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XXXVIII. Notes on the Construction of the Photophone. 

 By Professor Silvanus P. Thompson*. 



(1) IN the selenium photophone, light of varying intensity 

 J- is received upon a prepared surface of sensitive crys- 

 talline selenium, the electric resistance of which it thereby 

 changes. In the construction of the receiving " cell " it is 

 obvious that certain relations must hold between the dimen- 

 sions of the sensitive surface and the degree in which a given 

 quantity of light will change the electric resistance — relations 

 which ought to be observed in the construction of the instru- 

 ment, and which are certainly worthy of investigation. 



Professor Bell's typical selenium-cell consists of a small 

 cylinder about 2 inches in diameter and 2 J inches in length 

 (giving at most a superficial area of 15*8 square inches avail- 

 able) , built up of alternate disks of brass and mica, filled be- 

 tween the edges of the brass disks with selenium, and having 

 alternate brass disks connected up in multiple arc. This cell, 

 in his usual apparatus, is placed at the bottom of a parabolic 

 mirror. 



Certain experimental observations made in attempting to 

 repeat Prof. Pelfs experiments led the writer to query 

 whether this arrangement was the best possible one, and sug- 

 gested an investigation, of which the following paragraphs are 

 the chief points. 



(2) Theorem I. — With a given maximum of incident light 

 distributed uniformly over the surface, the change of electric re- 

 sistance in a selenium-cell will vary proportionally ivith its linear 

 dimensions, provided its parts be arranged so that on whatever 

 sccde constructed the normal resistance shall remain the same. 



Suppose there to be a cell of a certain size, having a certain 

 normal resistance (i. e. a certain resistance in the dark as 

 measured under a standard electromotive force), and presenting 

 a certain area of surface ; then, if a perfectly similar cell be 

 made on a scale n times as great (in linear measure each way), 

 the same total amount of light falling upon its surface will 

 produce n times as great a variation in the electric resistance. 



The proof of this theorem depends upon the law discovered 

 by Professor W. G. Adams f — namely, that the change in the 

 resistance of selenium is directly as the square root of the illu- 

 minating-power '. 



* Communicated by the Physical Society, haying- been read at the 

 Meeting on January 22. 



f Proc. Roy. Soc. vol. xxv. 1876, p. 113. 



