Page:Popular Science Monthly Volume 91.djvu/815

 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

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