Improvements in Electric Measuring Instruments.
Year: 1894

page_text: Date of Application, 6th June, 1504
Complete Specification Left, 6th Mar., 1895-Accepted, 13th Apr., 1895
Complete Specification Left, 6th Mar., 1895-Accepted, 13th Apr., 1895
PROVISIONAL SPECIFICATION.
Improvements in Electric Measuring Instruments.
WALTER THOMAS GOOLDEN and SYDNEY EVERSHED, both of Woodfield
Works, Harrow Road, W., Electrical Engineers, do hereby declare the nature of
this invention to be as follows:-
This invention relates to ohmmeters used for the direct indication of electric
resistance and has for its object the removal of certain difficulties which are met with
in the use of the several instruments for the same purpose which formed the subject
of Patent No. 2694; 1889. The difficulties referred to arise firstly from want of
perfect mechanical balance in the needle system, secondly from the tendency any
ordinary astatic needle has to become non-astatic from unequal demagnetiza-
tion of the needles, and thirdly from complete demagnetization of one or both
needles of an astatic system when a large current is accidentally caused to
traverse the current coils of an ohmmeter in the wrong direction. According to
our present invention the first difficulty is removed by suspending the needle
system on a single point as in a common magnetic compass, or by a nearly torsionless
thread as in many galvanometers; and the second and third difficulties are removed
by making the needle system of soft iron in the form of a tee, the vertical limb
being conveniently in the form of a tube, while the horizontal limb may be of
uniform rectangular section. The needle system is suspended by means of a cup
jewel fixed in the upper end of the vertical limb, so that the cup and needle system
may rest on a hard steel point carried on the end of a fixed vertical rod passing up
the hollow vertical limb. Surrounding the said vertical limb but not touching it
is fixed an exciting coil which serves to convert the needle system into an electro
magnet, having one pole at the lower end of the vertical limb and the other pole
divided equally between the ends of the horizontal limb. Thus the horizontal
limb having its two ends of like polarity and equal moment about the axis of
rotation becomes a perfectly astatic maguet, when a current traverses the fixed
exciting coil. Unless the axis of the vertical limb is exactly coincident with the
magnetic axis of the exciting coil the vertical limb will be attracted to the side of
the coil, and in order to keep it in stable equilibrium along the axis of the coil,
we fix a short piece of soft iron tube, of the same diameter as the vertical limb,
a short distance beneath the lower end of the latter and with its axis coincident
with the prolongation of that of the exciting coil and vertical limb. The magnetic
induction passing between the lower end of the vertical limb and the upper end of
the short piece of soft iron tube serves to hold the vertical limb in stable equilibrium
in or near the axis of the coil. The current coil is in two portions, each arranged
to act with the best effect on one pole of the needle. The pressure coil may be
similarly arranged, or advantage may be taken of the property by which an astatic
needle such as that described tends to set itself at right angles, or parallel, to an
oblong coil placed above or below the needle and with its magnetic axis coincident
with the axis of rotation of the needle system. Such an oblong coil forms a very
convenient pressure coil. Since a considerable resistance is commonly required in
series with the pressure coil of an ohmmeter, the exciting coil may be coupled in
series with and form part of the extra resistance required for the pressure coil.
The needle is then always magnetised in the proper direction whenever the

page_text: r
ohmmeter is in use, no matter in which direction the working electromotive force
is applied.
Dated this 5th day of June 1894.
WALTER THOMAS GOOLDEN.
SYDNEY EVERSHED.
Improvements in Electric Measuring instruments.
WALTER THOMAS GOOLDEN and SYDNEY EVERSHED, both of Woodfield
Works Harrow Road London W., Electrical Engineers, 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 :-
This invention relates to ohmmeters used for the direct indication of electric
resistance, and. has for its object the removal of certain difficulties which are met
with in the use of the several instruments for the same purpose which formed the
subject of Patent No. 2694 1889. The difficulties referred to arise firstly from
want of perfect mechanical balance in the needle system, secondly from the tendency
any ordinary astatic needle system has to become non-astatic in consequence of
unequal demagnetisation of the two needles ; and thirdly from complete
demagnetisation of one or both needles of the astatic system when a large
current is accidentally caused to traverse the current coils of an ohmmeter in the
wrong direction.
.
According to our present invention the first difficulty is removed by suspending
the needle system on a single point as in a common compass or by a nearly
torsionless thread as in many galvanometers; and the second and third difficulties
are removed by making the needle system of soft iron in the form of a T, the
vertical limb being conveniently in the form of a tube while the horizontal limbs may
be of uniform rectangular or other section. This form of needle system is shown
in Fig. 1 of the drawings accompanying this Specification which is a vertical
section through the centre of the needle system and coils of an ohmmeter made in
accordance with our present invention. V is a small tube of soft sheet iron forming
the vertical portion of the aforesaid T carrying a cup jewel J at its upper extremity
and freely supported on the fixed pin A which has a fine steel or iridium point at
the end on which the cup J rests. The horizontal limb, H, is made of a narrow
strip of sheet iron of uniform width and thickness and having a hole midway
between its ends in which the tube V is fitted and attached to H by soldering or
otherwise. The pin A and vertical tube V lie in the magnetic axis of a coil M
and when a current traverses M the needle system becomes magnetised so that one
pole S S is at the bottom of the iron tube while the other pole N N is at the two
ends of the iron strip H and these ends being thus of equal length and magnetised
similarly and to the same degree form an astatic combination such that when placed
in a uniform magnetic field they have no resultant turning moment about J as a
centre. The other pole S S of the system being cylindrical, and concentric with
the axis on which the needle system turns has of course no moment about that axis.
Thus the whole system is astatic.
The pin A is attached to a screw B which is threaded into the bracket D fixed
to the coil bobbin. Unless some suitable precaution is taken, the tube V being in
unstable equilibrium will be attracted towards the side of the coil M and coming
in contact with the pin A the free movement of the needle system will be hampered.
One way of preventing this is described in our Provisional Specification and consists
of fixing a short piece of iron tube K of the same diameter as V over the screw B
and with its upper end a short distance below the end of V. The attraction

page_text: between K and V is readily adjusted by sliding K up and down on B until V is
ust maintained in stable equilibrium in a vertical position. The iron tube K may,
however, be dispensed with and the needle system maintained in stable equilibrium
if the tube V is shortened so that its lower end is only a little more than halfway
through the coil M; this position of the lower end of V is indicated roughly by
the line L. A third way of maintaining stable equilibrium is by prolonging the
tube V upwards above the jewel J, or point of suspension and surrounding the
to increase
prolongation with a second fixed maguetising coil, which serves
the strength of the poles N N, and at the same time by its upward attraction not
only keeps the needle system stable but reduces the pressure of the jewel on the
supporting point.
The current coil of an ohmineter made in accordance with our present invention
is in two halves as indicated in Fig. 2 which is a diagram in plan. Each half of
the current coil C C, is wound on a flat bobbin shown in section in the figure,
similar in shape to the coils of an ordinary detector galvanometer. The infinity
position of the needle must be such that the poles N N are well within the current
coils and we find a total angular range of scale equal to 60° of arc allows the needle
to have its position for zero resistance parallel to the axes of the current coils,
and brings the position of the poles N N for infinite resistance well inside the
current coils.
The
The pressure coil is most conveniently made in the form of a flat oblong as
indicated by P, Fig. 2. This coil only requires one or two hundred turns of fine
wire, and can be made so thin that a place is easily found for it between the under
side of the current coils and the foundation plate F of the instrument.
pressure coil is arranged with its major axis at right angles to the needle N N.
and the current in it must be in the same direction as the current flowing in
the exciting coil M. The needle N N will then rest at right angles to the
major axis of the pressure coil when no current is traversing the current coils. In
the figures the current in the exciting coil M and pressure coil P is supposed to be
flowing contra-clockwise as indicated by the arrow, so that the ends of the
horizontal limb H of the soft iron T become north poles.
An index I, attached the jewel cap J, ranges over a scale divided to read in
ohms or megohms in the manner described in the Specification to Patent No. 2964,
1889, already referred to.
The indications of an ohmmeter made in accordance with our present invention
will be independent of the direction in which the electromotive force is applied to
the terminals of the instrument so long as any extraneous magnetic forces tending
to disturb the needle system produce a sensibly uniform field throughout the space
occupied by the horizontal limb of the needle, and when the field produced by
extraneous magnetic forces is not uniform the ohmmeter reading will be in excess
or defect of the correct value for the resistance under test according as the operating
electromotive force is applied in one direction or the other, and in such case the
mean of two readings taken with the electromotive force in opposite directions will
Thus whether the disturbing
give the true value of the resistance under test.
magnetic forces produce a uniform field or not the ohmmeter may be made to give
correct indications by the simple expedient of reversing the applied electromotive
force, and an ohmmeter made in accordance with our present invention may
therefore be described as a reversible ohmmeter.
In order that the needle system may be clearly shown it is shaded black where
seen in section in Fig. 1 and one of the current coils is omitted in this figure for
the same reason. In Fig. 2 the zero positions of the needle N N and index I are
indicated by dotted lines.
The dimensions of the various parts of an ohmmeter made as herein described
may be varied very widely but we find it generally convenient to make the
needle N N about two inches in length; Figs. 1 and 2 are therefore approximately
£11

page_text: ;
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
(1.) A reversible ohmmeter made substantially as described and for the purposes
set forth.
(2.) An astatic ohmmeter with a balanced needle system, for the purposes set
forth and substantially as described.
(3.) In an ohmmeter for the measurement of electric resistance, the combination of
a balanced and astatic soft iron needle system and exciting coil with pressure coils
and current coils for the purposes set forth and substantially as described with
reference to Fig. 1 and Fig. 2.
Dated this 5th day of March 1895.
WALTER T. GOOLDEN.
SYDNEY EVERSHED.