Improvements in Apparatus for the Measurement of Electrical Resistances.
Year: 1889

page_text: Date of Application, 15th Feb., 1889
Complete Specification Left, 23rd Sept., 1889-Accepted, 18th Jan., 1890
PROVISIONAL SPECIFICATION.
Improvements in Apparatus for the Measurement of Electrical
Resistances.
We, WALTER THOMAS GOOLDEN, and SYDNEY EVERSHED, both of Woodfield
Road, Harrow. Road, London, W., Electrical Engineers, do hereby declare the
nature of this invention to be as follows --
This invention relates to improvements in apparatus for measuring electrical,
resistances generally, and the insulation resistances of electrical circuits in
particular.
In carrying our invention into effect we use a modified form of ohmmeter having
one or more coils in shunt to the resistance to be measured (hereinafter called the
"pressure coil") and one or more coils (hereinafter called "current coils") in series with
the resistance. In ohmmeters as heretofore constructed the axes of the current and
pressure coils have been placed at right angles to each other, but according to our
invention they may be at any angle between 90 degrees and about 170 degrees, but
preferably at 145 degrees, so as to give the instrument a long scale. Where it is
difficult, for constructive reasons, to place the coil at such an angle we make either
the current, or the pressure coil, or both of thein, in two portions and place them at
right angles to each other so as to produce a resultant field at the needle having a
direction dependent on the distance of the two portions from the needle and the
ampere turns on each.
In order to make an ohmmeter indifferent to any extraneous magnetic field, such
field may be neutralised at the needle of the instrument by means of a large permanent
magnet in the well known manner, and in order to avoid the troublesome adjustment
of the large magnet every time the ohmmeter is used, or the extraneous magnetic
field changes in direction we mount the large magnet on a pivot, or cap and point,
and allow it to swing freely in a horizontal plane, its centre being either vertically
over, or under that of the needle, a clamp, or other device, serving to fix the large
magnet when it has taken up its position with its axis parallel to the direction of the
extraneous field.
When an ohmmeter is to be employed to measure very high resistances, such as
the insulation resistance of a telegraph cable, or electric light circuit, we prefer
that the deflecting force may be due to the current coils, and the controlling
force to the pressure coil; in other words the magnetic axis of the needle when at
zero may be parallel to the axis of the pressure coil. The deflection will then vary
as the current, that is as the conductivity of the circuit under test, but we prefer to
work the scale in convenient units of resistance, so that the usual zero corresponds to
infinite resistance, and is so marked, and the needle will only be parallel to the axis
Where it is desired to make the
of the current coil when the resistance is zero.
ohmmeter sensitive when near zero we preferably place the pressure and current
coils at right angles so as to cause the deflections near zero to be comparatively large
for very small currents in the current coils.
To obtain the necessary high electro-motive force for testing the insulation of
electric light and other circuits, we use a small magneto-, or dynamo-, electric
generator, driven by hand or clockwork, and wound so as to give the required
electromotive force, the generator being used in conjunction with a suitable ohmmeter
such as we have described above.
A convenient form of portable testing apparatus may be made by combining a
magneto-electric generator and ohmmeter in one box; the current coils being

page_text: provided, if desired, with separate terminals so that this part of the instrument may
be used as an ordinary "linesman's detector" when required, or the current coils may
have a second winding on them for this purpose.
Dated this 15th day of February 1889.
COMPLETE SPECIFICATION.
Improvements in Apparatus for the Measurement of Electrical
Resistances.
WALTER THOMAS GOOLDEN and SYDNEY EVERSHED, Electrical Engineers,
Woodfield Road, Harrow Road, London W. 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:-
We have in our provisional specification described the general nature of our
invention, consisting of an Ohmmeter having its pressure and current coils at a
greater angle than a right angle; a method for obtaining 2 magnetic fields at any
desired angle; a method for neutralizing the earth's magnetic field; a method for
using the coils of the Ohmmeter so that large resistances may be easily measured; and
a portable arrangement of ohmmeter and magneto generator.
We will now describe in detail each point of our invention, taking them in the
above order.
•
In Ohmmeters heretofore constructed the magnetic axis of the pressure coil was
at right angles to that of the current coil, so that when the needle had been deflected
about 60 degrees from the latter, the scale became too cramped to allow of accurate
reading. By displacing the pressure coil axis more than 90 degrees from the
current coil axis, we are able to use a much longer & therefore more open scale.
Fig. 1 shows, in section, a pressure coil and current coil arranged according to our
invention, where C is the current coil, made in 2 halves, in order to admit the needle
system, P. being the pressure coil having its axis inclined at an angle indicated in
the drawing by A., (which may conveniently be about 145°) from that of the current
coil as shown. By taking deflections in the right handed direction, the needle can
move through 90 degrees before the scale becomes too cramped. We find in practice
that 1-45° is the most convenient angle for general purposes; but if a more open
scale is required the angle must be increased, and where excessive sensitiveness is
required for a short range, the angle A may approach within 1° or 2° to 180°, the
needie being then almost in unstable equilibrium at the centre of its range.
It is sometimes inconvenient to arrange the coils as shown in fig. 1, as in order to
insert the needle and its axis, the pressure coil must not only be divided but capable
of rotation about a vertical axis, so as to bring the division opposite that in the
current coil.
The simple arrangement for producing 2 magnetic fields at any desired angle is
shown in fig. 2; here a portion . . of the pressure coil circuit is wound on the
current coil bobbin, and by a suitable proportion between the turns on p. y. and those
on p... the resultant magnetic field at 0. may be made to have any direction between
the lines and y. There are many ways of combining the 2 portions of the pressure
coil, but the method shown in fig. 2 is the most convenient. The principle of
producing a resultant magnetic field in any desired direction by means of 2 coils
at an angle may be applied to many other electric instruments, for example, Tangent
Galvanometers.
The errors due to the magnetic field of the earth may be eliminated by using
an astatic needle system, but it sometimes happens that the astatic system is
undesirable on account of the complication and extra weight of moving parts
it introduces. In such cases we use a large permanent magnet, pivoted on a
vertical axis either immediately under or over the needle of the ohmmeter, and at

page_text: such a distance as is found by trial to reduce the resultant field due to the earth's
magnetism at the needle to zero. Being pivoted, the magnet automatically adjusts
itself to the correct position. It is convenient to have some clamping device to
enable the magnet to be fixed after having taken up its position North & South.
The magnet should produce as uniform a field on the needle as possible, and this
object is best attained by the use of a straight bar magnet, free to swing in a
horizontal plane, and having its centre in a vertical line passing through the centre
of the needle.
The distance from the centre of the magnet to the centre of the needle should
be as nearly as possible six-tenths of the length of the bar magnet; the strength
of the magnet is then adjusted until it neutralizes the earth's field in the plane of
the needle.
The automatic neutralizing magnet described above may be applied with advantage
to all kinds of Galvanometers in which it is desired to reduce the directive force on
the needle to as small an amount as possible.
For the measurements of very high resistances such as insulation resistance of
electric mains, dynamos etc.; we modify the arrangement of the coils shown
in figs. 1 & 2, so as to make one end of the scale to correspond with infinity instead
of zero, as usual, while the reading at the other end of the scale corresponds with the
lowest resistance which it is desired to measure.
We effect this by the arrangement of coils & needle shown in fig. 3. The current
coil C. is made as large, and wound with as many turns of wire as possible and closely
surrounds the needle N. S., so that very small leakage currents may produce a
sufficiently strong magnetic field to overcome the friction of the needle system. The
pressure coil is in 2 portions P. P. placed symmetrically on either side of the current
coil, with their common magnetic axis at right angles to the magnetic axis of the
current coil, as in this case length of scale is of secondary importance compared with
sensitiveness to very minute leakage currents; & the needle should therefore begin
its range with its length at right angles to the magnetic axis of the current coils.
This
The index I. is attached to the needle at such an angle that while N. S lies in the
axis of the coils P. P. the index points to the end of the scale marked ** inf.
position evidently corresponds with zero current in the current coils, and therefore
with infinite resistance in the circuit under test.
An ohmmeter made as in fig. 3 may be calibrated by a series of observations of its
readings when connected to resistances varying from the lowest, to the highest which
the instrument is intended to indicate, but as resistances over 100,000 ohms are
expensive and often inaccurate, we prefer to calibrate by means of a set of com-
paratively low resistances, merely ensuring that the ratio of the current in the
current-coil to the current in the pressure coil, to give any observed reading, shall be
equal to what the ratio of the currents in the coils will be, when connected to the
resistance to be indicated by the observed reading. The actual value of the currents
used in calibration is immaterial so long as they bear the right ratio to each other for
each reading, and have sufficient force on the needle to overcome friction.
It is usually desirable to measure the insulation resistance of electric mains,
dynamos etc.; while under the full working difference of potential; and we therefore
use, according to our invention, a small magneto or dynamo-electric generator, of
simple construction, wound to give from 50 to 200 volts, in conjunction with the
ohmmeter as described & illustrated in fig. 3. A common shuttle armature with
a two-part commutator is well adapted for the purpose. Such an armature about 4"
in length and 14" dia. wound with about 5000 turns of wire, and driven at a speed
of from 2000 to 3000 revolutions per minute, will, when excited by permanent
magnets, weighing about 1 lb. each, give approximately 100 volts. The generator
and ohmmeter may be placed in one containing box, but as the field magnets of the
former are liable to introduce errors in the indications of the latter we prefer to keep
the 2 instruments separate. With the object of reducing such errors as far as
possible, we place the field magnets of the generator in such a position that their

page_text: stay-magnetic field is vertical at the needle of the ohmmeter when the boxes
containing the 2 instruments are placed on the same table.
It will be noticed that figs. 1, 2 & 3 show sections in a horizontal plane through
the instruments described. In fig. 4 the connections of the Ohmmeter & Generator
for the measurement of high resistances are strung. G is the generator. C. the
current coil of the ohmmeter, which can be shunted by means of the coil S, so as
to enable readings to be continued down to any desired limit in a well known
manner; P. is the pressure coil having a resistance R, made of German-silver
or Platinoid wire, connected in series with it, and L. L. represent the electric mains
under test.
As shown in the diagram the apparatus will indicate directly the insulation
resistance from between the mains. Where it is desired to measure the insulation
resistance between the mains and the earth, both mains are connected to one terminal
of the Ohmmeter, the other terminal being connected to earth.
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. An ohmmeter in which the pressure coil & current coil have their magnetic
axes at a greater angle than a right angle substantially as and for the purposes
set forth.
2. In an ohmmeter producing a resultant magnetic field and given direction by
means of 2 coils placed at an angle in a manner described.
3. In an ohmmeter neutralizing the effect of the earths magnetic field by means of
a permanent magnet free to move on a vertical axis, substantially as described.
4. In an ohmmeter for the measurement of large resistances, using the pressure
coil to control the needle, and the current to deflect it, substantially as described.
5. An instrument for the direct indication or measurement of resistances made,
substantially as described.
6. The combination of an instrument for the direct indication or measurement
of resistances with a portable dynamo or magneto-generator, substantially as
described.
Dated this 21st day of September 1889.
WALTER T. GOOLDEN.
SYDNEY
EVERSHED.