464 Mr. Snehamoy Datta on the Variation of 



with the result that the resistance of the cell gradually but 

 unsteadily increased. The unsteadiness may be due to small 

 fluctuations, usually 2 to 3 degrees, in the temperature of the 

 electric heater used for the purpose of annealing. The cell 

 being maintained at about 170° 0. overnight, was found next 

 morning to have increased its resistance considerably. 



For the experiment on the variation of resistance with 

 temperature, the selenium cell under test was enclosed in a 

 light-tight double-walled copper vessel whose temperature 

 could be controlled within half a degree by regulating the 

 heating current. The cell formed part of a circuit con- 

 sisting of a 4-volt accumulator and a calibrated galvanometer, 

 a key being inserted to complete the circuit at will. The 

 temperature of the cell was determined by the thermo- 

 couple, which formed part of the circuit of a second 

 calibrated galvanometer. Though this method is less 

 accurate than the potentiometer method, it was more con- 

 venient, and gave results accurate to about 1/10 of a degree 

 centigrade. 



In spite of the reputation selenium cells commonly have 

 of being subject to unaccountable variations, those used in 

 this investigation were found to be reliable, and the effect of 

 heat on them was quantitatively reproducible again and 

 again. But this required certain precautions to be taken, 

 which will be clear in the light of the following observa- 

 tions. In the course of this experiment, which has been 

 repeated over twenty times, there were definite indications 

 that the change in resistance is intimately connected \% ith 

 the time taken in producing the variation in temperature. 

 Not only does it depend on the rate of heating or cooling, as 

 the case may be, but the future behaviour of the cell depends 

 very much upon the period during which it is kept at a fixpd 

 temperature before being subjected to a variation in tempera- 

 ture. If a selenium cell after being annealed is brought 

 dowm to room temperature and maintained at that tempera- 

 ture for a considerable time, and then subjected to a change 

 in temperature, the change in resistance is much less than 

 when the cell is immediately before brought down from a 

 somewhat higher temperature. Or, in other words, the shape 

 of the resistance temperature curve is less steep when the 

 temperature is changed after a loug period of constant 

 temperature. And this has been noticed, not at room tem- 

 perature alone, but at all temperatures up to about 120° C. 

 It seems as if, after long being maintained at a constant 

 temperature, the selenium is sluggish in changing its 

 physical properties connected w T ith a variation in tempera- 

 ture. Hence, to get reproducible results, it is very important 



