378 



Prof. G. Sheard on the 



When the temperatures were below 400° 0., the melting- 

 point of cadmium iodide, the greater portion of the salt 

 inserted in A remained after the experiments had been 

 made. 



The effect of changing potential from positive to negative 

 values, and vice versa, on the ionization in tubes A and B, 

 was investigated at a temperature of 345-350° C. and a 

 pressure of 1*3 cm. The positive current-time relations 

 during the first twenty minutes of heating were identical 

 with those represented in curves 1 and 2, figure 3. The 

 steady current state, reached after 30-40 minutes' heating, 

 furnished an excellent opportunity of studying the effects 

 produced by a reversal of the electric field. 



Table II. 



1 div. = l X 10" 11 ampere. 





Tube A. 







Tube B. 





Time, 



Volts. 



Current. 



Time. 



Volts. 



Current, 



(Min.) 







(Min.) 







25 



+ 120 



46-0 



25 



+ 120 



96 



27 





43 2 



28 





9-6 



31 



-120 



6-8 



29 



-120 



46-0 



33 



+ 120 



55-3 



32 





31-0 



35 





39-2 



36 



+ 120 



8-9 



37 





34-3 



39 





73 



42 



-120 



7-2 



41 



-120 



33-0 



44 





3-4 



•13 





12-3 



46 





2-6 



45 





7'6 



48 



+ 120 



41-5 



49 



+ 120 



9-5 



52 





25-0 



53 





6-2 



55 





18'2 



57 





5-2 



59 





15-3 



60 





4-7 



An inspection of the results given in Table II. shows that 

 a reversal of potential in tube A from positive to negative 

 voltages gave negative current readings of about one-fifth to 

 one-tenth the positive values immediately preceding. When 

 the potential was again reversed, the positive readings were 

 temporarily increased, but fell off: asymptotically with the 

 time and ultimately assumed values fitting upon a curve 

 indicating the general decay with time of the positive 

 ionization. The corresponding treatment of B showed the 

 exact counterpart to the phenomena in A ; since, for example, 

 the reversal from a positive to a negative potential gave a 

 current about five times as great as the previous positive 



