I 88 SCIENTIFIC RECREATIONS. 



with its own little trumpet whatever was sung into it ; for all the waves 

 were produced by an electric current of a certain and uniform strength, and 

 therefore the sound waves were of the same size. 



But in 1874, Mr. Elisha Gray, of Chicago, improved Reiss' instrument, 

 and discovered a method by which the intensity or loudness of tones, as well 

 as their " pitch," were transmitted and reproduced. In this method he 

 employed electrical vibrations of varying strength and rapidity, and so was 

 enabled to reproduce a tune. Subsequently he conceived the notion of 

 controlling the vibrations by means of a diaphragm, which responded to every 

 known sound, and by this he managed to transmit speech in an articulate 

 manner. 



In 1876, Professor Graham Bell sent a Telephone to the Centennial 

 Exhibition at Philadelphia. Mr. Bell, according to the report, managed to 

 produce a variation of strength of current in exact proportion to the particle 



EARTH J[ 

 Fig. 191. Bell's Tp^phone (Receiver). 



of air moved by the sound. A piece of iron attached to a membrane, and 

 moved to and fro in proximity to an electro magnet, proved successful. The 

 battery and wire of the electro magnet are in circuit with the telegraph wire, 

 and the wire of another electro magnet at the receiving station. This 

 second magnet has a solid bar of iron for core, which is connected at one 

 end, by a thick disc of iron, to an iron tube surrounding the coil and bar. 

 The free circular end of the tube constitutes one pole of the electro magnet, 

 and the adjacent free end of the bar core the other. A thin circular iron 

 disc held pressed against the end of the tube by the electro-magnetic attrac- 

 tion, and free to vibrate through a very small space without touching the 

 central pole, constitutes the sounder by which the electric effect is reconverted 

 into sound. The accompanying illustrations (figs. 190, 191) show Mr. Bell's 

 Telephone as described. 



The Telephone, subsequently simplified by Professor Bell, is shown in 

 the two following illustrations (figs. 192, 193). The voice strikes against the 



