466 Mr. Snehamoy Datta on the Variation of 



reducing the resistance ; the portion of the curve parallel to 

 the ordinate marks the change in' resistance obtained by 

 keeping the cell at a fixed temperature till no further 

 change in resistance is observed *. The dotted curve thus 

 shows the ultimate resistances appropriate to various tem- 

 peratures. Curve II. is obtained with a different cell by 

 changing its temperature exceedingly slowly, so as not to 

 .show any resistance lag, the discontinuities in curve I. being 

 now absent. The form of the curve suggests that the varia- 

 tion of resistance with temperature is expressible by an 

 empirical equation of the ordinary type : 



The constants b and c are not general constants for selenium, 

 but depend on the annealing and the past history of the cell. 

 The values of b and c, having been derived by the solutions 

 of: equation supplied by the experimental data, Table I. shows 

 the agreement between the experimental data and those 

 calculated from the equation : 



R,= [65-01 x 0-86 * + -0029* 2 ] 10 5 . 



Temp, in 



cleg, centigrade. 



00 



3-3 

 6-5 



13-0 



20-0 



40 



600 



78-0 



87-0 

 1000 

 1200 

 137-0 

 160-0 

 170-0 



As shown in the curves, whatever may be the initial 

 resistances of the cells, their resistances (for cells of the 

 same dimensions) beyond 130° 0. very nearly coincide, and 



* The time allotted for making the change of temperature, e.g. from 

 A to B, was about 10 minutes. The subsequent time necessary before 

 the resistance became constant at C was of" the order of 25 minutes, 

 showing- that the process is a deliberate one not attributable to failure 

 of the thermo-couple to record the true temperature. 



Table I. 





K, (obs.) in ohms. 



R (cal.) in ohms 



660X10 5 



65-01 Xl0 5 



62-2 



62-20 



59-1 



59-54 



54-6 



54-32 



49-1 



48-97 



346 



35-24 



238 



2384 



15-1 



15-57 



11-9 



12-14 



8-0 



8-10 



3-5 



352 



1-9 



1-62 



1-7 



1-64 



2-0 



2-62 



