Experiments with Selenium Cells. 255 



scale by a current in the opposite direction (i. e. from anode 

 to kathode internally), returning to its former position as soon 

 as the magnesium was extinguished. This was not a thermo- 

 electric effect, for the approach of a hot brass rod caused a 

 smaller current from kathode to anode, which subsided slowly 

 and not suddenly when the rod was withdrawn. 



Photoelectric currents of this character had been previously 

 observed by Prof. Adams and by Mr. Fritts of New York, 

 but no explanation of their origin has been attempted. 

 Perhaps the behaviour of my cell may suggest the direction 

 in which the source of such currents should be looked for, but 

 I have not yet followed up the inquiry. 



In the illustration of lectures it is often desirable to be able 

 to exhibit experimental effects in a striking and attractive 

 manner. The movements of a spot of light upon a scale may 

 be convincing, but they are hardly impressive. Yet the 

 resistance of sensitive selenium cells is so high that the varia- 

 tion, under the influence of light, of currents passing through 

 them cannot easily be demonstrated to an audience in any 

 other manner than by their action upon a reflecting-galva- 

 nometer. With sufficient battery-power they may, however, 

 be made to work a delicate relay, and thus to produce indi- 

 rectly a great variety of startling effects. For example, a 

 selenium cell may be connected with a relay and a battery in 

 such a manner that a bell is rung when the cell is in the dark 

 and ceases to ring when the cell is illuminated ; the bell 

 circuit remaining closed when the current through the 

 selenium circuit is below a certain strength, and being broken 

 by the relay when the selenium current is increased by the 

 action of light. It is possible that if selenium cells could be 

 made to retain their sensitiveness for a reasonable time, an 

 arrangement of this kind might receive a practical applica- 

 tion. It might be used to give notice of the accidental ex- 

 tinction of railway signal-lamps or of ships'' lights ; and if the 

 connexions were so mude that light and not darkness set the 

 bell in action, it might be employed for the protection of safes 

 and strong-rooms, the mere light of a burglar's lantern being- 

 sufficient to give an alarm. 



Again, if the relay is connected with an electric lamp 

 instead of a bell, it constitutes a contrivance by means of 

 which a light is provided automatically when required, and 

 extinguished when no longer wanted. It gives us a lamp 

 which will light itself in the dark and put itself out in the 

 light. The relay can be adjusted so that the lamp may be 

 turned out when the external illumination reaches any desired 



