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BREGUET'S TELEPHONE.


Prof. G. Forbes gives the following description: The instrument which
I call Breguet's telephone is founded upon the instrument which was
described by Lipmann, called the capillary electrometer. The phenomenon
may be shown in a variety of ways. One of the easiest methods to show it
is by taking a long glass tube and bending it into two glasses of dilute
acid, and, the tube being filled with acid itself, a piece of mercury
is placed in the center of the tube. Then if one pole of a battery is
connected with one vessel of acid, and the other pole of the battery is
connected with the other vessel of acid, at the moment of connection the
bit of mercury will be seen to travel to the right or left, according to
the direction of the current. M. Lipmann explained the action by showing
that the electro-motive force which is generated tends to alter the
convexity of the surface of the mercury. The surface of the mercury,
looked at from one side, has a convex form, which is altered by the
electro-motive force set up when connection is made with the battery.
The equilibrium of the mercury is dependent upon the convexity, and
consequently when the convexity is disturbed the mercury moves to one
side or the other. Lipmann also showed that if a tube containing a bit
of mercury, and tapering to a point, is taken and dipped into acid, and
then the tube filled with acid, on one pole of a battery being dipped
into the tube and another into the acid the mercury will move up or
down, showing similar action to that which I have just described.