Resistance of Selenium Cells. 



33 



ness, the bulb of a thermometer being very near its surface. 

 The temperature of the air was 17° C, and the resistance of 

 the cell at the beginning of the 0hms 

 experiment was 140,000 ohms. 

 The bath was then very slowly 

 heated, and the resistance mea- 

 sured at every degree. At first 

 the rise was very rapid (see fig. 1) ; 

 then more gradual until the tem- 

 perature reached 23°, when the 

 maximum resistance of 150,000 

 ohms was attained. With conti- 

 nued heating the resistance fell, 

 slowly at first, then more rapidly, 

 then again slowly (as shown by 

 the curve), the final measurement 

 at 100° being only 16,000 ohms*. 



The same cell was afterwards 

 submitted to the combined action 

 of heat and light. A glass beaker 

 was fitted with a wooden cover, 

 to which the selenium cell was 

 attached so as to hang perpen- 

 dicularly inside the beaker ; the 

 beaker was placed in a sand-bath 

 which was heated by a Bunsen 

 burner, and the cell was illuminated by a powerful paraffin- 

 lamp at a distance of 30 centimetres. 



At 18° its resistance was only 19,000 ohms (see fig. 2). As 

 in the former case, the first application of heat was accom- 

 panied by a rise, though smaller and more gradual than before, 

 the maximum of 24,000 ohms occurring at about 29°. The 

 fall which followed was also very gradual, the resistance at 

 100° being 10,000 ohms, or only 14,000 less than the maxi- 

 mum, as against a difference of 140,000 in the former expe- 

 riment. 



In these experiments it might possibly be suspected that 

 the initial small rise of resistance is due to some accidental 

 disturbing cause, and does not point to any essential charac- 

 teristic of selenium, or rather perhaps of selenium cells. The 

 following experiment seems, however, to settle the point con- 

 clusively. One of my selenium cells (the exceptional one 



* When the cell was removed from the air-bath, its resistance in the 

 dark in air at 18° was found to have increased to 90,000 ohms. 



Phil Mag. S. 5. Vol. 15. No. 91. Jan. 1883. D 



220,000 



200,000 

 180,000 











160,000 







140,000 











