AN IMPROVED CONSTRUCTION OF GALVANOMETERS
Year: 1884

page_text: COMPLETE
SPECIFICATION,
I, CHARLES DENTON ABEL of N° 28 Southampton Buildings, Chancery Lane in
the county of Middlesex, Consulting Engineer and Patent Agent. do hereby
declare the nature of the said invention for "AN IMPROVED CONSTRUCTION OF
GALVANOMETERS " and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement :-
reference being had to the accompanying drawings, and to the figures and letters
marked thereon, that is to say :-
Galvanometers of ordinary construction have the disadvantage that the needle is
caused to oscillate, when a current is passed through the coil, which oscillations
prevent the easy and rapid reading of the indications. To obviate this, advantage
has been taken of the known property of a magnetic needle and a fixed piece of
metal to react upon one another, the oscillations of the magnet producing induced
currents in the fixed piece, which tend to bring the needle quickly to a standstill.
Copper being the diamagnetic body which possesses this property in the highest
degree, this metal has heretofore been used for the said purpose. But when the
intensity of the current passing through the conductors is subject to very sudden
changes, or when it is desired to obtain an instantaneous measurement of a current,
the above means are not sufficient for the purpose, as the needle still makes a
number of oscillations before it comes to rest.
According to the present invention, this difficulty is entirely obviated by
arrangements in which soft iron or other magnetic metal or alloy is employed in
place of the copper.
Iron, being the most magnetic body, produces on the needle a much more energetic
action, stopping the oscillations in a very much shorter time than is the case with
copper.
The piece of iron thus applied must be so arranged as not to produce upon the
needle any other action that might destroy, partly or wholly, the deviations
produced by the current.
In place of iron may also be used other magnetic metals, as for instance nickel
or cobalt or alloys containing such metals in sufficient quantity to enable them to
act magnetically upon the needle.
The accompanying drawings shew arrangements by which such metals or alloys
can be employed according to this invention. Figs. 1, 2 and 3 shew respectively a
transverse section, an elevation and a plan of one arrangement.
It is not necessary to give a complete description of the galvanometer; B B are
the coils, A is the needle mounted on the axis C, which carries an index D moving
over a graduated dial E. The index carries a weight p for bringing it to the
vertical position when not acted upon by a current. The needle A is situated in
the centre of a circular ring F of bronze or brass of sufficient thickness to be
accurately turned in a lathe, which ring is provided internally either with a solid

page_text: ring of soft iron a, or with several coils of iron wire; the internal diameter is such
that the ring, although approaching quite close to the ends of the needle, does not
interfere with its free motion.
Fig. 4 shews another arrangement in which a third coil H is added, through
which the electric current is made to pass, and which is intended to impart to the
soft iron needle an initial magnetism when it is yet in such a position as not to
be magnetised by the coils that produce the deviation.
The space within the coils being very small in this case, the neutralising ring
cannot be placed within it; the following arrangement is therefore employed. On
the needle axis, outside the coils, is fixed or formed a disc of soft iron G, which is
.connected magnetically to the needle by a soft iron tube, should the needle axis
itself be of non-magnetic material. Under the action of the bobbin H, the disc G
will become magnetised, constituting the one pole, the needle becoming the opposite
pole; the circular neutraliser F is then applied round the disc G. Figs. 5, 6 and 7
shew respectively a front view, a side view, and a plan of an arrangement which
allows of the apparatus. being used in an inclined position without affecting its
action. The coil J, which is circular and of small thickness, is supported upon a
cylinder I, and carries a circular piece K having two appendices which pass down
inside the coil and base and which carry two axes. The upper axis, situated at
about the middle of the coil, carries the needle L and two small screwed arms
which are provided with adjustable weights and serve to regulate the apparatus,
in order that it may be in a state of equilibrium irrespective of its centre of
oscillation. ...
The index M fixed on the same axis, indicates the deviations upon a dial N.
The neutralising ring O is fixed to the piece K by two lateral lugs, as shewn at
figs. 5 and 7.
The lower axis carries a small grooved pulley P on which is wound
a pliable thread, the upper end of which is fixed at R', while its lower end carries a
weight Q.
The electric current has the effect of causing the index to move to the right or
left in raising the weight Q, which always acts with the same force, whatever may
be the inclination of the instrument.
All the parts are of non magnetic material except the needle, the neutralising
ring and the axes, which can be of steel. In place of the arms R R¹ may be used
a grooved eccentric, over one side of which the weighted thread passes, its upper
'end being fixed at a, fig. 8.
'
Lastly, an eccentric may be used arranged in a similar manner, but the periphery
of which, in place of being circular, is formed in accordance with a curve
proportioned to the increase of the deflecting power of the current, so as to obtain
indications upon the dial proportional to the strength of current.
Having thus described the nature of this invention and in what manner the
same is to be performed,, as communicated to me by my foreign correspondent,
I claim
The construction of galvanometers wherein the oscillations of the needle are
neutralised by means of a ring of soft iron or of other magnetic metal or alloy, so
arranged as either to act directly upon the needle or upon a piece of metal
connected thereto, substantially as herein described.
Dated this 18th day of Januaru 1991