﻿Measurement of Electromotive Force. 



5G1 



1%. 2. 



Lord Rayleigh (/. c. pp. 294-5). With the key given the 

 sound of each tap is wheezing, so that the overtones come 

 into play. The enormous advantages of notes of high pitch 

 over those of low pitch for purposes like the present has been 

 shown both by Ferraris and, more remarkably, by Lord 

 Rayleigh. 



In addition to the Bell telephone I made use of the duplex 

 form* shown in fig. 2. Here A A is an ordinary horse-shoe 

 magnet provided with two similar 

 U-shaped armatures of soft iron 

 B, B'. These carry the telephone 

 coils a, a', the terminals of which 

 lead to four clamp-screws (not 

 shown), so that a current may be 

 passed through the coils either in 

 series, or in parallel, or differen- 

 tially. Between the coils the dis- 

 coid hollow sound-chamber C C, 

 the walls of which are thin ('016 

 centim.) circular plates of iron, is 

 secured by clamping the wide tube 

 D, which conveys the sound to the 

 ears, to the flat-topped upright G. 

 A side view of the sound-chamber 

 with its efflux-tube is shown de- 

 tached at CD'. The whole is 

 soldered firmly together at the 

 edges, these being spaced by a 

 ring of copper wire. 



The armatures B, W are adjust- 

 able, and may be moved as close 



to the disk C C as desirable (set screws not shown). The 

 plates of C vibrate symmetrically, changing the form from 

 biconvex to biconcave, and I obtained the best results with 

 chambers 4 centim. in diameter and about 0*2 centim. deep. 

 When the chamber is more shallow the magnetic field is 

 strengthened, but the sound probably encounters too much 

 resistance in passing out into the air. 



In view of the large resistances R and r (fig. 1), the coils 

 of the telephone were made of 0*01 centim. wire, wound so 

 that as many as 3000 turns could be brought to bear on each 

 armature, with resistances as high as 300 ohms (see tabic 

 below). The advantage of the double form is to be found in 

 the ease with which the coils may be replaced by others, in 



Duplex telephones were first introduced by Elisha Gray, 



