Page:Popular Science Monthly Volume 90.djvu/164

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��Popular Science Monthly

��Making Photo-Electric Cells to Work from a X»ight

THE generating of an electric current by the action of heat upon a point of contact of two dissimilar metals,

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��General principles of the thermo-electricity which is set up by difference in temperature

generally recognized by the term "ther- mo-electricity," is perhaps well known to those engaged in experiments in the field of electricity. The action and general construction of a thermo-electric or "heat" battery, is shown in Fig. i, in which the points designated by A are strips of copper connected in series at the contact points with the iron strips B, which are the alternate junctions, the latter being heated or placed in boiling water, while the former are kept cool by immersion in ice water. The arrows indicate the direction of the current, which increases in proportion to the number of pairs of junctions and the difference in temperature of the hot and cold contact points.

That light also possesses this mys- terious power of producing an electric current is known to but a few investiga- tors, and perhaps if more widely known would result in a practical research that would add much to our knowledge, and might lead to the solution of the problem as to the nature of electricity. This light method of producing electricity is from what is called photo-electric cells.

A simple photo-electric cell may be constructed by forming two cylinders of tin-foil of such size that one may be placed inside the other, the inner tin- foil being wrapped in blotting paper to shade it from the light. The two cylin- ders are then immersed in plain water and connected with a sensitive galvanom- eter consisting of a compass placed over a small coil of insulated wire, as shown at

��C, Fig, 2. Ordinary sunlight or the light from burning magnesium is allowed to fall on the outer cylinder D, a current of electricity being set up as shown by the deflection of the compass needle, and flowing from the shaded cylinder to the "light" cylinder.

An interesting and efficient cell of this type has been constructed by Saur and termed the "impulsion cell." As shown at Fig. 3, it consists of a solution of common salt, copper sulphate and water. A plate of silver sulphide, E, is immersed in this solution and also a porous pot, F, containing a little mercury in the bottom into which dips a sheet of platinum G. The glass vessel is then placed in an opaque box as in Fig. 4, which should be closed to exclude all light. On connecting the circuit through a galvanometer a weak current will be set up as shown by the needle.

When, however, one side of the box container is opened and light from the sun allowed to fall on the silver plate, there will be noticed a considerable in- crease of the current strength, which will decrease with the mere passing of a cloud over the face of the sun, thus demon-

���A simple photo-electric cell formed of two cylinders of tinfoil one inside of the other

strating the sensitiveness of the cell. This cell may be put to practical appli- cation as a "light-recorder" by connect- ing through a tracing point running on a strip of lined paper.

It may be of interest to note another "light" cell perfected by Riggollot who used copper oxide plates, i in. long by ^2 in. wide. The plates were dyed with malachite green which seemed to in-

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