Prof. G. Forbes gives the following description: The instrument which
I call Breguét'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.
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
Breguét, 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.
At the top I always got sound, but at the side I got no sound, although
the mercury was shaking. I then tried to see how feeble a current was
audible in the telephone. An assistant tapped the tube while I stood out
of the way, and where I could not see. I got him to tap it gentler and
gentler, and could hear the most feeble tap. A pellet of paper was next
dropped from various heights down to an inch, and each tap was perfectly
audible in the telephone. I tried many methods, and one, purely
accidentally chosen, was a piece of glass tube which I had drawn out
into a tube about 2 mm. diameter, and then nearly closed the end of
it. I have that tube here, and you will see what an ill-shapen and
ugly-looking tube it is, but it is one of the best tubes I ever got; and
finally, I found that small bits of thermometer tube, which were simply
closed at their ends with a blow-pipe, gave very good results, and I was
able to make them useful for various purposes. I then tried mounting a
tube on the end of a speaking-trumpet and speaking to the mercury, but
got no effect. In every place where I attached the glass tube itself
to a sounding-board I got a very accurate reproduction. I put one on a
piece of ferrotype plate, and that gave really the best result I ever
got. The tube was fastened with sealing-wax, and with it I got excellent
speech heard in a Bell receiver. I tried putting in a large number of
these tubes, all in quantity, on the bottom of a ferrotype plate, but
with no advantage. I have not yet tried putting them in series, one
behind the other, so as to increase the electro-motive force, but I
think that probably would be an improvement; of course it would require
many vessels of acidulated water to dip into. The most distinct
articulate speech was obtained from an ordinary ferrotype telephone
plate, secured at the edges, and one of the glass tubes you see here
attached to it.
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