252 Mr. S. Bidwell on some 



effective one is that which I suggested ten years ago 

 ('Nature/ vol. xxiii. p. 58). A fine copper wire is wound 

 upon an oblong strip of mica from end to end, the number 

 of turns being about 20 to the inch : a second wire is then 

 wound on, parallel to the first, the turns of the second wire 

 alternating with those of the first ; great care is taken that 

 the two wires do not touch each other anywhere. A thin 

 layer of melted selenium is spread over one surface of the 

 mica, filling the spaces between the wires. The film is 

 first quickly cooled by placing the mica upon any con- 

 venient cold surface, then it is crystallized by heating at a 

 temperature of about 200°, and, lastly, it is annealed by 

 gradually lowering the temperature for several hours. A 

 cell made in this manner, with a surface of about 10 sq. cm., 

 is generally found to have a resistance in the dark of from 

 50,000 to 100,000 ohms. As a rule, those cells which have 

 an unusually low resistance are less sensitive to light than 

 others. In a cell of average quality the resistance is reduced 

 about 50 per cent, by the light of an ordinary gas-flame at 

 the distance of one foot. 



I made a number of cells on this plan in the years 1880 

 and 1881. Up to the end of 1882 they were frequently 

 used, and no material falling off in their sensitiveness was 

 noticed. Thirteen of these cells were laid aside until 1885. 

 They had then for the most part deteriorated more or less, 

 one or two of them being quite useless. They were again 

 put away until the present year, 1890, when it was found 

 that only one of the thirteen retained its original sensitive- 

 ness and resistance unimpaired.* A second one worked 

 fairly well with a feeble current, its resistance with a single 

 Leclanche cell being in the dark 14,000 ohms, and when 

 illuminated by a gas-flame at 12 inches, 8000 ohms ; but 

 with a battery of six cells the resistance became variable and 

 unsteady, as if there were a loose contact somewhere, and a 

 continuous boiling noise was heard in a telephone introduced 

 into the circuit. The resistance of the remaining eleven 

 cells had fallen enormously, in some cases to less than 10 ohms, 

 and they were quite insensitive to the action of light. An 

 attempt was made to restore most of them by melting down 

 the selenium and recrystallizing and annealing ; but though 

 the resistance could in this manner be brought up again to a 

 high point, the sensitiveness of the renovated cells was in all 

 cases found to be very poor. 



Prof. Adams states (Phil. Trans, vol. clxvii. p. 348) that 



* This cell had the unusually high resistance of 400,000 ohms, reduced 

 by the light of a gas-jet at one foot to 190,000 ohms. 



