Improvements in Electric Measuring Instruments.
Year: 1898

page_text: Date of Application, 14th Nov., 1898-Accepted, 23rd Dec., 1898
COMPLETE SPECIFICATION.
Improvements in Electric Measuring Instruments.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by Elihu Thomson,
residing at Swampscott, in the County of Essex, State of Massachusetts, United
States of America, Electrician); and in what manner the same is to be performed,
to be particularly described and ascertained in and by the following statement:-
In testing electrical apparatus, or in measuring the energy passing in an electric
current, it often becomes necessary to make simultaneous readings on volt and
ammeters, and where the instruments are mounted on separate bases, it is diffi-
cult for a single observer to get simultaneous readings. Under ordinary condi-
tions, this may not make any substantial difference, but where special accuracy
is desired, or where the conditions change rapidly, the error may be sufficient to
materially affect the final results.
It is therefore a common practice while testing to employ two men who, on
a given signal, take readings from the volt and ammeters; this is objectionable,
on account of the increased cost of the test, and also for the reason that it is not
always possible to get the additional man.
This invention has for one of its objects to overcome the objection above pointed
out, and to provide an instrument which is so arranged that it is capable of
making separate indications, as of volts and amperes for example, and this either
separately or simultaneously, the arrangement being such that the two portions
of the instrument are entirely separate in their action.
The invention also relates to certain details of construction more fully described
and claimed hereinafter.
In the accompanying drawings, which show an embodiment of the invention,
Fig. 1 is a plan view of an instrument, with the cover mounted thereon; Fig. 2
is a plan view of the same with the cover and scale plate removed; Fig. 3 is a
section taken on line 3-3 of Fig. 2, and Fig. 4 is a diagram of the circuit
connections.
In order to make the instrument double reading, two separate moving systems A
and B, are provided, in the present instance, the moving systems are designed
to indicate volts and amperes, but these may be modified in any desired way
without departing from the spirit of the invention, as for example, by arranging
one of the moving systems to indicate watts; or the moving systems may both
indicate volts, or both amperes.
To make the instrument as compact as possible, it is preferred to employ a
common field-magnet for both moving systems, which magnet in the present
instance is arranged to form an astatic couple, as it possesses certain well known
advantages over other types, but if desired any one of the ordinary forms of field-

page_text: magnet may be employed. In carrying out this feature of the invention, two bar
magnets C and C are mounted on the insulating base 1) and retained in place by
the clamps E. The outer ends of the bars are curved slightly at c so as to form
pole pieces. Situated above the magnets C and C¹ is a corresponding pair of
bar-magnets C and C which are held in place by clamps E. These magnets are
disposed with respect to each other so that the lower one C and the upper one C²
form one pair, and the lower one C and the upper one the other pair.
Mounted for oscillating movement between the pole pieces of these magnets
are the moving coil systems A and B, and as they are usually similar in con-
struction with the exception of the size of wire employed in winding,, a description
of one of them will be sufficient. The coils F which are separately wound, and
similar in construction, are mounted on a disc F¹ made of copper or other good
conducting material, and are arranged to oscillate between the poles of the per-
manent magnets. The disc F1 is secured to a shaft F2, which is supported at its
ends by the jewel bearings f; the bearings being mounted in the clamps E and E¹.
Since the disc is made of good conducting material, eddy currents will be induced
therein whenever it cuts the lines of force of the magnets; which will tend to
reduce the oscillations of the moving system, and make the instrument more
dead beat.
Situated on top of the magnet clamp E¹ is a square plate G, preferably of
diamagnetic material, in which are mounted the fixed pins or supports G¹ and G².
Between the pins. G¹ and G2 and the shaft F2 are two spiral springs H and H¹
which oppose the action of the moving system and return it to zero, and also act
as conductors to convey current to and from the coils F.
Situated between the upper and lower bars of the magnets, and insulated there-
from, is a resistance I composed of a number of insulated turns of high-resistance
material, as German silver for example. Connected in shunt around this resist-
ance are the coils of the moving system B, which are adapted to indicate the
amount of current flowing in the mains of a system by measuring the drop in
potential across the resistance. The moving system A located at the upper end
of the instrument is employed to measure the voltage of a system, and is included
in series with two coils of resistance wire S and S1, which coils are wound on
spools and the spools mounted on the insulating base.
Four binding posts are provided for the instrument, T and T for the volt
meter element A, and T2 and T3 for the ammeter element B.
Situated above the working parts of the instrument is a scale plate which is
common to both instruments; this is a very advantageous arrangement, and par-
ticularly where electrical tests are being conducted, for it permits the person
taking readings from the instrument to observe the position of both index needles
or-pointers J, J at one and the same time.
The scale plate is divided into two parts I and I2, and the parts are supported
by the studs and which are mounted on the insulated base D. The portion I'
of the scale plate is at a different level than the portion 12; this is to permit the
needle J to pass under the needle J¹ in the operation of the instrument. For
convenience of manufacture, the scale plate is cut into two parts, but instead of
this, it may be made in a single piece and slotted to receive one of the pointers;
it is of course evident that one portion should be a trifle higher than the other,
as before.
The needle J is connected to the under side of the disc F which supports the
moving element of the ammeter, the needle J¹ is connected to the moving element
of the voltmeter.
In order to give the needles as wide a range of movement as possible, the
graduated scale K for the needle J is situated near the centre of movement of the
needle J; and the graduated scale K¹ for the needle J¹ is situated near the centre
of movement of the needle J. With this arrangement, it will be seen that the
needles cross one another near the central portion of the scale, but as the needle J'
occupies a position above that of needle J, they can readily pass.

page_text: If it is unnecessary to limit the external dimensions of the instrument, the
centres of movement may be separated by a greater amount and the needles J
and J¹ so arranged that they do not cross one another, in which case the graduated
scales K and K¹ will be situated on the same side of the center line as their
respective pointers.
The scales K and K in either case could be graduated in any desired manner,
the customary way being to calibrate the instrument and to mark the scale for the
different potential or watt yalues.
Surrounding and protecting the working parts of the instrument is a sheet metal
casing L which is provided with two openings L' and L2 permitting the inspection
of the needles J and J. The central portion L3 of the cover is so arranged that
it covers up the slot or division in the scale plate.
It will readily be seen that by the arrangement shown the instrument is made
very small and compact, and that the person can at a glance read the indication
of both needles. It will be evident that this invention in some of its aspects,
is independent of the particular type of instrument to which it is applied.
Having now particularly described and ascertained the nature of our said inven-
tion (as communicated to us from abroad) and in what manner the same is to be
performed, we declare that what we claim is;-
1-Two electric measuring instruments attached to a single or common base
and having their indicating needles so located with relation to a scale plate that
independent readings may be taken at the same instant, substantially as
described.
2-Two electric measuring instruments attached to a single or common base,
in combination with a single dial plate provided with scales for the index needles
of both instruments, substantially as described.
3-Two electric measuring instruments each provided with an index needle
operatively connected with the rotary or movable part of its own instrument,
said rotary or movable parts being located in the magnetic fields between the oppo-
site ends of a pair of energizing magnets, substantially as described.
4 Two electric measuring instruments attached to a common base, with their
movable parts located in the magnetic fields between the opposite ends of two
magnets in combination with index needles and a single dial plate provided with
one or more scales towards which both of said needles point during their entire
range of movement, substantially as described.
5-A combined instrument for measuring electrical currents, consisting of two
instruments enclosed in a single casing and a single scale plate common to the
movable parts of said instruments, substantially as described.
6-Two electrical measuring instruments having movable index needles J and J¹
attached to movable coils F, F located in the magnetic fields between pole-pieces
of the permanent magnets C and C, in combination with a dial plate common to
both index needles, and independent binding posts TT, TT, having circuit
connections with the movable coils, all of said parts being rigidly attached to a
common base of support, substantially as described.
7-In an electric measuring instrument, the combination of a plurality of
separate moving systems, capable of independent movement and mounted on a
common base with an astatic field-magnet which is common to all of the moving
systems.
8-In an electric measuring instrument, the combination of a plurality of
separate moving systems, capable of independent movement and mounted on a
common base, an index needle for each moving system, the path of travel of the
needles being so arranged that the needles cross each other as they pass over the
scale, and a field magnet which is common to all of the moving systems.
9 In an electric measuring instrument, the combination of a field-magnet, two
separate moving systems capable of independent movement situated at opposite

page_text: ends of the field-magnet, a scale for each moving system located near the centre
of movement of the opposite system, and needles for the moving system which
cross one another as they move over their respective scales.
Dated this 25th day of October 1898.
THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED.
A. R. MONKS,
W. MARTINDALE,
} Directors.
ALFRED CLEMENTS,
Assistant Secretary.