Lipmann further showed the reverse effect, that if a piece of mercury be
forcibly pressed, so as to alter the convexity of its surface, such
as by bringing it into a narrower part of the tube, then there is an
electro-motive force produced.

It occurred to me, and no doubt it did to Breguet also, that if we speak
either against the surface of the glass tube, and caused the tube to
vibrate, or if the mercury were caused to vibrate in the manner I have
shown, we ought to be able to introduce a varying current in the wires
which might have sufficient electro-motive force to produce audible
speech in a Bell telephone. Further, the same instrument, since varying
electro-motive force affected the drop of mercury and produced varying
displacement, ought also to act as a receiving instrument, and should
vibrate in accordance with the currents that arrive. My experiments
have only been in the way of using the instrument as a transmitter; but
Breguet, I find, used it as a receiver as well as a transmitter, though
I am not aware that M. Breguet made any actual experiments so as to
produce articulate speech. I presume that this was done, although I have
not come across any description of the experiments, and it was for that
reason that I thought possibly some account of my own experiments might
be interesting to the members of the Society. The first tubes that I
used were bits of glass tube about a centimeter diameter, and simply
drawn out to a tapering point. I have the tubes here. The first
experiment I tried was by tapping the glass tube so as to mechanically
shift the position of the mercury, and by listening on the telephone for
the effect. For a long time, at least an hour, I could get no effect at
all. At last I got a sound, but could not understand how it was that at
one time of tapping I could not hear, while at another time it was quite
loud.