The Electrical Engineer
May 6, 1891
An Electrolytic Clock
by Nikola
Tesla
If
a delicately and pivoted and well-balanced metal disc or cylinder be
placed in a proper plating solution midway between the anode and
cathode, one half of the disc becomes electro positive and the other
half electro negative. Owing to this fact metal is deposited on one, and
taken off from the other half, and the disc is caused to rotate under
the action of gravity. As the amount of metal deposited and taken off is
proportionate to the current strength, the speed of rotation, if it be
small, is proportionate to the current.
The
first device of this kind was operated by me early in 1888, in the
endeavor to construct an electric meter. Upon learning, however, that I
had been anticipated by others, as far as the principle is concerned, I
devised the apparatus illustrated in the accompanying engraving. Here F
is a rectangular frame of hard rubber which is fastened upon a wooden
base. This frame is about — inch thick, 6 inches long and 5 inches
high.
On
both of its upright sides are fastened thick metal plates which serve as
the electrodes. These plates are held firmly against the rubber frame by
the binding posts T T and T1 T1 On the lateral sides of the frame are
fastened the brass plates B and B1, respectively, of the same shape as
the rubber frame F. These brass plates serve to keep in place two plates
of polished glass, and the vessel is hermetically sealed by placing a
soft rubber washer under and above each of the glass plates. In this
manner the plates may be screwed on tight without fear of breaking them.
The
plating solution, which in this case is a concentrated solution of
sulphate of copper, is poured in through an opening on the top of the
rubber frame, which is closed by a plug R.
In
the centre of the vessel is placed a light and delicately balanced
copper disc D, the axis of which is supported by a capillary glass tube
which is fixed to one of the glass plates by means of sealing wax, or
other material not attacked by the liquid. To diminish the friction as
much as possible, the capillary tube which serves as a bearing contains
a drop of oil. The centre of disc should be equi-distant from both the
electrodes. To one side of the axis of the disc is fastened a very light
indicator or pointer consisting preferably of a thin glass thread. The
glass plate next to this pointer has a circle with the usual hour
divisions engraved upon it, as on a clock dial. This circle may be
movable so that it can be put in any position relatively to the pointer.
If the dial is not movable then a thin wire of annealed iron may be used
as on a pointer. The wire should then be so placed that it is exactly in
the centre of the solution. By means of a horse-shoe magnet the disc may
then be rotated and set in proper position.
The
copper solution being carefully poured in, and the plug R replaced, the
terminals of a constant current battery are connected to the
binding-posts TT1, and from time to time the rotation of the disc is
observed. A shunt is connected to the other two binding-posts TT1, and
by varying the resistance of this shunt, or other disc, the speed of
rotation is regulated until it is made to correspond to the division of
the dial; that is, until, for instance, one turn is made in 12 hours.
Obviously
this instrument was not devised for a practical purpose. Neither will it
be quite exact in its indications. There are certain errors, unavoidable
from the principle; for instance, the friction, which cannot be
completely overcome. But the device is interesting as a means of
indicating time in a novel manner. It will, however, be found that by a
careful construction, constant current, and a temperature compensator,
it may be made to rotate with almost perfect uniformity. The current
density should, of course, be very small to secure the best results, and
the disc of about 3 inches diameter should turn once in 6 hours. It is
probable that with a silver solution and a silver plate better results
would be obtained.
It is very interesting to note the appearance of the solution and disc in such a narrow transparent vessel. The solution appears a clear blue, one side of the disc seems to be silver white in a certain position, and the other half is dark like tarnished silver. There is no line of demarcation, but the shades melt beautifully together.