148 



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, 



ne.i 



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- 



