246 



SCIENCE. 



UPON SUBSTITUTES FOR SELENIUM IN ELECTRICAL 

 RECEIVERS. 



At the time of my communication to the American 

 Association the loudest effects obtained were produced by 

 the use of selenium, arranged in a cell of suitable con- 

 struction, and placed in a galvanic circuit with a tele- 

 phone. Upon allowing an intermittent beam of sunlight 

 to fall upon the selenium a musical tone of great in- 

 tensity was produced from the telephone connected 

 with it. 



But the selenium was very inconstant in its action. It 

 was rarely, if ever, found to be the case, that two pieces 

 of selenium (even of the same stick) yielded the same 

 results under identical circumstances of annealing, etc. 

 While in Europe last autumn, Dr. Chichester Bell, of 

 University College, London, suggested to me that this 

 inconstancy of result might be due to chemical impurities 

 in the selenium used. Dr. Bell has since visited my labor- 

 atory in Washington, and has made a chemical examina- 

 tion of the various samples of selenium I had collected 

 from different parts of the world. As I understand it to 

 be his intention to publish the results of this analysis very 

 soon, I shall make no further mention of his investiga- 

 tion than to state that he has found sulphur, iron, lead, 

 and arsenic in the so-called " selenium," with traces of 

 organic matter ; that a quantitative examination has re- 

 vealed the fact that sulphur constitutes nearly one per 

 cent, of the whole mass ; and that when these impurities 

 are eliminated the selenium appears to be more constant 

 in its action and more sensitive to light. 



Prof. W. G. Adams* has- shown that tellurium, like 

 selenium, has its electrical resistance affected by light, and 

 we have attempted to utilize this substance in place of 



Fig. 7. 



selenium. The arrangement of cell (shown in Fig. 7) 

 was constructed for this purpose in the early part of 

 1880; but we failed at that time to obtain any indica- 

 tions of sensitiveness with a reflecting galvanometer. 

 We have since found, however, that when this tellurium 

 spiral is connected in circuit with a galvanic battery 

 and telephone, and exposed to the action of an inter- 

 mittent beam of sunlight, a distinct musical tone is 

 produced by the telephone. The audible effectis much 

 increased by placing the tellurium cell with the battery 

 in the primary circuit of an induction coil, and placing 

 the telephone in the secondary circuit. 



The enormously high resistance of selenium and the 

 extremely low resistance of tellurium suggested the thought 

 that an alloy of these two substances might possess inter- 

 mediate electrical properties. We have accordingly mixed 

 together selenium and tellurium in different proportions, 



♦Proc. Roy. Soc, vol. xxiv, p. 163. 



and while we do not feel warranted at the present time in 

 making definite statements concerning the results, I may 

 say that such alloys have proved to be sensitive to the ac- 

 tion of light. 



It occurred to Mr. Tainter before my return to Wash- 

 ington last January that the very great molecular disturb- 

 ance produced in lamp-black by the action of intermittent 

 sunlight should produce a corresponding disturbance in 

 an electric current passed through it, in which case lamp- 

 black could be employed in place of selenium in an elec- 

 trical receiver. This has turned out to be the case, and 

 the importance of the discovery is very great, especially 

 when we consider the expense of such rare substances as 

 selenium and tellurium. 



The form of lamp-black cell we have found most effec- 



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N Fig. 8. 



tive is shown in Fig. 8. Silver is deposited upon a plate 

 of glass, and a zigzag line is then scratched through the 

 film, as shown, dividing the silver surface into two por- 

 tions insulated from one another, having the form of two 

 combs with interlocking teeth. 



Each comb is attached to a screw-cup, so that the cell 

 can be placed in an electrical circuit when required. The 

 surface is then smoked until a good film of lamp-black is 

 obtained, filling the interstices between the teeth of the 

 silver combs. When the lamp-black cell is connected 

 with a telephone and galvanic batter)', and exposed to the 

 influence of an intermittent beam of sunlight, a loud mu- 

 sical tone is produced by the telephone. This result seems 

 to be due rather to the physical condition than to the na- 

 ture of the conducting material employed, as metals in a 

 spongy condition produce similar effects. For instance, 

 when an electrical current is passed through spongy plat- 

 inum while it is exposed to intermittent sunlight, a distinct 

 musical tone is produced by a telephone in the same cir- 

 cuit. In all such cases the effect is increased by the use 

 of an induction coil ; and the sensitive cells can be em- 

 ployed for the reproduction of articulate speech as well as 

 for the production of musical sounds. 



We have also found that loud sounds are produced from 

 lamp-black by passing through it an intermittent electri- 

 cal current ; and that it can be used as a telephonic re- 

 ceiver for the reproduction of articulate speech by electri- 

 cal means. 



A convenient mode of arranging a lamp-black cell for 



