An Instrument for the Measurement of Electric Ourrents.
Year: 1893

page_text: Date of Application, 1st Feb., 1893
Complete Specification Left, 1st Nov., 1893-Accepted, 23rd Dec., 1893
PROVISIONAL SPECIFICATION.
An Instrument for the Measurement of Electric Ourrents.
T, WILLIAM THOMSON, Baron Kelvin of Larga, Professor of Natural
Philosophy in the University of Glasgow, North Britain, do hereby declare the
nature of this invention to be as follows:-
In this instrument I use two small equal coils of fine wire rigidly connected with
their central planes in one, and their wires so joined, either in series or in parallel
that the current runs in opposite directions through them. The framework
carrying these coils is supported so as to be free to turn round a horizontal axis
which may be either in the line joining the centros or perpendicular to that line,
and is guarded against other motions. The finest pivots in jewelled holes do not
give sufficient freedom for the propor motion, and the alignment of knife edges to
rest on two planes is difficult in construction and liable to be lost by some slight
strain in use or in carriage. I therefore use either an axe-edge supported on a
central bearing plane, or a concave knife-edge and a straight knife-edge fixed in
proper positions with the edges at their bearing points as nearly as may be
tangential to and coincident with the axis of the proper motion; and supported by
two fixed hooks well rounded and polished and of hard material. Two fixed coils
are placed so as to act respectively on these two movable coils after the manner of
the single fixed coil and the single movable coil in Weber's electro-dynamometer
but with motions round a horizontal axis instead of round a vertical axis as in the
instrument of Weber.
To provide the electric connection for the electric current through the movable
coils I use preferably two flat spiral springs like the hair-spring of a watch but of
non-magnetic metal. These springs serve also for the purpose of giving return
force to the movable system, varying very nearly in simple proportion to the angle
of its displacement from its position of equilibrium.
The indications of the instrument may be read either by a pointer showing on a
scale, or by a mirror.
Dated this 31st day of January 1898.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicant's Agents.
COMPLETE SPECIFICATION.
An Instrument for the Measurement of Electric Currents.
1, WILLIAM THOMSON, Baron Kelvin of Largs, Professor of Natural
Philosophy in the University of Glasgow, North Britain, 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 :—
In this instrument I use two small equal coils of fine wire rigidly connected with
their central planes in one, and their wires so joined, either in series or in parallel,
that the current ruus in opposite directions through them. The framework
carrying these coils is supported so as to be free to turn round a horizontal axis,
which may be either in the line joining the centres or perpendicular to that line,
and is guarded against other motions. The finest pivots in jewelled holes do not
give sufficient freedom for the proper motion, and the alignment of knife-edges to
rest on two planes is difficult in construction and liable to be lost by some slight

page_text: strain in use or in carriage. I therefore use either an axe edge, supported on a
central bearing plane, or a suspension on a conoave knife edge and straight
knife-edge fixed in proper position with the edges at their bearing points as nearly
As may be tangential to and coincident with the axis of the proper motion; and
supported by two fixed hooks well rounded and polished and of hard material.
Two fixed coils are placed so as to act respectively on these two movable coils after
the manner of the single fixed coil and the single movable coil in Wober's electro-
dynamometer, but with motions round a horizontal axis instead of round a vertical
axia as in the instrument of Weber.
To provide the electric connection for the electric current through the movable
coils I use preferably two flat spiral springs like the hairspring of a watch, but of
non-magnetic metal. Those springs serve also for the purpose of giving return
force to the movable system, varying very nearly in simple proportion to the angle
of its displacement from its position of equilibrium.
The indications of the instrument may be read either by a pointer showing on a
scale, or by a mirror.
In order that my invention may be properly understood and readily carried
into effect I have hereunto appended an explanatory sheet of drawings in the
several figures of which corresponding parts are marked with the same letters of
reference.
Figure 1 is a front elevation.
Figure 2 is a sido elevation.
Figure 3 is a horizontal section on the line. y.
Figuro 4 is a vertical section showing the method of supporting movable
system.
In this instrument two small coils a and b of fine wire are used which are rigidly
connected by a bridge e with their central planes in one. The wires of these coils
are so joined either in series or parallel that the current runs in opposite directions
through thom. The coils and framework are supported so as to be free to turn
round a horizontal axis, which may be either in the line joining the centres or
perpendicular to that line. In the instrument illustrated on the accompanying.
sheet of drawings the axis round which the coils turn is in the line joining the
centres of the coils.
The method of supporting the coils and framework is shown at Figure 4. I use
at one end of the framework a concave knife edge, as shown at d, and at the other
end of the framework a straight knife-edge as shown at e. The concave knife-edge
and straight knife edge are supported by two fixed hooks and g of hard material.
The line joining the bearing points is coincident with the axis round which the
coils move. Two fixed coils and are placed so as to act respectively on these
two movable coils:
To provide the electric connections for the electric current through the movable
cuils I use preferably two flat spiral springs ; and k like the hairspring of a watch,
but of non-magnetic metal. One end of each spiral spring is connected through
the framework to the wire of the fine coils. These. spiral springs serve also for the
purpose of giving return force to the movable system. The ends of the fixed coil
are brought down to two terminals and m. One end of the movable coils is
metallically connected by the spiral spring to the fixed coil, while the other
end of the movable coils is insulated from the fixed coil i and is connected through
the spiral spring & and wire n to the terminal o. The indications of the instrument
may be read by a pointer p and scale q, or by a mirror.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is :-
1. The instrument described for mensuring electric currents.
2. In electric measuring instruments

page_text: system by which the difficulty of obtaining alignment in knife-edges and the
frictional error of a pivot suspension are obviated, while the position of the axis of
the movable system relatively to the fixed coils is made constant.
Dated this 31st day of October 1898.
BOTTOMLEY & LIDDLE,
154, St. Vincent Street, Glasgow, Applicant's Agents.