410 Mr. R. Sabine on some Electrical Experiments 



be opened to admit light to the uncovered face of the selenium. 

 A pole of platinum foil was placed in the tube, and distilled 

 water sufficient to nearly cover the selenium plate. This 

 selenium (galvanic) element, when in the dark, gave an elec- 

 tromotive force of 0*112 volt, the selenium being positive to 

 the platinum. On admitting diffused daylight, the direction 

 of the current was changed, the selenium becoming negative 

 to the platinum, with an electromotive force of 0*056 volt; so 

 that by the admission of diffused daylight the selenium sur- 

 face had become very much less electropositive than it was in 

 the dark. 



Two similar plates of selenium were prepared and placed 

 side by side in a suitable cell, which was enclosed in a light- 

 tight case with two shutters, by means of wmich light could be 

 admitted to one or the other of the plates at pleasure. Distilled 

 water being poured into the bottom of the cell, so as to reach 

 about three fourths up each plate, the electromotive force 

 between them was measured with an accumulator, discharge- 

 key, and galvanometer. 



Both plates in the dark gave a very slight current. Then 

 light was admitted by one of the shutters being opened. The 

 plate on which the light fell instantly became electronegative. 

 The consecutive readings were : — 



-0-05 volt 

 -0-04 „ 

 -0-03 „ 



Then that shutter was closed and, after a few minutes, the 

 other opened. The readings were now : — 



+ 0-09 volt 

 + 0-08 „ 

 + 0-07 „ 



On connecting a galvanometer direct between the poles and 

 observing the deflection, it was found that the current imme- 

 diately following each reversal was higher, and that it subsided 

 to a lower reading in a short time. This is probably due to 

 polarization of the plates, and is exactly what might be ex- 

 pected. 



Action of Light and Heat the same. — The next experiment 

 was to ascertain if the effect of light upon the surface is the 

 same or the reverse of that of heat. Two short lengths of 

 glass tube, a a and a! a! ', fig. 4, were stopped at the lower 

 ends with corks, b and] b f , through which strips of platinum 

 foil were passed. Across the tops of the vertical tubes 



