Improvements in Electrical Measuring Instruments.
Year: 1899

page_text: Date of Application, 7th Feb. 1899 -Accepted, 20th May, 1899
COMPLETE SPECIFICATION.
Improvements in Electrical Measuring Instruments.
JOHN WILLIAM FLOWLR, of 59, Haitham Road, Camden Road, in the County of
London, Electrical Engineer, and THOMAS EDWARD GAMBRELL, of Grasmere,"
Allfathing Lane, Wandsworth, in the County of London, Electrical Engineer,
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 an instrument for measuring electic curients of the D'Arsonval, or moving
coil type, its sensibility depends, other things being equal, on the number of turns
of wie in the coil, and the intensity of the magnetic field in which it moves.
The essence of this patent consists in the use of a coil very thin in proportion
to its other dimensions, enabling the poles of the magnets between which it moves
to be brought very close together, and thus producing an intense magnetic field.
The required number of turns in the coil aie obtained by varying its diameter,
and within small limits, the thickness also, of the coil.
In Figs 1, 2, and 3, an instrument for measuring very small currents, or resist-
ances, is shown Fig. 1, is a sectional elevation on the line ab of the plan, Fig 3;
and Fig. 2, is a front elevation. C, C are two sets of powerful permanent magnets,
their pole faces being brought to a horizontal plane. 1) is a massive soft iron
armature, its thickness being large in proportion to the width of the gap between
the poles of the magnets, C, C, it is brought close and parallel to the pole faces by
adjusting screws indicated at h, h, h, h. In the thin horizontal space left, the pair
of coils, E, E, move, being properly connected to assist one another when an electric
current is passed through them, by means of the brass stems, G, G, to which their
ends are soldered The coils are wound of fine copper wire, in moulds. the
required number of turns having been made, they are impregnated with pure
paraffin wax, so that on cooling, a solid coil of the shape of the mould is produced.
They are supported between two thin plates of copper, separated by a shaped
block of ebonite, in which the stems G, G, are screwed. These plates of copper also
serve to damp the vibration of the coils by the eddy currents induced in them
by the magnets, when in motion. Either stem as required, may carry the mirror, F,
by which readings of the coils' position are taken or a light aluminium pointer
may be attached, moving along a suitable scale. The coil system is supported by
thin strips of phosphor bronze, S, S, which also carry the current to and from
the coils, and provide the necessary controlling force towards zero. Suitable adjust-
ments are provided for centring, raising and lowering the coils and for bringing
the reflected ray of light, or aluminium pointer to zero. The spring at M, keeps
an uniform tension on the suspensions, when the coils are raised or lowered. The
sensibility of the instrument can be varied between wile limits by means of
the screws at h. h, h, which raise the armature 1), from the magnets C. C, so
weakening the magnetic field in which the coils move The armature may be
replaced by another set of magnets, inverted over the first: the poles being so
relatively arranged as to give the required form to the magnetic field, i.e., "N to
S," and Sto N. Suitable supports and adjustments for the magnets must be
:

page_text: provided in this case. Suitable base, supports, cover, means of levelling, and
accessories are provided for the instrument, according to the purpose for which it
is intended. These are partly shown in Fig 3, which is a part plan of the instru-
ment; in it P, is the base board, 0, a glass shade, and N, one of the three levelling
screws The coils may be wound an moulds round, elliptic, sector-shaped,
rectangular or square in shape, as required for the various purposes and types of
instruments to which they are applied, extreme' thinness, in proportion to other
dimensions, being the principle involved in each.
Figs 4 and 5, show an elevation and plan respectively of a form of coil applicable
to an instrument having only one permanent magnet and armature, in the space
between which the coil A can move It is supported between two thin plates of
copper carried by an ebonite clip B, in which the brass stems for suspension, con-
nections and mirror are screwed There is also screwed into it a light brass arm D,
on which can be screwed in and out, the brass counterweight C, by which the coil
is balanced. S, S, shows the phosphor bronze suspension strips. A vane may be
attached to it, dipping in liquid to further damp vibrations, if the copper sheath
prove insufficient.
A thin flat coil suitable for the measurement of low resistances may be con-
structed by cutting a plate of copper by a fret saw, or other means, with a spiral,
continuously, from circumference to centre, giving any required number of turns.
A second plate exactly like the first may be reversed and laid upon it separated
by a thin layer of insulating material: their centies being pinned or soldered
together the current thus circulating in both spirals in the same direction.
By enclosing the coil in a thick sheath of copper it can be weighted to any
requned extent for the measurement of transient currents, or statical charges of
electricity, as in other ballistic galvanometers
Having now particularly described and ascertained the nature of our said
invention, and in what manner the same is to be performed, we declare that what
we claim is:-
CLAIMS.
1 In all these forms the new principle claimed is the use of a coil whose
thickness is very small in proportion to its other dimensions, so enabling an intense
magnetic field to be produced with comparatively small permanent or other
magnets.
2. The variation of sensibility of an instrument containing a coil as above
described, by altering the distance between the armature, or inverted magnets, and
the fixed magnets below, thus increasing, or decreasing the intensity of the
magnetic field produced, in which the coil moves.
Dated this Thirtieth day of January 1899.
JOHN W FLOWER.
T. E. GAMBRELL.