EDISON'S ACOUSTIC INVENTIONS. 



133 



Fig. 12 illustrates a microphone having ten plates of silk, a mixture 

 of dextrine and lampblack having been previously worked into the 

 pores. 



In Fig. 13 fifty disks, D, of protoxide of iron, are shown inclosed in 

 a glass tube. 



A novel form of transmitter, used by Mr. Edison in his experiments, 

 is shown in Fig. 14. The semi-conductor is a collection of small frag- 



r/ '//V>////,//^ 



Fig. 13. 



Fig. 13. 



Fig. 14. 



ments of cork, covered with plumbago. It can be used with or without 

 a diaphragm. 



The instrument shown in Fig. 15 acts both as a transmitter and 

 receiver. The solid carbon of the transmitter is here replaced by silk 

 fibres coated with graphite. Its action as a receiver is probably due to 

 the attraction of parallel currents ; the volume of the whole being con- 

 tracted during the passage of a current through F. 



In the accounts which have been published of experiments with 

 the microphone, the statement has frequently been made. that minute 

 sounds are actually magnified by it, in the same sense that minute ob- 

 jects are magnified by the microscope. A little reflection will show, 

 however, that there is no real analogy in the action of the two instru- 

 ments. The sound that is heard in the receiving-instrument of the 

 microphone, when a fly is walking across the board on which the trans- 

 mitter is placed, is not the sound of the fly's footsteps, any more than 

 the stroke of a powerful electric bell, or sounder, is the magnified sound 

 of the operator's fingers tapping lightly, and it may be inaudibly, upon 

 the key. This view of the subject readily explains why the microiDhone 

 has failed to realize the expectations of many persons, who, upon its 

 first exhibition, enthusiastically announced that by its aid we should 

 be able to hear many sounds in Nature which had hitherto remained 

 wholly inaudible. 



Short-Circijitixg Telephones.— a number of the telephones in- 

 vented by Mr. Edison may be classed together as short-circuiting or 



