OMETERS
Year: 1884

page_text: COMPLETE SPECIFICATION.
.
I, JAMES BLYTH; M.Á. of Anderson's College, Glasgow, in the County of
Lanark, North Britain, Professor of Natural Philosophy and Mathematics do
IMPROVEMENTS IN
hereby declare the nature of my said invention for
GALVANOMETERS and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:
"
.
My said Invention consists principally in employing an endless or circular
solenoid as the medium by which an electric current is made to act on a small
suspended magnet or needle for the purpose of obtaining an indication or
measurement of the strength, of the current as shown by the deflection or angular
movement of the magnet. The endless solenoid is formed by taking a straight
solenoid or helical coil of insulated copper wire and bending it into a circular
forin, like an anchor ring or annulus of circular section. Such a solenoid forins
à closed magnetic shell and can consequently exert no magnetic force outside of
itself, so that the mere use of the instrument does not tend to produce any change
in it or in any permanent magnet or magnets which may be combined with the
solenoid. A small magnet or a magnetised needle is placed within the solenoid at
any point in its circular centre line, and is suspended or carried by a fine wire or
fibre passing between two adjacent convolutions of the solenoid; in such a way as
to be free to be deflected or noved angularly by the action of an electric current
passing through the wire forming the solenoid; and so as to communicate the
angular movement to a light external pointer arranged to move over a suitably
graduated scale. For weak currents the simple endless solenoid may be used, but
for stronger currents a permanent magnet may be applied to act directively either
on the magnet or needle within the solenoid or on a magnet or needle outside of
the solenoid but fixed on the same axis as the one within the solenoid: Or instead
of using a permanent magnet a volute or helical spring may be applied to the axis
of the small magnet or needle, means being provided for increasing or diminishing
the action of the spring to suit currents of different strengths.
The construction or arrangement of the parts of my improved galvanometer may
be varied; but in order that my Invention may be properly understood a few
modifications embodying my improvements are shown on an accompanying sheet
of explanatory drawings and are bereinafter described. In these drawings the same
reference numerals are used to mark the same or like parts wherever they are

page_text: Figure, 1, on the accompanying sheet of drawings is a plan of a simple form of
my improved galvanometer. Figures 2, 3, and 4, show modes of applying
permanent magnets, and figures, 5 and 6, show a modification in which more or
less resistance to the deflecting action of the current. can be applied by means of
an adjustable spring.
In the example shown in figure, 1, what, may be termed the frame or body of
the instrument consists of a shallow brass bobbin or pulley, 7, having the endless
solenoid, 8, fixed round its core and between its disks. This solenoid, 8, may be
secured by passing a thread or fine cord a few times round through the internal
annular space and drawing it tight and knotting it so as to bind the inner sides of
the convolutions against the core of the bobbin.7. The ends, 9, of the insulated
copper wire forming the solenoid, 8, are twisted together and connected to
terminals to which conductors 'may be secured in the usual way for passing the
current to be measured through the instrument. The small magnet or needle on
which the current acts is not shown in figure, 1, but is placed as shown at, 10, in
figures, 2, 3, and 6, being fixed, to a stiff axis, 11, of fine wire placed at right
angles to the plane of the solenoid (that is, of its circular centre line). In the
example shown in figure, 1, the axis, 11, is pivoted in lower and upper centres and
has fixed to it above the upper disk of the bobbin, 7, a light pointer, 12, and also
a light volute spring, 13, which last serves principally to move the pointer, 12, to
zero when a current is not passing through the solenoid. The pointer; 12, moves
over a graduated arc, 14, showing degrees of deflection to which may be added an
experimentally determined scale of " amperes." The scale, 14, pointer, 12, and
spring, 13, may be protected by a glass cover as shown at, 15, in figures, 2, 3,
and 6.
: 4
In the modification shown in vertical section in figure, 2, the endless solenoid, 8,
is represented as coiled upon a wooden annulus, 16, as a core, a part of this core
being of course removed where the small magnet or needle, 10; is placed. In this
example a permanent magnet, 17, is placed with its poles at the inner and outer
sides of the solenoid, 8, at the part where the small magnet or needle, 10, is placed
for the well understood purpose of exerting a certain directive force to partially
resist the action of the current passing through the solenoid. In the example
shown in vertical section in figure, 3, a permanent magnet, 18, is applied in a
different way. The axis, 11, of the small magnet or needle, 10, is continued
downwards so as to have another small magnet or needle, 19, fixed to it below or
outside of the solenoid, 8, this lower magnet or needle, 19, being. placed between
the poles of the permanent magnet, 18, which is in the form of a volute or flat
spiral as shown separately in figure, 4. In figure, 3, there is also shown attached
to a continuation of the axis, 11, of the small magnet or needle, 10, a small paddle
immersed in liquid in a vessel, 20, to restrain oscillation according to a well known
plan. And the same figure shows the axis, 11, of the small magnet or needle, 10,
as suspended by a fibre or thread attached to an adjusting screw, 21.
In the example shown in figure, 5, figure, 6, being a vertical section of the part
where the small magnet or needle, 10, is situated, the axis, 11, has a voluté
spring, 13, fixed to it by its inner end, whilst the outer end of the spring fixed
to a spurwheel, 22. A pinion, 23, gears with the spurwheel, 22, and can be turned
by means of a projecting spindle so as to apply more or less strain to the axis, 11,
whilst a pointer, 24, fixed to the spurwheel, 22, shows on a graduated scale, 25, to
what extent the spring, 13, is coiled up. When a current is being tested or
measured the strength must be calculated from the indications of both pointers, 12,
and 24, or the scale, 25, of the latter pointer may have on it multipliers or other
numbers for directly correcting the indications shown on the other scale, 14.
It is an advantage of my improved construction that when a spring, 13, is used
it can be made of steel, neither it nor any external controlling magnets such
as 17, 18, being affected by a current passing through the solenoid, 8.
When weak currents are to be measured the modifications shown in figure, 1, is
used, but with the small magnet or needle, 10, suspended by a fibre as shown in

page_text: strength can then be determined by means of the following equations :
2 n C cos. H sin. 0.
r
=
whence C
= Hr tan. 0.
2 n
in which equations n = number of convolutions in the solenoid; .= radius of
circular centre line of the solenoid; H horizontal intensity of the earth's
magnetism; = angle of deflection; and C = the strength of the current. The
constant of the instrument is consequently which can easily be determined with
-
2 n
+
.
accuracy.
When strong currents are to be measured the permanent magnets are used or
the adjustable spring, 13, as herein before described to partially resist the current,
and the calculations are of course to be modified accordingly. Any of the forms of
my improved galvanometer (if without the fibre suspension) may be placed in a
horizontal, vertical, or inclined position; and if the instrument is placed with the
axis, 11, of the small suspended magnet or needle, 10, parallel to the line'of magnetic
dip, magnetically astatic arrangements can be dispensed with.
When the endless solenoid is made with very fine insulated wire wound on an
annular wooden core the instrument may be used as a "voltmeter" for showing
difference of potential in "volts." -
Having thus particularly described the nature of my said Invention and in
what manner the same is to be performed I proceed to ascertain the distinctive
feature of novelty thereof by declaring that what I claim is :-
•
The combining of an endless or circular solenoid with other parts substantially
in the manner and for the purposes hereinbefore described.
Dated this seventeenth day of March 1884.
EDMUND HUNT,
Applicant's Agent. _