234 Mr. Shelford Bidwell on the 



§ 2. Selenium Cells. 



Any device consisting essentially of a mass of crystalline 

 selenium in combination with two metallic electrodes, and so 

 arranged as to expose a relatively large surface for light to 

 act upon is commonly, but not very appropriately, termed a 

 " Selenium Cell/' So many inquiries have reached me as to 

 the construction of the sensitive cells which I have at different 

 times exhibited before this Society and elsewhere, that I 

 venture to think a short account of the process may be use- 

 fully given here. 



A slip of mica about 55 rnillim. long, 17 millim. wide, and 

 0*4 millim. thick, is sandwiched between two semi-cylinders 

 of boxwood about 18 millim. in diameter and of somewhat 

 greater length than the mica, the ends being held together by 

 screws. The whole is smoothed down in a lathe until the edges 

 of the mica are flush with the surface of the wood. A screw of 

 16 threads to the centim. (40 to the inch) is cut upon the 

 middle portion of the cylinder for a length of 45 centim. The 

 mica is then removed from the wood, and two small holes are 

 drilled near each of its ends. An annealed copper wire 0*19 

 millim. in diameter (No. 36 s.w.G.) is tightly wound around 

 the notched portion of the mica, the turns being made to 

 occupy alternate notches, and its ends are secured by passing 

 them through two of the drilled holes. A second wire is then 

 wound into the other notches, its turns alternating with those 

 of the first. These wires constitute the electrodes, and great 

 care is taken that they do not touch each other at any point. 



The next step is to apply the selenium. A brass plate 

 2 millim. thick is supported upon an iron tripod and covered 

 with a sheet of thin mica ; upon this is laid the prepared 

 mica slip, small weights being placed upon its ends to steady 

 it. The surface of the wired portion of the slip is evenly 

 covered with 1 gramme (or a little less) of powdered vitreous 

 selenium, and a small Bunsen burner is lighted beneath the 

 brass plate. In a few minutes the greater part of the Se will 

 melt : some of it, however, will probably crystallize, forming 

 hard grey lumps, and when this occurs the heat must be 

 continued, and if necessary increased, until all the grey 

 lumps have disappeared. Then the Se is worked smoothly 

 and uniformly over the slip with a steel spatula, particular 

 care being taken that the surface is covered quite up to the 

 edges. This operation is often found a troublesome one, but 

 if the temperature be properly regulated, there is little difficulty 

 about it. When the temperature is too low, hard crystalline 

 lumps are formed ; when it is too high, surface-tension causes 



