Year: 1886

page_text: I JAMES THORNE, of 85 Gracechurch Street, in the City of London, Merchant,
do hereby declare the nature of this invention and in what manner the same is to
be performed, to be particularly described and ascertained in and by the following
statement:-
This invention relates to apparatus for indicating the speed of rotating shafts,
and is based upon the discovery of Arago that, when a copper disc is rotated in a
horizontal plane beneath a magnetic-needle, the said needle will be deflected in the
direction of rotation of the disc, the amount of deflection being proportionate to
the speed of rotation.
In my improved speed indicator, the needle or pointer is deflected by means of
induced electric currents produced by the rotation, in a magnetic field, of a
cylindrical sheath or drum of copper or similar conducting material. I prefer to
enclose in the said sheath or drum an armature of soft iron, which is capable
of rotation independently of the said sheath or drum, and which either carries the
needle or pointer or is geared therewith in any convenient manner.
In the accompanying drawing, I have shewn how my said invention may be
conveniently and advantageously carried into practice.
Figure 1 is a front elevation of a speed indicator constructed according to my
said invention.
Figure 2 is a similar view, the dial being removed to show the internal
mechanism.
Figure 3 is a vertical transverse central section of the said indicator.
Figure 4 is a sectional elevation showing the magnet and the copper sheath with
the armature therein.
Figure 5 is a section on the line x, x, Figure 4.
Like letters indicate corresponding parts throughout the drawing.
a is a case or box in which the mechanism is enclosed. b is a horse-shoe magnet,,
which is firmly secured at b¹ to the said case. c is the copper sheath or drum,
which is mounted upon a shaft or spindle d carried in a bearing e formed on, or
attached to, the case a, and which is arranged to be rotated between the poles of
the magnet b. On the shaft d is fixed a pulley f to which motion is to be imparted
by a belt from the shaft whose speed is to be indicated.
g is the armature, which is fitted to rotate upon a stud or shaft h fixed in a
bracket h¹ attached to the case a. The said stud or shaft is insulated from the
armature, and is in axial alignment with. the shaft d. The said armature and
sheath are thus arranged to turn upon or about a common axis, but the armature
is independent of the said sheath and is only moved by the action upon the said
armature of the induced currents generated by the rotation of the said sheath in
the magnetic field.

page_text: In the apparatus shewn in the drawing, a toothed segment i is fixed upon the
boss g¹ of the armature and gears with a pinion k fixed upon the arbor of the
needle or pointer l. If desired, the needle or pointer may be fixed upon the boss gl
of the armature g, but, in such case, the range of indications will be inore restricted
than when a segment and pinion is used. Moreover other gearing may, if desired,
be substituted for the said segment and pinion for imparting motion from the
armature to the needle or pointer.
The armature g, under the influence of the magnet b, becomes itself a magnet.
The poles of these two magnets are marked N, S, n, 8. When the sheath or drum c
is rotated, between these poles, electric currents are induced in the said sheath or
drum, and the action of these currents upon the armature tends to rotate the latter
in the same direction as the said sheath or drum. The said armature, however,
by reason of magnetic attraction, tends to resume its normal position. The result
is that the armature is turned through an angle proportionate to the speed of
rotation of the said sheath or drum. And this result is not affected by any loss or
diminution of strength in the magnet, because such loss or diminution of strength
is equal in both poles, consequently the currents induced by the rotation of the
sheath or drum will be proportionately less, and the deflection of the armature and,
therefore, of the needle or pointer will still be proportionate to the speed of
rotation of the sheath or drum.
The dial of the instrument is so graduated that the pointer will correctly indicate
thereon the speed of the shaft with which the instrument is connected. The said
dial is either graduated to indicate revolutions per minute, or it is graduated to
indicate miles per hour to adapt it for use on locomotives or other vehicles.
According to a modification of my invention, I dispense with the magnet b, and
use a permanent magnet instead of the armature g, but, in this case, it is necessary
to employ a spring connected with the magnet, which spring will tend to cause the
said magnet to resume its normal position.
It is obvious that the construction of my improved instrument may be somewhat
modified, if desired, without departing from the nature of my said invention.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I wish it understood
that I claim :-
First. A speed indicator wherein the needle or pointer is operated by means of
induction currents produced by the rotation, in a magnetic field, of a sheath or
drum of copper or similar conducting material.
Second. The combination, with the needle or pointer, of an armature connected
therewith either directly or through the medium of suitable gearing and enclosed
in a sheath or drum of copper or similar conducting material arranged to rotate in
a magnetic field, substantially as, and for the purpose, set forth.
Third. The modification of my invention wherein I dispense with the large
magnet and employ a permanent magnet in place of the arinature, substantially as
set forth.
Fourth. The improved speed indicator consisting of the parts constructed and
combined substantially as described with reference to the accompanying drawing
and operating as above specified.