connected with the Telephone. 291 



Let the disk d, fig. 5, be impressed forwards by a sonorous 

 vibration, it will generate in the coil c a positive current, 



Fig. 5. 



2' 



which, flowing through the line, will pass through the coil c' 

 and attract the disk d'. Now the effect of induction is to re- 

 tard or prolong the effect of the positive current 1 ; but the 

 motion of the disk d ! has itself produced a current in the re- 

 verse direction to the first current ; and this neutralizes the 

 prolongation due to induction, and so helps to clear the line 

 for the next signal, which passes through precisely the same 

 process ; and hence the vibrations of the second disk tend to 

 produce currents which diminish materially the effects of induc- 

 tion, and so render possible conversation to distances that far 

 exceed anticipation. 



The extreme delicacy of Bell's apparatus has been shown in 

 various ways ; for instance : — 



Extract from a letter from T. A. Edison, dated November 

 25, 1877. 



" I made a pair of telephones that work with copper dia- 

 phragms : it is on the revolving-copper-disk principle of 

 Arago. 



" I find that a copper diaphragm may replace the iron in 

 Bell's. Copper must be -^ in. thick. It is very low with copper 

 in both ; but if the receiver is one of the regular kind, and the 

 transmitter is a copper diaphragm, you can carry on conver- 

 sation with ease both ways ; but with the pair I have made 

 the talking is loud, as I have several dodges on it." 



I repeated these experiments ; but the effect was so feeble as 

 to be scarcely distinguishable, and, although interesting from 

 a scientific point of view, it was of no practical value. 



Mr. James Blyth has independently repeated the experi- 

 ment, and has shown that wood, paper, and india-rubber pro- 

 duce similar effects. These effects are probably due to the 

 fact that diamagnetic bodies have a similar though feebler 



U2 



