Improvements in Ohmmeters and other Electrical Measuring Instruments.
Year: 1895

page_text: Date of Application, 13th Mar., 1895-Accepted, 15th June, 1895
COMPLETE SPECIFICATION.
Improvements in Ohmmeters and other Electrical Measuring
Instruments.
We, CHARLES WILLIAM SCOTT CRAWLEY and ALFRED SOAMES, both of
16 Red Lion Street, Clerkenwell in the County of Middlesex, 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 :-
In ohmmeters of the well known type with two coils set at an angle to each other
one traversed by the current from a source of E.M.F. through the resistance to be
measured and the other by the current from the same source through a fixed
resistance, a resultant field is set up whose direction depends on the resistance to be
measured. To indicate the direction of this field and thus the unknown resistance,
a soft iron needle is generally used which takes up its position in the direction of
the field and moves a pointer over a scale which can be marked directly in
resistance. It is of great importance that this needle should not be affected by the
earth's field otherwise the readings will vary with the orientation of the instrument.
To effect this freedom from action of the earth's field we employ a needle system
hereafter described which gives extremely perfect astaticity, i.e. is unaffected or
practically unaffected by the earth's field. Instead of working with two concentric
coils which form a joint or resultant field, we generally prefer to use two coils
joined up in circuit with a fixed and the unknown resistance as before, but in
different planes, and act on one end of the needle with one coil and on the other end
with the other coil; a method which is rendered feasible by the use of a practically
astatic needle.
We construct the needle of a magnet bent on itself either twice at right angles
or other equivalent way, and suspend or pivot it so that the line of suspension is in
the plane passing through the poles of the magnet, and the poles are equi distant
from the axis of suspension. Where greater power is desired we employ two such
magnets back to back but preferably not touching each other. The magnets are as
evenly magnetised as possible. We do not confine ourselves to the use of this form
of magnet for ohmmeters only but may use it in any electrical measuring instru-
ment, in which a very well astaticised needle system is desired. We are aware that
a needle consisting of a single magnet bent twice at right angles to itself and
suspended by a fibre has been described before.
In the following drawings like letters apply to like parts.
Fig. 1 is a plan of an ohmmeter constructed on the above invention. Fig. 2.
elevation and part section of the saine.
Fig. 3 a double & Fig. 4 a single needle.
Fig. 5 a diagram of the circuits of the usual type.
Fig. 6 diagram of a needle as applied to a resultant-field form of instrument.
A A are the coils in series with the resistance to be measured, X. BB the coils
at an angle to AA and in series with the fixed resistance R. (R is not essential as
the resistance of B B may itself be sufficient.) V is the battery or other source
of E.M.F. The upper end of the needle is acted on by A A and the lower by BB;
the effect of one being more or less at about right angles to that of the other and
consequently the whole needle system including the pointer takes up a position
depending on the ratio of the currents in the two coils and therefore on the ratio of

page_text: the resistance in the two circuits. A pointer P attached to the system indicates on
the scale S, which is calibrated to show directly the resistance X. We may of
course use the single needle shown in Fig. 4 and only one A and one B coil.
Fig. 4 shows the needle system with one magnet as previously described consisting
of a magnet NS bent twice at right angles on itself and with the two poles N & S
in the same plane with the axis of rotation and equi distant from it. Fig. 3 shows
a double needle system formed of two such needles. Instead of setting the two
coils A & B at considerable angle to each other they may obviously be parallel or
at a very small angle in which case they are joined up so that while one coil
attracts the end of the magnet that it acts on, the other coil repels the end of the
magnet that it acts on; so that the effect of the two coils on the needle system is the
same as previously described viz. that the tendency of one coil is to set the needle
system at an angle to the position in which the other coil tends to set it, and the
position of the needle indicates the resultant of the two actions. In Fig. 6 A & B
are any two coils at an angle to each other which when traversed by a current of
electricity causes a resultant field in the direction of the arrow and the lower limb
of the astatic magnet N S. sets itself in and thus indicates the direction of this
field.
our said
Having now particularly described & ascertained the nature of
invention and in what manner it is to be performed we declare that what we
claim is;-
(1.) In an ohmmeter the combination of two coils or pairs of coils acting on the
ends of an astatic permanent magnet needle system and indicating directly the
resistance measured.
(2.) In an ohmmeter the combination of two coils or pairs of coils in different
planes acting on the ends of an astatic needle system substantially as described.
(3.) In an electrical measuring instrument the use of an astatic needle substantially
as described to determine the direction of a magnetic field.
(4.) In an electrical measuring instrument the use of a needle system formed of
two hent magnets, substantially as described, to obtain astaticity.
(5.) The ohmmeter substantially as described.
Dated this 19h 1M1 1005
C. W. S. CRAWLEY.
ALFRED SOAMES.