003mm. 

 0-0003 mm. 

 00003 mm. 



A similar displacement of the maximum emission towards 

 higher pressures at lower temperatures was found by Garrett 

 to be exhibited by aluminium phosphate. 



This maximum emission at a certain pressure does not 

 appear to occur at a low temperature with a specimen of the 

 salt which has been heated for a long time at a high tempe- 

 rature. A specimen which had been heated for considerable 

 periods at temperatures as high as 1160° C. gave the 

 following values of the thermionic current at a series of 

 pressures at 730° C. : 



Pressure (mm.) ... -25 -45 13 1 73 2*2 12 -75 '45 "26 -14 012 



Current 25 38 42 41 42 40 48 47 40 20 17 



(l=4xl0~ li amp.) 



