EDISON'S ACOUSTIC INVENTIONS. 



135 



behind eacli of these stands a siniihir plate as at P, held at its centre 

 by an adjusting screw. The outside row of plates is electrically con- 

 nected with each other and with the battery, which goes to the Hne. 



SiQ. 16. 



Fig. 20. 



Fio. 21. 



When the inside row of plates vibrates under the influence of a sound, 

 the distance between the plates varies and changes their static capacity. 

 In Fig. 21 the plates are arranged as in the ordinary form of a con- 

 denser. An initial pressure is put upon them by a screw passing 



