216 Dr. J. Moser on the Microphonic 



This experiment shoivs that between the copper and the 

 selenium , or rather the cuprous selenide, there is only a slight 

 and imperfect contact. 



But the same is the case in the photophonic cell. In 

 this also the three layers — copper, cuprous selenide, sele- 

 nium — follow one after the other. The plates give only an 

 enlarged view of what is to be observed in the photophonic 

 cell on a smaller scale, and with more difficulty, by the eye. 

 Thus we see that in the photophonic selenium cell also we 

 have only a similar slight and imperfect contact, which is to 

 be influenced by radiation. It was first by this experiment 

 that it became obvious to me that a microphonic effect is the 

 essential part of the action of the selenium cells. 

 yt^ I was confirmed in this conclusion, that the selenium photo- 

 phone is a microphone, when I learned that Mr. Sumner Tainter 

 had constructed a photophone in which selenium was replaced 

 by carbon. Indeed his apparatus, the zigzag line filled with 

 carbon on the silver-coated glass plate, is nothing else than a 

 microphone. If we remember the apparatus described by Mr. 

 Hughes as a thermoscope, we understand that the selenium 

 photophone of Messrs. Graham Bell and Sumner Tainter 

 agrees in its principle with the carbon photophone of the 

 latter; and, again, this is in its main features the same as the 

 thermoscope. ^ % 



Thus my attention became now more directed to those 

 passages in the literature where the degree of the resistance 

 at the surfaces of contact between the selenium and the metal, 

 in comparison with the total resistance, is discussed. 



In 1875, before the invention of the microphone, Dr. 

 Werner Siemens observed a high resistance at the surfaces 

 of contact. He arrived at the conclusion " that an essential 

 part of the resistance of the selenium is in its limiting layers 

 at the surfaces of contact " *. 



Mr. Sabine is of the same view. Finding, e. g., in one 

 piece of selenium with several transversal platinum wires the 

 resistances of the junctions to be 429, 479, 498, and 428 

 megohms, the resistance of the selenium itself between the 

 wires, however, much smaller (22, 13, and megohms), he 

 remarks : — " It is clear from these measurements that a large 

 portion of the observed resistance of a so-called selenium 

 resistance may, and frequently does, reside in the junctions, 

 and not in the selenium. Therefore the larger we make the 

 surface of contact between the platinum and the selenium, 

 the less likely are we to find an otherwise sensitive piece of 

 selenium rendered comparatively insensitive by the introduc- 

 * Pogg. Ann. clix. p. 140, 



