Improvements in Galvanometers with Suspended Coils.
Year: 1887

page_text: Date of Application. 18th June, 1887
Complete Left, 17th Mar., 1888
Complete Accepted, 15th June, 1888
Improvements in Galvanometers with Suspended Coils.
WILLIAM EDWARD AYRTON, Fellow of the Royal Society, Professor of Applied
Physics at the City and Guilds of London Central Institution Exhibition Road
London, and JOHN PERRY Fellow of the Royal Society, Doctor of Science, Professor
of Mechanics and Engineering at the City and Guilds of London Technical College
Finsbury do hereby declare the nature of this invention to be as follows :-
Our improvement applies to galvanometers in which the current is measured by
the deflection of a coil suspended in a magnetic field. Usually with such galvano-
meters the coil is caused to return to its zero position on the stoppage of the current
by its being suspende. with a bifilar suspension, or by a wire which the coil twists
when it is deflected. Hence the stronger the magnetic field the more will the coil be
deflected for a given current, other things being the same, therefore the sensibility of
such a galvanometer varies temporarily when it is brought near a strong magnet or
a large mass of iron. Further if the normal magnetic field, which causes the coil to
deflect when a current is sent through it, be produced by a permanent magnet the
sensibility of such a galvanometer will diminish as the permanent magnet grows
weaker.
These defects we remove, and so render the sensibility of a suspended-coil-
galvanometer constant by not merely using a bifilar or torsional suspension, but
instead by causing the controlling couple, or couple that brings the needle to zero
when the current is stopped to be proportional to the strength of the controlling field.
In one form of our improved instrument we suspend the coil with a silk fibre
suspension having no appreciable torsional rigidity, and in another we support the coil
on a pivot, and in both cases in addition to the large magnet which is attached to the
instrument to produce the magnetic field we rigidly attach one, or more, small steel
magnets to the suspended coil the axis of motion of these magnets and their magnetic
axes being in the plane of the coil. For simplicity let us suppose that one such small
magnet is attached to the coil, then it tends to place the coil with its plane along the
lines of force, whereas a current passing through the coil tends to place it with its
plane at right angles to the lines of force. The coil, therefore, takes up a definite

page_text: position depending on the strength of the current, and this position will not bo
sensibly altered by a variation in the strength of the field, since a diminution, or
increase, in the strength of the field diminishes, or increases, both the moment of the
controlling couple and the moment of the deflecting couple in about the same ratio,
both being very nearly proportional to the strength of the controlling field. Hence
any weakening of the large magnet will not affect the sensibility of the instrument.
Electric connection with the suspended coil is made by its ends dipping into mercury
cups.
In some cases we attach one or more small magnets to the coil in addition to the
bifilar or torsional suspension.
If not merely the strength of the field in which the coil is suspended be altered, but
also the direction of the lines of force, for instance if the galvanometer be placed
close to a powerful dynamo machine then the zero of the instrument may be altered
although not the sensibility. But this causes no inconvenience since in our instrument
the large magnet can be turned about an axis passing through the axis of suspension
of the coil, and fixed in any particular position. To use our galvanometer near any
powerful magnet the large magnet attached to the galvanometer is first turned round
slightly until the pointer attached to the coil points to zero, and then the deflections
produced by any number of amperes, or fraction of an ampere, will be nearly
invariable.
Our arrangement is useful for the general measurement of currents and notably
it may be used in ammeters or galvanometers graduated to read off in amperes a
current passing through them, and in voltmeters or galvanometers graduated to read
off in volts a potential difference maintained at the galvanometer terminals.
Dated thin Dischtsanth dose of Tuna 1987

page_text: COMPLETE SPECIFICATION.
Improvements in Galvanometers with Suspended Coils.
WILLIAM EDWARD AYRTON, Fellow of the Royal Society, Professor of Applied
Physics at the City and Guilds of London Central Institution, South Kensington,
and JOHN PERRY, Fellow of the Royal Society, Doctor of Science, Professor of
Engineering at the City and Guilds of London, Technical College Finsbury 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:
Our improvement applies to Galvanometers in which the current is measured by
the deflection of a coil suspended in a magnetic field. Usually with such galvano-
incters the coil is caused to return to its zero position on the stoppage of the current
by its being suspended with a bifilar suspension, or by a wire which it twists when
it is deflected. Hence the stronger the magnetic field the more will the coil be
deflected for a given current, other things being the same, therefore the sensibility
of such a galvanometer varies temporarily when it is brought near a strong magnet
or a large mass of iron. Further if the norinal magnetic field which causes the coil
to deflect when a current is sent through it be produced by a permanent magnet the
sensibility of such a galvanometer will diminish as the permanent magnet grows
weaker.
This defect we remove and hence we render the sensibility of a suspended coil galvano-
meter practically constant by not depending solely on a bifilar or torsional suspension
for the controlling force, but by using in addition to the bifilar or torsional suspension,
or in place of the bifilar or torsional suspension, a magnetic control the whole being
so arranged that the controlling couple, or couple that brings the needle to zero when
the current is stopped, is proportional to the strength of the magnetic field that causes
the deflection.
In one form of our improved instrument we suspend the coil with a silk fibre
suspension having no appreciable torsional rigidity, in another we support the coil
on a pivot, or on a pivot at the top of the coil and on another pivot at the bottom,
and in all cases in addition to the large permanent magnet which is attached to the
instrument to produce the magnetic field, or in addition to a stationary coil conveying
a current when the deflecting field is so produced, we rigidly attach one or more
steel magnets to the suspended coil with their magnetic axes parallel to the plane of
the suspended coil.
For simplicity let us suppose that one such small magnet is attached to the coil
then it tends to place the coil with its plane along the lines of force whereas a current
passing round the coil tends to place it with its plane at right angles to the lines of
force. The coil, therefore, tends to take up a definite position depending on the
strength of the current, and this position will not be sensibly altered by any temporary
or permanent variation in the strength of the field since a diminution or increase in
the strength of the field diminishes or increases both the moment of the controlling
couple and the moment of the deflecting couple in the same ratio, both being pro-
portional (for a given current passing round the coil) to the strength of the magnetic
field in which the suspended coil is placed.

page_text: This method of compensating for alterations in the strength of the magnetic field
will be the more perfect the more nearly the small magnet, which is rigidly attached
to the coil, is fixed at the place where the resultant force due to the magnetic field
and the current in the coil acts on the coil. This place, however, our experiments
shew is liable to vary in consequence of the strength of the different parts of the
magnetic field occupied by the different parts of the coil not varying proportionately
when the field is disturbed either by the presence of some powerful extraneous magnet
or by an alteration in strength of the permanent steel magnet usually attached to
such a form of galvanometer to produce the magnetic field. This dificulty we over-
come, and thus render the compensation practically perfect, by attaching not one,
but instead a number of small hard steel permanent magnets to the coil with their
axes all parallel to one another and to the plane of the coil. In this way each portion
of the suspended coil is directly acted on by two forces, one due to the action of the
magnetic field at that spot on the current passing round the suspended coil, the other
due to the action of the magnetic field at the same spot on the small hard steel
permanent magnet attached to that portion of the coil. Hence any ordinary variation
in the strength of the magnetic field does not affect the ratio of these two forces.
If not merely the strength of the field in which the coil is suspended be altered,
but also the direction of the lines of force, for instance if the galvanometer be placed
close to a powerful dynamo machine, then the zero of the instrument may he altered
but not the deflection of the coil for a given current. But this causes no incon-
venience since in our instrument the large permanent magnet, or the stationary coil,
which produces the magnetic field can be turned about an axis passing through the
axis of suspension of the coil, and fixed in any particular position. To use our
improved form of galvanometer near a powerful magnet the largo magnet, or the
stationary coil, attached to the instrument is first turned round slightly until the
pointer attached to the coil points to zero and then the number of amperes or fractions
of an ampere producing any particular deflection will be invariable.
In some cases we make electrical contact with the two ends of the moving coil by
the ends dipping into stationary mercury cups but we prefer to use instead two
spirals of fine wire so wound as to produce little or no resistance to the rotation of
the coil.
Our arrangement is useful for the general measurement of currents, for telegraph
receiving instruments like Sir W. Thomson's syphon recorder where the coil moves
in a magnetic field produced by a permanent magnet or by an electro-magnet
magnetised by an independent current, and notably our arrangement is is important
in ammeters or galvanometers graduated to read off in amperes a current passing
round them, and in voltmeters or galvanometers graduated to read off in volts the
potential difference maintained at the galvanometer terminals.
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 :-
In galvanometers with suspended coils the attachment of one or more small magnets
to the coil for the purpose of preventing the sensibility of the galvanometer being
altered by either a temporary or a permanent variation of the strength of the
magnetic field or of any part of the magnetic field in which the coil is placed.