8x6 THE POPULAR SCIENCE MONTHLY. 



light of the moon, and Werner Siemens discovered that, in certain ex- 

 tremely sensitive varieties of selenium, heat and light produced oppo- 

 site effects. In Siemens's experiments special arrangements were made 

 for the purpose of reducing the resistance of the selenium employed. 

 Two fine platinum wires were coiled together in the shape of a double 

 flat spiral in the zigzag shape, and were laid upon a plate of mica so that 

 the disks did not touch one another. A drop of melted selenium was 

 then placed upon the platinum-wire arrangement, and a second sheet 

 of mica was pressed upon the selenium, so as to cause it to spread out 

 and fill the spaces between the wires. Each cell was about the size of 

 a silver dime. The selenium-cells were then placed in a paraffine bath, 

 and exposed for some' hours to a temperature of 210 Cent., after 

 which they were allowed to cool with extreme slowness. The results 

 obtained with these cells were very extraordinary ; in some cases the 

 resistance of the cells when exposed to light was only one fifteenth of 

 their resistance in the dark. 



Without dwelling further upon the researches of others, I may say 

 that the chief information concerning the effect of light upon the con- 

 ductivity of selenium will be found under the names of Willoughby 

 Smith, Lieutenant Sale, Draper and Moss, Professor W. G. Adams, 

 Lord Rosse, Day, Sabini, Dr. Werner Siemens, and Dr. C. W. Siemens. 

 All observations by these various authors had been made by means of 

 galvanometers ; but it occurred to me that the telephone, from its ex- 

 treme sensitiveness to electrical influences, might be substituted with 

 advantage. Upon consideration of the subject, however, I saw that 

 the experiments could not be conducted in the ordinary way, for the 

 following reason : The law of audibility of the telephone is precisely 

 analogous to the law of electric induction. No effect is produced dur- 

 ing the passage of a continuous and steady current. It is only at the 

 moment of change from a stronger to a weaker state, or vice versa, 

 that any audible effect is produced, and the amount of effect is exactly 

 proportional to the amount of variation in the current. It was, there- 

 fore, evident that the telephone could only respond to the effect pro- 

 duced in selenium at the moment of change from light to darkness, or 

 vice versa ; and that it would be advisable to intermit the light with 

 great rapidity, so as to produce a succession of changes in the con- 

 ductivity of the selenium, corresponding in frequency to musical vibra- 

 tions within the limits of the sense of hearing. For I had often noticed 

 that currents of electricity, so feeble as to produce searcely any audible 

 effects from a telephone when the circuit was simply opened or closed, 

 caused very perceptible musical sounds when the circuit was rapidly, 

 interrupted, and that the .higher the pitch of sound the more audible 

 was the effect. I was much struck by the idea of producing sound by 

 the action of light in this way. Upon further consideration it appeared 

 to me that all the audible effects obtained from varieties of electricity 

 could also be produced by variations of light acting upon selenium. I 



