188 Mr. S. Bidwell on the Sensitiveness 



was due to the action of radiation as such, any change of 

 resistance resulting from the incidental rise of temperature 

 being quite inappreciable. 



Cell No. 2. — This was constructed in a somewhat different 

 manner. A piece of silver-foil was laid upon the surface of 

 the mica before the two wires were wound round it, and 

 instead of having prepared sulphur spread upon one face, the 

 whole was immersed in pure melted sulphur for a few minutes, 

 and then carefully drained. Before this treatment the silver 

 wires were of course short-circuited by the foil, but the liquid 

 sulphur penetrated between them, forming a film of sulphide; 

 and when cold, the resistance of the cell was about 100,000 

 ohms. Though this cell turned out to be somewhat less sen- 

 sitive than the other, it seemed likely that, on account of its 

 comparatively low resistance, it might be successfully used 

 for a photophonic experiment. It was therefore connected in 

 circuit with a battery of ten Leclanche cells and a telephone, 

 and exposed to a rapidly interrupted beam of light. The 

 telephone at once gave out a musical note, which was nearly 

 as loud as that produced by a good selenium cell under 

 similar circumstances. 



The behaviour of this cell under changes of temperature 

 was the same as that of the other. 



Cell No. 3. — A mixture, consisting of equal parts of sub- 

 limed sulphur and precipitated sulphide of silver, was melted 

 and spread on one surface of a slip of mica, around which two 

 silver wires had been wound as before. No foil was used in 

 this case. The resistance of this cell was diminished by radia- 

 tion, but increased in a very marked manner by rise of tempe- 

 rature. A paraffin lamp, at a distance of 18 inches, produced 

 a steady diminution of the resistance. When the lamp was 

 placed at a distance of 10 inches, the galvanometer-needle first 

 moved in a direction indicating a further fall of resistance ; 

 but after a few seconds, when the temperature began to rise, 

 it turned in the opposite direction. On moving the lamp 

 6 inches nearer, there was at once a large deflection in the 

 direction of increased resistance, the temperature-effect com- 

 pletely predominating over that of radiation. 



Cell No. 4. — A strip of silver-leaf was attached to a glass 

 plate by means of gold size, and the middle part of it was 

 exposed to the vapour of boiling sulphur until both surfaces 

 were completely blackened. The resistance of this cell was 

 high, but for a few days it was extraordinarily sensitive, a 

 reflected beam of sunlight instantly effecting a diminution of 

 80 per cent, in its resistance. About a fortnight after it was 

 made, its sensitiveness had greatly fallen off. 



