_Electro Tuning Forks and their Uses._--On a former occasion I described
an instrument to which, in 1873, I gave the name _Electro-Tuning Fork_,
and which is nothing else than a tuning fork whose motion is kept up
electrically in such a way as to last indefinitely, provided that the
elements of the pile are renewed gradually, and that from time to time
the metallic contact is changed, which causes, at every oscillation, the
current to pass from the pile into the magnet, which keeps up the
vibration.
We reproduce herewith, in Fig. 1, a cut showing in projection one of the
simplest forms of the apparatus.
[Illustration: FIG. 1.--CONSTANT VIBRATOR.]
If we imagine the platinum or steel style, s, of the figure to be done
away with, as well as the platinized plate, I, and its communication
with the negative pole of the pile, P, we shall have the ordinary
instrument kept in operation electrically by the aid of the
electro-magnet, E, the style, s, the interrupting plate, I, and the
pile.
If we preserve the parts above mentioned, the instrument will possess
the property of having vibrations of a constant amplitude if sufficient
energy be kept up in the pile. In fact, when the amplitude is
sufficiently great to cause the style, s, to touch the plate, I, it
will be seen that at such a moment the current no longer passes through
the electromagnet, and the vibration is no longer maintained. The
amplitude cannot exceed an extent which shall permit the style, s, to
touch I.
Under such conditions, the duration of the vibrations remains exactly
constant, as does also the vibratory intensity of the entire instrument.
The measurement of time, then, by an instrument of this kind is, indeed,
as perfect as it could well be.
This complication in the arrangement of the apparatus has no importance
as regards those tuning forks the number of whose vibrations exceeds a
hundred per second, for in such a case these are given an amplitude of a
few millimeters only; but it would be of importance with regard to
instruments whose number of vibrations is very small, and to which it
might be desirable to give great amplitude; for then, as I have long ago
shown, the duration of the oscillation would depend a little on the
amplitude, but a very little, it is true.
I shall not refer now to the applications of these instruments in
chronography, but will rather point out first the applications in which
they are destined to produce an effective power.
For this purpose it is necessary to make them pretty massive. The number
of the vibrations depends upon such massiveness, and it is necessity to
know the relation which exists between these two quantities in order to
be able to construct an instrument under determinate conditions. I made
in former years such a research with regard to tuning forks of prismatic
form, that is to say, of a constant rectangular section continuing even
into the bent portion where the parallel branches are united by a
semicylinder, at the middle of which is the wrought iron rod as well as
the branches. The _thickness_ of the instrument is the dimension
parallel to the vibrations; its _width_ is the dimension which is
perpendicular to them, and its _length_ is reckoned from the extremity
of the branches up to the middle of the curved portion.
It is found that the number of vibrations is independent of the width,
proportional to the thickness, and very nearly inverse ratio of the
square of the length, provided the latter exceeds ten centimeters.
If we represent the length by l, the thickness by e, and the number
of vibrations by n, we shall have the following formula:
n = k x ( e / l squared )
in which k is a constant quantity whose value depends upon the nature
of the metal of which the tuning fork is made.
This constant varies very little from steel to malleable cast iron, and
it may be taken as equal to 818270.
Thus, then, we have a means of constructing a tuning fork in which two
of the three quantities, n, e, l, are given in advance. Experience
proves that no errors are committed exceeding one or two per cent.
It is seen from this that there is a means of increasing the mass of the
instrument without changing anything in the thickness, the length or,
consequently, the number of vibrations, and this is by increasing the
_breadth_.
It is in this way that I have succeeded in having long massive tuning
forks made of malleable iron, giving no more than 12 to 15 vibrations
per second, and vibrating with perfect regularity. Fig. 2, annexed,
shows one of these instruments of about 55 centimeters length, whose
breadth, E, is from 5 to 6 centimeters, and which makes about fifteen
double vibrations per second only.
[Illustration: FIG. 2.--THE ELECTRICAL TUNING FORK.]
This number might be still further reduced, but at the expense of our
being led to exaggerate the longitudinal dimensions of the apparatus in
such a way as to make it inconvenient. The object may be attained more
simply by loading the branches with slides supporting leaden weights, M,
of 500 grammes each. By fixing these slides at different points on the
branches, the number of vibrations can be made to vary from simple to
double, and even triple. Thus, by fixing them at the extremity of the
branches the number of the vibrations is reduced to 5 or 6.
There will be seen in the figure the electro-magnet which keeps up the
vibration. This is formed of three simple electro-magnets, whose bobbins
have a resistance of no more than 10 ohms, and which are united in
series. The interrupting plate, P, against which the style, s, rests
at each vibration, is capable of a forward movement, or one of recoil,
by the aid of a screw, V, and of an eccentric movement which is produced
by a small handle, m, and during which its plane remains invariable.
This arrangement permits the point of contact of the style and plate to
be varied without changing the precision with which the contact takes
place, and all the points of the plate to be slowly used in succession
before replacing it. The motion is produced by means of a relatively
weak pile, whose poles are connected to the terminals, A and A'. Three
Callaud elements of triple surface, renewed one after the other every
month at the most, are sufficient to keep up the vibrations
continuously, day and night, without interruption, and that too even
when the instrument is employed in producing a small mechanical power,
as we shall see further on.
We have now seen how electro-tuning forks may be constructed of large
dimensions, of large mass, and giving a small number of vibrations per
second.
Such instruments are well fitted to perform the role of electrical
interrupters, and it was in such a character that one of them figured in
the Exhibition of the Upper School of Telegraphy as a type of an
interrupter for testing piles.
When it is desired to test a pile to ascertain the practicability of
employing it in telegraphy, it is necessary to make it perform a work
which shall be as nearly as possible identical with that which it will
be called on to do, until it is used up, to estimate the duration of
such work, to measure regularly the constants of the pile, the
electro-motive power, and the internal resistance. Usually, in
telegraphy, this work consists in sending over a line of a certain
resistance intermittent currents, through the intermedium of suitable
manipulators. It suffices then to cause the branches of the electro
tuning fork to play the role of one of these manipulators. For doing
this the tuning fork carries two insulating ebonite or ivory strips, B B
(Fig. 3), which, at every oscillation, abut against vertical brass
springs, r. Each of these latter is located in front of the platinized
point of a screw, v, which is affixed to a small metallic tongue. The
springs and tongues are insulated from each other, and are mounted on a
piece which may be moved by a screw, V, so as to cause the springs of
the strips, B B', to approach or recede according to the amplitude of
the instrument's vibrations. Each spring and tongue is connected with
terminals affixed to the base of the apparatus. One of the poles of one
element, P, of the pile is connected with the tongue and corresponding
screw, while the other pole is connected with the screw in front of it
through the intermedium of a galvanometer, g squared, which gives the
intensity of the intermittent current, and of a resistance coil,
b squared, which performs the role of an artificial telegraph line. The
apparatus being set in operation, it will be seen that the current from
the pile is emitted once at every vibration.
Thus there may be exhausted as many pile elements as there are springs,
and that, too, simultaneously; and the contacts of the screws and
springs can be regulated in such a way that the duration of the
emissions shall be the same for all.
At the laboratory of the School of Telegraphy one of these instruments
has operated without interruption, day and night, during eighteen
months.
[Illustration: FIG. 4.--VERY RAPID ELECTRIC TUNING FORK]
The apparatus shown in Fig. 4 is also an interrupting electro-tuning
fork, but it makes a much greater number of vibrations than the
preceding, and may serve for other electric tests.
The operation of the tuning fork is kept up electrically by the aid of
the screw, v, and the corresponding plate; of the style, s, and of
the fine wire spiral spring, f, both insulated from the fork, from the
electro-magnet, N, and from the two wires, F F', which communicate with
a pile.
The interrupting system is symmetrical with the first. It consists of
the style, s, of the spiral spring, f, of the screw, v, and of the
plate that this carries at its extremity. The terminal, B, which carries
the spring, f, and the rod which carries the screw being insulated
from each other, it is only necessary to cause to terminate therein the
extremities of a circuit comprising one pile, in order to produce in the
circuit a number of interruptions equal to that of the tuning fork's
vibrations. Provided the lengths of the springs, f and _f'_, are
proper, such vibrations will not be altered.
Moreover, the instrument is so arranged as to produce vibrations whose
_duration can be varied at pleasure and kept constant_ during the whole
time the experiments last. This is done by modifying the _amplitude_ of
the vibrations; for the greater the amplitude, the longer likewise the
duration of the contact of the style, s, on the corresponding plate,
and the shorter the duration of the interruption. In order to modify the
amplitude, the action of the electro-magnet on the branches of the
apparatus is made to vary. To effect this, the electro-magnet is made
movable perpendicularly by the aid of a screw, V, between two slides, so
that the core, N, may be moved with respect to the median line of the
branches, and even be raised above them. Its action diminishes,
necessarily, while it is being raised, and the amplitude of the
vibrations likewise diminishes gradually and continuously. It may thus
be made, without difficulty, to vary from two to three tenths of a
millimeter to three or four millimeters or more.
But it is not sufficient to cause the amplitude to vary; it is necessary
to measure it and to keep it constant at the value desired.
[Illustration: FIG. 5]
The measurement is effected by the aid of a very simple apparatus that I
have before described under the name of the _vibrating micrometer_. This
is a small square of paper carrving a design like that shown in Fig. 5,
and which is seen in Fig. 4 glued to one of the masses, M, which serve
to vary the number of the instrument's vibrations. This figure is in
fact, an angle, one of whose sides is graduated into millimeters, for
example, and the other forms the edge of a wide black band. The apex of
the angle is above and the divided side is perpendicular to the
direction of the vibrations.
Under such conditions, when the fork is vibrating, the apex of the
angle, by virtue of the persistence of impressions upon the retina,
_seems_ to advance along the graduation in measure as the amplitude of
the vibrations increases. If an angle has been drawn such that the slope
of one of its sides to the other is one-tenth, it is easy to see that
for each millimeter passed over _apparently_ by the apex of the angle,
the amplitude will increase by two-tenths of a millimeter.
This is the way, then, that the amplitude is measured. On another hand,
it suffices to keep the apex of the angle of the micrometer immovable,
in order to be sure of the constancy of the tuning fork's amplitude; and
this is done, when necessary, by causing the screw, V, to move slightly.
The instrument represented in Fig. 4 is, moreover, fixed to a support
devised by Mr. A. Duboscq, so as to make it possible to give the tuning
fork every position possible with respect to a vertical plane; to raise
it or lower it, and to move it backward or forward so that it may be
employed for chimography, and in all those experiments in which
electro-tuning folks are used.