3§o 
Recent Advances in Telegraphy . [J uly 9 
surrounding the two poles of a powerful permanent magnet. 
These coils are connected up together, and to the terminals 
1 1, cn the mahogany stand s. Immediately in front of the 
poles of a magnet is a membrane stretched on a ring, R. 
This membrane carries an oblong piece of soft iron cemented 
to it just opposite the poles of the magnet. A suitable 
acoustic cavity, or mouthpiece to speak into, o, fitted with 
three screws for tightening up the membrane, complete the 
apparatus. The sender speaks into the mouthpiece in an 
elevated voice, and the membrane, vibrating in unison, 
carries the piece of soft iron — which is really a movable 
armature — to and from ’the poles of the magnet. This has 
the effect of inducing a magneto-electric current in the coils 
of wire, M M, which are connected up to the line. The 
strength of this induced current varies continuously, “ as 
nearly as may be, in simple proportion to the velocity of a 
particle of air engaged in constituting the sound.” It 
travels along the line, and passes through the receiver at 
the distant station, evoking there the sounds which gave 
a 
rise to it. The receiver is even more simple than the sender, 
and its action is the reverse. It consists of a tubular electro- 
magnet, m, encased in iron to concentrate its power as much 
as possible on the circular disk armature, a, which is so fixed 
as to be free to vibrate over its cavity. It is connected up 
in circuit with the line by the terminals shown. The in- 
duced current coming from the sending station passes through 
the coil of this electro-magnet, and sets the thin disk arma- 
ture into sonorous vibrations, which are distinctly and clearly 
heard as a reproduction of the original sounds. 
In his address to the physical section of the British Asso- 
ciation at Glasgow last year, Sir William Thomson thus 
