624 SCIENTIFIC RECORD FOR 1885. 



Helmholtz. {Wied. Ann., xxi, 209; J. Phys., December 1885, TI, rvr, 

 578.) 



Bidwell Las constructed a voltaic cell having a solid electrolyte. He 

 exhibited to the London Physical Society a cell consisting of plates of sil- 

 ver and copper, between which was contained a mixture of one part of cop- 

 I)er sulphide and five of sulphur. The electro motive force was 0-07 volt 

 and the internal resistance 6,537 ohms, and the current readily deflected 

 the needle of a reflecting galvanometer so as to throw the light off the 

 scale. A second cell was made with a copper plate on which copper sul- 

 phide had been pressed and a silver plate resting on silver sulphide on the 

 surface of the copper suljihide. The silver plate was brushed over with a 

 dilute solution of sulphur in carbon disnlphide and then heated till the 

 free sulphur was driven off. On putting the cell together it produced a 

 current of 6,800 micro-amperes through an external resistanceof 0-2ohni. 

 The copper and silver plates were each 2h by 2 inches, and the thickness of 

 the two layers of sulphide 0.05 inch. The electro-motive force of the cell 

 is 0-053 volt, and its internal resistance is therefore about 7 ohms. 

 {Nature, August, 1885, xxxil, 345 ; PJiil. Mag., October, 1885, V, xx, 

 328.) 



The interesting subject of the behavior of selenium to light as regards 

 its electric resistance has been examined by several investigators. AVer 

 ner Siemens has described the electro-motive action of certain selenium 

 cells sent to him by Fritts, who first observed in them the fact that, 

 when ])laced in circuit with a galvanometer, an electric current flowing 

 from the gold leaf to the base plate was generated by the action of 

 light. Siemens confirms this observation and regards it as of the great- 

 est scientific imi)ortance. Since obscure thermal rays are without effect, 

 the action cannot be a thermo-electric one. Moreover, the electromotive 

 force developed is proportional to the intensity of the light. He con- 

 cludes, therefore, that " here we meet for the first time with an instance 

 of the direct conversion of the energy of light into electrical energy." 

 {Ber. Al: Berl., February, 1885; Phil. Mag., April, 1885, V, xix, 315; 

 Am. '). Sci., June, 1885, III, xxix, 495.) 



Bidwell has investigated very carefully the phenomena of selenium 

 cells and is disposed to regard the change of resistance by the action 

 of light as a phenomenon of electrolysis. Since the selenium in the 

 cells has always undergone a prolonged heating in contact with the me- 

 tallic terminals, selenides of these metals may exist within the selenium, 

 forming a kind of network, thus affording the conducting material 

 throughout the mass. A cell constructed of silver wires and sulphur 

 containing some silver sulphide was sensitive, its resistance being re- 

 duced to one-third by burning a jMece of magnesium wire near it. In 

 the electrolysis of silver 8uli)hide, however, sulphur itself would be de- 

 posited on the metallic plate, and the resistance would be enormously 

 increased, unless under the action of light this sulphur united with the 

 silver. Moreover, he finds the specific resistance of seleniuja tp be 2,500 



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