Improvements in Electric Measuring Instruments.
Year: 1896

page_text: Date of Application. 19th May, 1896-Accepted, 27th June, 1898
COMPLETE SPECIFICATION.
Improvements in Electric Measuring Instruments.
I, JAMES DEVONSHIRE, Secretary of The British Thomson-Houston, Limited,
of 38, Parliament Street, Westminster, London, (a communication from abroad
by Elihu Thomson, Professor of Physics and Chemistry, of Swampscott, in the
State of Massachusetts United States of America,) 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:-
My invention relates to improvements in electric measuring instruments, which
are adapted e'ther for use upon alternating or upon continuous current circuits,
and which may be made to give indications of potential as volt-meters, or of
current as ammeters. They are, however, particularly designed for use upon
alternating circuits where the presence of iron in the instruments might be
objectionable. The instrument is adaptable also to be used as a watt-meter by
having a portion of its coils traversed by a current in series with the work and
other portions of its coils traversed by currents in shunt to the work, as is
common in such instruments.
The accompanying drawings show embodiments of my invention.
Fig. 1 is an end elevation of the instrument; Fig. 2 is a plan view;
Fig. 3 shows the arrangement of the circuits and coils which form the elements
of the instrument when used as a voltmeter;
Fig. 4 shows a modification of structure;
Fig. 5 is a separate view of the modified system of coils, the coils being shown
on edge,
Fig. 6 is an arrangement of circuits adapting the instrument for use as an
ammeter, and
Fig. 7 is an arrangement of circuits adapting the instrument for use as a watt-
meter.
In Fig. 1 and 2 a base B, B of insulating material preferably, carries upon it a
metal frame, F, F, open in the center and having supports at J, J, containing
jewels for the reception of the axle, carrying the movable parts of the instrument.
The index 1 is carried on the shaft in any way, and in Fig. 2 is shown as resting
on the zero of the scale S. Below the index is a spiral spring, like the hair spring
of a watch; to an arm A, centrally pivoted on the frame F, the outer end of the
spring is attached, while its inner end is secured to the shaft which carries the
moving parts. Upon the shaft are also carried two connections, as at a, b, made
of fine silver foil wrapped around the shaft several times, so that the shaft may
turn freely, which are used for introducing currents to the movable part of the
apparatus, also mounted upon the shaft. These various features will be referred
to in other figures.
A button or handle H pivoted in the frame F, carries a little arm with a bristle
or strip of flexible material D which can be turned into contact with the upper end
of the shaft for stopping too violent oscillations of the needle or index. A circuit
closing button or switch K is mounted on the base; its connections are shown in
Fig. 3. There is also a plug switch P, Fig. 2, for plugging in or plugging out of
circuit a resistance coil, which is mounted on the instrument at any convenient
point, as in the circular dotted lines d, intermediately below the scale. The
terminals are at T, T', a set of ordinary binding posts.

page_text: Referring now to Fig. 3, the connections of the instrument, and other parts of
its structure not easily seen in Figs. 1 and 2, are shown. T, Tare the terminals,
as before, the switch K may be depressed so as to close a circuit through the
instrument, while the plug switch P, P may be made to insert the resistance R or
to cut it out, as when the plug is at the left, the resistance R will be in circuit,
and if it is at the right, the resistance R will be cut out. A fixed set of coils and
a movable set are used. The fixed set may be made up of two or more sets of
coils C, C, preferably an upper and a lower set, which are securely held by proper
supports in the frame F, Figs. 1 and 2. These coils are shown as semi-circular in
outline, and are flat semi-circles mounted in coincident planes, the two coils of the
upper set forming a circular coil with a double diameter, and the two coils of the
lower set also forming a circular coil with the windings parallel with those of
the upper coil. These four semi-circular coils carry a current, but the current is
reversed in direction in one of the coils of the upper set relatively to the other
coil, and in one of the lower set relatively to the other of the lower set; it is
however, maintained parallel and in the same direction in the corresponding coils
of the upper and lower sets, so that the magneto-motive force of the upper and
lower arallel coils will be in the same direction, but reversed in the two halves of
the system. These fixed coils are closely approximated, with just enough space
for the placing and free movement of another movable set Q of coils, similar in
structure and relation; the direction of current in the coils Q, Q is also reversed,
such coils being put into circuit by the little silver foil strips surrounding the
shaft, and indicated at a, b, Fig. 1. The two movable coils are connected in
series, giving thus two free terminals, while the current therein is reversel in its
direction of flow in one relatively to the other. The position of the coils,
relatively, when the needle or index I, Figs. 1 and 2, is at zero, is preferably
about that shown in Fig. 3, and the connection of the coils of the movable set
and the fixed set is so made that the current traversing them tends to throw the
Icovable set around from the position indicated in right-handed rotation, and into
parallelism with the fixed set. This will cause the index to move through a large
angle, as the magnetizing effects of the coils are at the start nearly opposed, while
at the end of the scale they are nearly in harmony. The torque so obtained
registers the strength of current flowing, and is, of course, opposed by the
spring Z, when the instrument is constructed with a spring, while it may be
opposed by gravity if the instrument is set up with its coils in a vertical plane.
This latter effect is obtained by loading the movable system a little, so as to have
the center of gravity a little below the center or axis of the moving coils.
winding of the movable coil may be connected in series with the other coils, or in
parallel for voltmeter purposes the series arrangement is to be preferred. The
relative section of its wire may be made smaller than that of the fixed coils. Its
turns need not be so many as in the fixed coils, and the material of its winding
may, if desired, be of aluminum, to secure lightness, since the matter of higher
specific resistance is of no consequence.
The
A connection to a source of electric potential, with the plug switch P arranged
to cut out the resistance R, would render the instrument sensitive to the lower
potentials, but for greater accuracy and for adapting the instrument to read
moderately high potentials, such as 100 volts or more, a large resistance R,
preferably non-inductive, is employed and the plug switch is arranged to cut in
the resistance in series with the coil system. With the system of coils as indicated
in Fig. 3, the tendency of the scale is to be rather open in the middle of its range
or near thereto, which is an advantage obtained in voltmeter measurements as
ordinarily constructed for measuring potentials on electric lighting circuits, it
being desirable to have the greatest sensitiveness at or near the ordinary voltage
or average potential, so that slight departures therefrom may be readily noted.
The especial advantage of the arrangement of coils and circuits of the moving
and fixed sets, is that the instrument is entirely astatic, and not influenced by stray
fields into which it may be brought. This is due to the fact that the system is

page_text: virtually a double magnetic circuit, or a magnetic circuit closed on itself, with
magneto-motive forces exerted in two opposite directions, so that a stray
magnetic field would affect one field of the instrument to diminish it, but at
the same time would proportionally increase the other portion of the magnetic
circuit,
While the fixed coils have been shown as nearly semi-circular, their shape may
be modified to secure particular effects, as when it may be desirable to secure a
scale in which the divisions are more uniform, indicating certain units of increment
of current or potential. Thus in Fig. 4 the fixed coils are shown in solid lines,
while the semi-circular movable coils are shown in dotted lines, the fixed
coils C, C, C, C being of the shape shown, that is, the central portion is narrowed.
The effect of this is to reduce the rate of change and also the leverage or the
deflecting forces upon the movable coil system when the coils are at or near the
positions shown in Fig. 4, as when the index I is near the middle of the scale. By
taking advantage of this deformation or differing configurations of coils, the
instrument may be adapted to read as desired, and its divisions made, relatively to
one another more or less uniform.
In Fig. 5 I show a view of the shaft and the two coils Q, Q, the coils being
seen on edge, while the very fine flexible silver foil connections are indicated
at a, b curled around and free from the shaft, the terminals a, b are carried to the
fixed connections of the circuit, as usual.
In Fig. 6 I show diagrammatically the arrangement of the movable and fixed.
coils which it is desirable to adopt in making the instrument for use as an
ammeter for alternating currents. In this case the movable coils are in series
with each other but in shunt to the other coils, or to the fixed system C, C, and I
also provide a variable resistance V, non-inductive in character, for changing the
resistance in circuit with the fixed coils C, C, thereby adjusting the magnetic
relations of both fixed and movable coils. In this arrangement, as it is to be used
as an ammeter for measuring current flow, it is desirable to keep the section of
the coils C, C as large as possible, and hence they are made of much larger wire
than the movable coils, that is, a larger fraction of current is caused to flow
through the fixed system than through the movable system in order to avoid the
necessity for carrying much current through the flexible connections to the
movable system. Adjustments adapting the instrument to work with varying
frequency may be made at V so as to keep the self induction of the two sets of
coils nearly the same.
In using the instrument as an ammeter it still preserves its astatic character,
owing to the disposition of the coil being the same as before, so far as the
relations of current and magneto-motive force as affected by stray fields are
concerned.
This is true also when the instrument is used as a watt-meter, in which case the
circuit arrangements are those of Fig. 7. Here the fixed coils are in series with
the work while the movable coil system Q is in derived circuit with a high
resistance R, non-inductive in character, connected in the derived circuit. This is
the well known watt-meter arrangement so far as the mere matter of connecting
the coils is concerned, and in such a watt-meter, in most instances, the size of wire
in the coils C, C will be selected to carry the load current while the wire in the
coils Q will be relatively fine. So also, the actual space occupied by the coils C, C
and the weight of metal in them may be made much greater than that of the
movable coils, it being desirable to keep down the weight of the movable element
to avoid the effect of momentum.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed as communicated to me from
abroad I declare that what I claim is:
1. In an electric measuring instrument, sets of fixed coils, each set composed of
a plurality of coils so arranged that the current traverses the respective coils of

page_text: each set in relatively opposite directions, and passes through similar coils in the
several sets in the same direction, in combination with a movable set of coils
also composed of a plurality of coils so wound that the current passes in opposite
directions in the respective coils, and circuit connections for the different sets
of coils.
2. In an electric measuring instrument, the combination of two sets c fixed
coils, each set composed of two flat semi-circular coils, each coil having a
diametral portion, and the coils in each set being so wound that the current passes
in opposite directions in the respective coils of each set, but in the same direction
in the coils having the same relative position in the different sets; in combination
with a movable member composed of two flat semi-circular coils having adjacent
diametral portions, and so wound that the current traverses the respective coils in
opposite directions, and circuit connections for the several sets of coils.
3. In an electric measuring instrument, the combination of two sets of fixed
coils, each set composed of two flat semi-circular coils, each coil having a diametral
portion, and the coils in each set being so wound that the current passes in
opposite directions in the respective coils of each set, but in the same direction in
the coils having the same relative position in the different sets; in combination
with a movable member composed of two flat semi-circular coils having adjacent
diametral portions, and so wound that the current traverses the respective coils
in opposite directions, an indicator for the movable set of coils, and circuit
connections for the several sets of coils.
4. An electric measuring instrument made without iron, comprising sets of
fixed coils in which the current passes in respectively opposite directions in the
several coils, passing in the same direction in similarly placed coils of each set,
and a movable element composed of two similar coils in which the current passes
in respectively opposite directions, an indicator attached to the movable coils,
circuit connections for the coils, a resistance in the circuit, and means for cutting
in and cutting out the resistance.
5. In an electric measuring instrument, the combination of two sets of fixed
coils, each set composed of two flat semi-circular coils, each coil having a diametral
portion, and the coils in each set being so wound that the current passes in
opposite directions in the respective coils of each set, but in the same direction in
the coils having the same relative position in the different sets; in combination
with a movable member composed of two flat semi-circular coils having adjacent
diametral portions, and so wound that the current traverses the respective coils in
opposite directions, a spring opposing the motion of the movable coils and circuit
connections for the several sets of coils.
Dated this 19th day of May 1896