Making a Selenium Cell for the Laboratory 



The resistance of the cell depends upon 

 its size and the spacing of the wires 



By Charles E. Mullin 



BECAUSE the electrical conductivity of 

 selenium is dependent upon the 

 strength of the light illuminating it, 

 experiments with this substance are par- 

 ticularly interesting. Selenium is not the 

 only substance possessing this property, but 

 it is by far the most sensitive of any yet 

 discovered. A "selenium cell," as the 

 apparatus utilizing 

 this property is 

 called, may be con- 

 structed in the shop 

 or laboratory at very 

 little expense. 



Two types of cells 

 are used, one of a 

 comparatively low 

 resistance, the other 

 of a higher re- 

 sistance. Both types 

 possess a certain 

 amount of inertia; 

 that is, they do not 

 respond instantly to 

 a slight change of 

 illumination, and 

 this characteristic is 

 intensified with ex- 

 cessive or over-il- 

 lumination. 



It may be well to 

 make a little study 

 of the element sele- 

 nium before building 

 the cell, as the cell 

 must receive a care- 

 ful heat treatment 

 after assembling. 

 Selenium is a non-metal of the sulphur 

 group and is generally found associated 

 with it in nature. Like sulphur it exists 

 in several forms, the "A" selenium be- 

 ing that used for the cell. The "A" 

 selenium is a dark grayish-black, crys- 

 talline solid, possessing a metallic luster. 

 It is insoluble in carbon disulphide and 

 has a specific gravity of 4.8. It begins 

 to melt at 217 deg. C. and is a liquid at 

 250 deg. C. At higher temperatures in 

 the air it is oxidized. 



The "B" selenium, which is the most 

 stable form, is a dark reddish-brown, 



specific gravity 4.5, soluble in carbon 

 disulphide, from which it crystallizes in 

 prismatic crystals. It melts at 217 deg. C. 

 and boils at 700 deg. C. A third variety, 

 specific gravity 4.26, is found in two forms, 

 the one electro positive and insoluble in 

 carbon disulphide, the other electro- 

 negative and soluble in carbon disulphide. 

 It fuses at 100 deg. 

 C. and when sudden- 

 ly cooled becomes 

 vitreous. When 

 heated to 270 deg. 

 C. and suddenly 

 cooled to 180 deg. 

 C, at which tem- 

 perature it is kept 

 for several hours, it 

 is converted to the 

 "A " selenium. On 

 heating the "B" va- 

 riety to 150 deg. C. 

 it changes to the 

 "A" variety, with 

 the evolution of heat. 

 The chemical prop- 

 erties of selenium are 

 very similar to those 

 of sulphur. These 

 properties are given 

 here that the experi- 

 menter may better 

 understand the heat 

 treatment which the 

 completed cell must 

 receive in order to 

 render it sensitive to 

 light. 



The material needed for a fair-sized low 

 resistance type of cell is as follows: 



15 ft. strip brass, l /i in. wide by 1-16 in. thick 

 2 brass or iron bolts, 4 in. long by 3-16 in., with 

 2 washers and 2 nuts each 



Y2 oz. selenium, stick preferred 

 I piece mica, 8 by 12 in., very thin. 



Cut from the brass 50 strips, 3 in. long, 

 ^/2 in. wide by 1/16 in. thick, as shown in 

 Fig. 1, each with a 3/16 in. hole, % in. from 

 the end. Fifty brass washers, x /2 in. square, 

 with a 3/16-in. hole in the center, should 

 also be cut from the 1/16-in. brass, as 

 shown in Fig. 2. Fifty strips and washers 



Dimensions and 

 patterns of the 

 parts for making 

 a selenium cell for 

 laboratory use 



799 



