Improvements in Instruments for Measuring Electric Currents.
Year: 1893

page_text: COMPLETE SPECIFICATION.
[Communicated from abroad by EDWARD WESTON, of Newark, New Jersey,
United States of America, Electrician.]
Improvements in Instruments for Measuring Electric Currents.
I, HENRY HARRIS LAKE, of the firm of Haseltine Lake & Co., Patent Agents,
45 Southampton Buildings, in the County of Middlesex, 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:-
The invention relates to an electrical measuring instrument and consists in the
following improvements and the construction thereof:
First, devices for dampening or retarding the motion of the index so that the
instrument will be "dead-beat or substantially so.
Second, an arrangement of the scale-plate of the instrument whereby it may be
adjusted in place without disturbing the working parts of the apparatus, and also
whereby its indications may be rendered visible in obscure localities.
One form of the retarding device for the index is represented in Figs. 1 and 2.,
Another form is shown in Fig. 3, and the construction of the scale-plate is repre-
sented in Figs. 4 and 5.
Referring first to Figs. 1 and 2, F and H are the pole pieces of a permanent
magnet, the remainder of which it is not necessary to show. This magnet is
enclosed in a case or box, a part of which is indicated at A. There is therefore a
L is a cylinder
strong magnetic field in the cylindrical space between F and H.
of paper or light metal which is supported by the tube N, which tube is carried by
leaf-springs, O, P. Within the tube is a spiral spring, R, the free end of which is
fastened to the short rod e. Rode is attached to a yoke, T, which is supported by
pivot pins in the block W carried by bracket a. The yoke T carries the index-
needle V, which may move over any suitable scale. On the cylinder L is a coil, K,
through which the current to be measured passes. As a consequence of the passage
of the current, a reaction occurs between the field produced around the coil K and
the permanent magnetic field existing between pole pieces F and H; and hence the
coil and cylinder L will move bodily in an axial direction and assume a position
dependent upon the difference of potential between the instrument terminals. When
the cylinder L moves, as, stated, it communicates rotary motion to the needle V
through the spring R, rod e and yoke T; and therefore the extent of movement of
the cylinder will be indicated by the extent of movement of the needle V over its
scale, and as this scale may be graduated in suitable electrical units, the pressure
or strength of the current passing through the coil K may thus be measured and
indicated.
I have described this instrument simply as one to which the dampening device,
to which I will now refer, may be attached. It will be understood that the use of
said dampening device is not limited to its employment to that particular form of
instrument, but that it may be applied to any instrument having a moving index
and rotary index-shaft, such as e.
On said shaft e I secure a disk, h, of diamagnetic material, preferably aluminium.
This disk is received between the arms of a small permanent magnet, i, which is
supported upon a bracket, ;, of non-magnetic material.
This construction is convenient and preferable, but any other arrangement
whereby the polar extremities of a magnet are disposed on each side of the rotating
disk may be employed.

page_text: In accordance with well-known laws, the movement of a diamagnetic body, when
moved transversely the lines in a field of force, is retarded; and, in the present
case, the retardation of the rotation of the disk h is sufficient, or may be adjusted
so as to be sufficient, to overcome the vibration of the needle V or of the needle
and its associated parts due to inertia.
It is to be understood that the retarding effect of the magnetic field upon the
disk e is not to be sufficient 'to interfere with 'the movement' of the index V due to
the passage of the current, but simply sufficient to prevent the vibration of that
needle on each side of its proper indication, which vibration is due to its own inertia,
or to that of the needle and the movable parts connected with it.
The form of dampening device which is represented in Fig. 3 may be applied to
the instrument already described and shown in Figs. 1 and 2; and Fig. 3
represents simply the movable cylinder L of such an instrument, together with the
coil K thereon. In order to produce a dampening or retarding effect upon 'tliis
cylinder, I wind directly upon it and inside the coil K another coil, f, the terminals
of which are connected together as shown at g. The use of this coil depends upon
the well-known law of Lenz that, in all cases of electro-magnetic induction, the
induced currents have such a direction that their reaction tends to stop the-motion
which produces them. In the present case, the closed conductor fmoves' with 'the
cylinder L'in the magnetic field; and as a consequence a current is induced in said
coil which tends to stop its motion. While the current in the instrument through
coil K therefore is strong enough to move that coil, and hence the cylinder, 'to a
new position 'and 'to overcome the counter effect of the induced current in the
coil due to that motion, the strength of the induced current, on the other hand,
in the coil due to vibrations of the cylinder L.produced simply by its own inertia
is sufficient to overcome the energy of that motion and so bring the cylinder
instantly, or very 'nearly instantly, to 'rest. It is not essential that the conductor f
should be connected merely at its 'terminals inasmuch as short circuit may be made
at various points along its length. So also it may be disposed outside the coil K as
well as inside. Neither is it necessary that it should be carried on the same support
as the coil K, so long as its motion is caused by that coil; nor need it be in the
same magnetic field as coil K, as it may be located in some other 'magnetic field
and still exercise its retarding effect.
Referring to Figs. 4 and 5, it is of great advantage to construct the scale of an
electrical measuring instrument'so that it will be removable without disturbing the
other parts of the apparatus, especially when the instrument is being calibrated.
This I accomplish in the following manner :
1 represents the case of an instrument the working parts of which are here
not shown. They may, however, cause, for example, the vibration of an index-
needle or the movement of a spot of light, or govern any known mode or means
of indicating some particular reading on the face of the scale. In the wall, 2, of
the instrument there is an opening which receives the scale-plate 4. Plate 4 has
Hanges, '5, which rest against the exterior of the wall, and have openings' to receive
pins, 6, which maintain the scale-plate in definite position. On the inner-side of
the plate is inscribed the scale-marking, for which purpose the plate may be
enameled or 'covered with paper or any other material, or the plate may be of
inetal with "the marking engraved directly upon it; or I may make the scale-
plate 4 in the form of a 'rim or frame which receives a separate iplate of glass or
other translucent material, in which case a source of illumination is arranged
behind the scale so that the latter then becomes visible through the opening in the
front wall of the case.
It will be readily apparent that the scale-plate 4 can be removed from the case
and the scale-marking inscribed or altered in any desired way without disturbing
or affecting the working parts of the instrument; and also that by providing a
translucent scale-plate the indications thereon may be rendered visible at a con-
siderable distance when the instrument is disposed in obscure locality]

page_text: Having now particularly described and ascertained the nature of the invention
and in what manner the same is to be performed, as communicated to me by my
foreign correspondent I declare that what I claim is
1. A dampening device for electrical measuring instruments containing a rotary
or vibratory shaft and an index-needle actuated thereby, consisting in a disk of
diamagnetic material upon said shaft and moving when said shaft rotates through
a magnetic field.
2. A dampening or retarding device for electrical measuring instruments
containing an index-controlling body movable transversely the lines of force of a
magnetic field, consisting in a closed conductor supported by said body and arranged
in said magnetic field.
3. An electrical measuring instrument having a scale-plate, either opaque or
translucent, detachably secured to said instrument and arranged so as to be
accessible and removable without disturbance of the working mechanism of said
instrument.
1000
*HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.