3 D 
THE TELEGRAPH. 
that a musical sound always accompanies the changes of 
magnetic forces in a bar of steel mounted in such a manner 
as to give out its acoustic vibrations. 
In 1861, M. Phil. Beiss, of Friedrichsdorf near Homburg, 
discovered that a vibrating diaphragm may be thrown into 
movement by the human voice in such a manner that the 
rhythm and pitch of the sounds can be reproduced at a 
distance by means of an electro-magnet. 
In 1874, Elisha Grey invented a method of electrical trans¬ 
mission, by which the intensity of the sounds, as well as their 
pitch and rhythm could be reproduced at a distance. He 
afterwards conceived the idea of controlling the formation of 
waves of electricity by means of the vibrations of a diaphragm 
capable of lending itself to all the modifications of the human 
voice. He thus solved the problem of the transmission and 
reproduction of articulate speech by an electric conductor. 
In 1876, Professor A. G. Bell invented his now well-known 
form of telephone, in which the transmission and reproduction 
of articulate speech is accomplished by means of superposed 
magneto-electric currents. In the same year, Dolbear con¬ 
ceived the idea of substituting permanent magnets for the 
electro-magnets and batteries previously employed, and of 
using the same apparatus for both sending and receiving 
instrument, instead of using two instruments of different 
constructions. 
In 1877, Edison applied to the telephone the discovery, 
made by himself a few years before, of the variation of the 
resistance of carbon and other inferior conductors when they 
are subjected to changes of pressure. By this means, he could 
not only vary the force of the battery current, in unison with 
the raising or lowering of the yocal sounds, but also obtain a 
louder and more distinct articulation. 
Since then, Gower, Pollard, Hughes, A. Breguet, Crossley, 
Paul Lacour, Preece, Blake, and many others have contributed 
to the development of this invention, which Sir William 
Thompson has called the wonder of wonders . 
Into this little instrument, scarcely larger than the mouth 
of an ordinary ear-trumpet, the interlocutor speaks to his 
correspondent by setting up vibrations in a plate of iron. 
This iron, acting on the electro-magnet, produces currents of 
