Electric Currents by Sulphur Cells. 329 



might possibly be found in the lead, iron, and arsenic which 

 are contained as impurities in ordinary commercial selenium. 

 Little direct evidence was offered in support of this view; 

 but it was shown that sulphur, when mixed with a certain 

 proportion of sulphide of silver and arranged in the form of a 

 "cell" with silver electrodes, exhibited many of the pro- 

 perties of crystalline selenium, especially that of having its 

 electrical resistance temporarily diminished under the influ- 

 ence of light. Analogy therefore tended to confirm the opinion 

 which I had been led to entertain. 



While observing the secondary or polarization-currents 

 which are generated by sulphur cells (as by those made with 

 selenium), after being disconnected from a battery, certain 

 effects were noticed which seemed to indicate that when the 

 electrodes consisted of two different metals, a sulphur cell 

 might be capable of originating and maintaining an indepen- 

 dent or primary current. Experiments were therefore made 

 w T ith the object of investigating this point ; and the present 

 paper contains an account of the results obtained. I have 

 hardly attempted to connect them together by any complete 

 theory: of some of them, indeed, I can offer no explanation 

 whatever ; others appear to be in direct opposition to what 

 might have been expected. But, so far as I have been able 

 to ascertain, they are entirely novel, and of sufficient interest 

 to be worthy of record. 



(1) A slip of mica was wound with two parallel wires of 

 silver and copper 1 millim. apart, and melted sulphur con- 

 taining a small quantity of precipitated sulphides of silver and 

 copper was spread over one surface. It is not known what 

 proportion of the sulphides was contained in the mixture, 

 because the bulk of them sank to the bottom of the crucible 

 in which the sulphur was melted. When cold, the cell was 

 connected with a reflecting galvanometer, and was found to 

 generate a small but steady current, indicated by a deflection 

 of about 20 scale-divisions. When the connections of the 

 cell were reversed, the current was reversed. It w T as therefore 

 not due to any thermo-effect in the circuit. The direction of 

 the current was from silver to copper through sulphur : its 

 strength was diminished by exposure to light, and increased 

 by rise of temperature. Connecting the free ends of the 

 silver and copper wires and heating the junction, it was found 

 that the thermo-current thus produced passed (as usual) from 

 copper to silver through the junction. The increase of cur- 

 rent by heat was therefore not to be accounted for by thermo- 

 electric action, for that would produce the opposite effect. 



