IMPROVEMENTS IN GALVANOMETERS
Year: 1884

page_text: I, ARTHUR PRINCE CHATTOCK, of No 2 Armstrong Terrace, Charlton, in the
County of Kent, Electrician do hereby declare the nature of said invention for
"IMPROVEMENTS IN GALVANOMETERS" to be as follows :-
=
My invention has for its object improvements in galvanometers and relates to
that type known as tangent galvanometers. The tangent galvanometer consists of
a circular coil of wire of one or more turns at the centre of which is suspended a
small magnetic needle. On a current of electricity passing through the coil (when
the plane of the latter is in the magnetic meridian) the needle is deflected and the
tangent of the angle of deflection measures the ratio of the strength of current to
the strength of the permanent magnetic field wherein the needle hangs.
Now in a galvanometer according to my invention the coil is suspended as well
as the needle, and the tendency it experiences to turn is measured by a spring
which is made to oppose this tendency keeping the coil at the zero point. The
angle through which the spring is turned measures the product of the strength of
current and the strength of the magnetic field, thus the ratio and the product of
the strength of current and of field being measured at the same time the value of
the current is obtainable.
The following is the mathematical solution of the problem.
Let C be the strength of current, H the strength of the permanent magnetic.
field (due to the Earth etc) R the mean radius of the coil, n the number of turns,
T the turning "moment" due to a twisting of the spring through one degree:
(formed once for all by allowing a mass of known moment of inertia to vibrate
when suspended in the place of the coil;) the deflection by the current of the
needle from its position of equilibrium, and t the deflection of the spring required
to bring the coil to zero.
For two simultaneous deflections & and 6 we have :-
When is the ratio of the circumference of a circle to its diameter the expression
under the first root contains nothing but quantities which are constant for a given :
instrument and are also measurable...
1
The expression under the second root contains the two variable quantities to
be read when the instrument is used. Consequently my instrument measures the
current absolutely.

page_text: In order to simplify the calculation I mark on the dials which shew respectively
the deflection of the needle and the torsion of the spring, the logarithins of the
tangents of the degrees, and the logarithms of the degrees.
In using the instrument the numbers on the dials are simply read off, added
together, and their sum, on an accompanying table gives the value of the current.
In one form of galvanometer according to my invention the coil is suspended
from a part of the framing and at its lower portion carries a spiral spring connected
to a spindle upon which is a dial plate near the centre of the coil, and below the
dial over which the needle moves. As a current which would produce a readable
deflection of the coil would deflect the needle too far, I balance a part of the effect
of the coil upon the needle by a second coil hereinafter called "the balancing coil
which is capable of adjustment about a fixed horizontal and also about a vertical
line.
"
To destroy the reaction of the needle upon the suspended coil, I fix near it a
second magnet whose effect can be varied by adjustment. A convenient plan is to
form it of two polarized bars connected like the halves of a pair of scissors, and
then its power can be regulated by varying the angle of the bars until it exactly
counterbalances the needle. The holder in which the needle hangs and to which
the adjusting magnet and the dial are fixed is turned by a milled head in taking a
reading, at the same time that the torsion spring is turned by a second milled head.
The needle dial is turned until the needle occupies the same position on it that
it did before the current flowed, and the angle 4 is read off by a fixed pointer
which also serves at its lower end as the pointer for the other dial. The current is
led to the suspended coil by mercury cups.
By permanent magnets, or electro-magnets excited by the current to be measured
or otherwise, and suitably arranged to produce a uniform magnetic field at the
place where the coil hangs (or to produce on the needle and on the suspended coil
the same effect as a uniform field would produce) a stronger field can be obtained
than that due to the earth. And if such an artificial field be used with the instru-
ment, the dimensions of the coil may be considerably reduced, at the same time
that part or the whole of the balancing coil may be dispensed with.
In another form of my galvanometer I arrange the spring of known strength to
connect the needle with the suspended coil, instead of to connect the suspended coil
with the framework of the instrument.
The constant of the instrument is then determined in the following manner :—
The suspended coil is clamped to the framework-The moment exerted by the
coil (with the current flowing through it) on the needle (when the latter hangs in
the plane of the coil) is then measured by the spring-from this I obtain the
following equation:-
Crªn Cos α =
= M Sin a
from which
M tan a
C =
from these two equations the moment of the needle comes to be :-
M=

page_text: The value of M obtainable in this way, need only be found occasionally, as it
will remain fairly constant.
Thus, keeping the needle clamped to the coil, as described above, a current can
be measured by simply allowing it to deflect the coil through an angle a from the
meridian-The equation being:-
Current
-
tan ax Constant
In this, as in the other form of my galvanometer, the magnetic field can be
strengthened, and the dimensions of the instrument consequently reduced, by using
permanent or electro magnets, as described above.

page_text: I, ARTHUR PRINCE CHATTOCK, late of No 2 Armstrong Terrace, Charlton, in the
County of Kent, now of 21 Annandale Road, East Greenwich in the same county
Electrician, do hereby declare the nature of my invention for "IMPROVEMENTS IN
GALVANOMETERS," and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:-
My invention has for its object improvements in galvanometers and relates to
that type known as the tangent galvanometer. The tangent galvanometer consists
of a circular coil of wire of one or more turns at the centre of which is suspended a
small magnetic needle. On a current of electricity passing through the coil, (when
the plane of the latter is in the magnetic meridian) the needle is deflected and the
tangent of the angle of deflection measures the ratio of the strength of current to
the strength of the permanent magnetic field wherein the needle hangs.
Now in a galvanometer according to my invention, the coil (called hereinafter the
swinging coil) is suspended as well as the needle and the tendency it experiences to
turn is measured by a spring which is made to oppose this tendency, keeping the
coil at the zero point. The angle through which the spring is turned measures the
product of the strength of current and the strength of the magnetic field. Thus,
the ratio and the product of the strength of current and of field being measured at
the same time, the value of the current is obtainable.
The following is the mathematical solution of the problem :-
Let C be the strength of current, H the strength of the horizontal component of
the permanent magnetic field (due to the earth etc) R, the mean radius of the coil
the coil being wound so that the ratio of the number of layers of wire, to the
number of wires per layer is as 11 to 9. R₂ the square root of the mean of the
squares of the radii of the separate layers in the coil. N the number of turns on
the coil, T the turning "moment" due to a twisting of the spring through unit
angle (found once for all by allowing a mass of known moment of inertia to vibrate
when suspended in the place of the coil), 6 the deflection by the current of the
needle from its position of equilibrium, and t the deflection of the spring required
to bring the coil to zero. For two simultaneous deflections, t and 8 we have:-
where is the ratio of the circumference of a circle to its diameter.
The expression under the first root contains nothing but quantitities which are
constant for a given instrument and are also measurable.
The expression under the second root contains the two variable quantities to be
read when the instrument is used. Consequently my instrument measures the
current absolutely.
It is only necessary to make this double measurement once for any given strength
of magnetic field in which the instrument may be placed; for, the "constant" of
the instrument having been determined thereby, currents may be measured by
simply using the instrument as an ordinary tangent galvanometer.

page_text: In order to simplify the calculation of the current value I mark on the dials which
shew respectively the deflection of the needle and the torsion of the spring, the
logarithms of the tangents of the degrees and the logarithms of the degrees. On
the needle dial plate are also marked the natural tangents of the degrees and the
degrees themselves. In using the instrument the numbers on the logarithm scales
are simply read off, added together, and their sum on an accompanying table gives
the value of the current
The "constant" of the instrument, referred to above, is determined as follows:-
Having measured any convenient current in the above manner I adjust the
deflection of the needle by means of a controlling magnet supplied on the instru-
ment (as in the Obach Tangent Galvanometer) until the tangent of the deflection is
some convenient multiple of the value of the current flowing-The "constant" of
the instrument is then the quotient of this current by the tangent of the deflection
it produces, and values of other currents may be obtained by simply multiplying.
the tangents of the deflections they produce by this " constant." My instrument is
thus a direct reading instrument.
"
In one form of galvanometer according to my invention the swinging coil is sus-
pended inside a hollow ring to protect it from air currents. At its lower portion it
carries a spiral spring, of German silver or other non-magnetic substance, connected
to a spindle which carries the dial plate upon which I mark the logarithms of the
degrees. In grooves round the circumference of the hollow ring in which the
swinging coil" hangs I wind wire in such a manner that, on a current being sent
through it and the "swinging coil" in opposite directions, the needle at the centre
of the instrument is unaffected. This coil is called hereinafter the " balancing coil."
The perfect adjustment of this "balance" is achieved by leading the current a few
times round the needle through a horizontal coil which is capable of being tilted
about a horizontal axis.
This coil is connected in series with and forms part of the "balancing coil." It is
tilted in one direction or the other until the balance is obtained.
Further, I wind round the hollow ring coils of wire which have effects on the
needle respectively of 10th Tooth 1000th &c of the effect of the balancing coil for any
given current. The purpose of these coils is to increase the range of the instrument.
They are adjusted to the exact values of 10th Tooth 1000th &c by means of hori-
zontal adjusting coils as in the case of the "balancing" coil. It is convenient to
make these horizontal adjusting coils concentric, to make the suspended needle their
common centre, and to enclose them and the needle in the box which protects the
latter from air currents. The ends of all these separate coils are best brought to
brass blocks on a connection board and connected up as required by plugs. When
more than one of these coils are used, they are connected in series.
To destroy the re-action of the needle upon the "swinging coil" I fix near it a
second magnet whose effect can be varied by adjustment. A convenient plan is to
form it of two polarized bars connected like the halves of a pair of scissors and then
its power can be regulated by varying the angle of the bars until it exactly counter-
balances the needle. This fixed magnet is however, not absolutely necessary. The
effect of the needle on the coil is exceedingly small, and, except when very great
accuracy is required, is quite negligible. It can be dispensed with even in the latter
case, by simply turning the needle (clamped) parallel with the axis of the coil, while
the reading on the latter is being taken, and afterwards unclamping the needle and
measuring its deflection. The holder in which the needle hangs, and to which the
adjusting magnet and the needle dial are fixed is turned by a milled head in taking
a reading. The torsion spring, when in use, is turned by a second milled head.
The needle dial is turned until the needle occupies the same position on it that it
did before the current flowed, and the angle is read off by a fixed pointer which
also serves at its lower end as the pointer for the other dial.
The current may be led to the swinging coil by mercury cups, or, if very fine.
wire is used on the coil, through weak springs.

page_text: The controlling magnet mentioned above is at right angles to and turns about a
horizontal axis which is coincident with the axis of the "swinging coil."
Referring to the accompanying sheet of drawings Fig 1 is a half sectional
elevation of my improved galvanometer and Fig 2 is a similar view at right angles
to Fig 1. A is the needle provided with a damping wing of mica. It is suspended
by a fibre and carries an index B to shew its movements. This index moves under
the scale C which is marked in log. tan. degrees in natural tan degrees and in degrees.
The scale can be turned by the milled head D, and is read by means of the fixed
index E. F is the swinging coil suspended at G and adjusted to zero by the
spring H and milled head H. The angular movement of the spring is read off the
scale K by the index E, and the zero point is determined by the index E¹. The
current enters and leaves the swinging coil at the mercury cups I and I'. J is the
balancing coil wound on a hollow ring. J is an adjusting coil (forming part of the
balancing coil) and can be tilted or inclined upon its axes J. The other coils of
values th&c of the balancing coil are not shewn in the drawing. They are wound
in the same grooves as the balancing coil and are in every rospect similar to it
except in their effect on the needle,-this effect being proportional to the number of
turns of wire and inversely proportional to the radius of the wire. L is the
connection plate provided with terminals for the swinging and balancing coils.
M. is the controlling magnet. N is the adjusting magnet.
When the galvanometer leaves the instrument-maker's hands the following
adjustments are necessary :-
The instrument (without the controlling magnet) is set up and levelled so that
the needle stands at zero and swings freely. The " swinging coil" being then
parallel to the needle will hang in the magnetic meridian. The controlling magnet
is now put on and its position adjusted until the needle is unaffected whether the
magnet is vertical or horizontal (In the latter case its north-seeking pole must point
southwards.) The magnet is next taken off and the needle deflected to about 45°
by means of a current in the coils or by a second magnet. The controlling magnet
on being replaced is turned to such a position that the deflection of the needle is
unaltered. In this position the magnetic axis of the controlling magnet is vertical
and to this position it must always be brought before the constant of the instrument
is determined (unless it be taken off altogether.)
"
Next; the magnet (if there be one), used for counterbalancing the effect of the
needle on the "swinging coil" should be adjusted. The needle (clamped) is first,
turned so as to lie along the axis of the "swinging coil," a current is sent through
the latter, which is then brought to zero by the torsion of the spring. The needle
is now turned through a right angle, and the magnet adjusted by varying the
angle between the two bars of which it is composed until the "swinging coil"
again stands at zero.
After this the "balancing coil" should be adjusted to destroy the effect of the
"swinging coil" on the needle, by sending through the two coils the strongest
current they are designed to carry (the current flowing in opposite directions
through the two coils) and by adjusting the horizontal coils mentioned above
as being in the circuit of the "balancing coil" until the needle romains in the same
position whether the current flows or not.
When these adjustments are finished the other coils on the hollow ring can be
brought to their proper values of 10th 11th &c of the "balancing coil" by tilting
their respective horizontal coils, rising at the same time some other galvanometer
in the circuit which will give values proportional to the currents flowing in the
said coils,
In making the preliminary measurement of current for the purpose of finding
the "constant" of the instrument the balancing coil is arranged in series with the
swinging coil so as to oppose it; and one of the other coils of 16th or 7th the
strength of the swinging coil is also arranged in series with it to deflect the needle.
The reason of this is that a current which would produce a readable deflection
in the torsion spring would be far too strong to give a convenient deflection of the

page_text: 2 tun a
and consequently the value of the current is obtainable in absolute measure.
The value of M need only be found occasionally as it will remain fairly constant.
In using the instrument the needle is kept clamped to the "swinging coil" and
a current is measured by simply allowing it to deflect the coil through an angle a
from the meridian, the equation being:
The advantage of this form of my instrument over the one described above lies
in the fact that the strength of the magnetic field may vary without affecting the
accuracy of the readings, so that no preliminary reading is necessary to determine
the "constant" as in the other form of the instrument. Any error due to a change
in the direction of the field can of course be eliminated by taking two readings with
the current flowing in opposite directions.
A convenient arrangement of the second form of my instrument is to suspend
the "swinging coil" as before within a hollow ring, making the latter moveable
about a vertical axis through its centre and turning it so as to follow the movements
of the "swinging coil" when in use.
By arranging that the "swinging coil" always occupies the same (zero) position
in the hollow ring, the turning of the latter is equal to the deflection of the
"swinging coil" and a dial fixed to the ring gives this deflection conveniently by
means of a pointer fixed to the base of the instrument. The dial is graduated in
degrees and tangents of degrees.
The "constant" being proportional to the sine of the angle which the magnetic
axis of the needle makes with the vertical, we can, by turning the needle about the
•

page_text: axis of the "swinging coil," arrange that the constant shall be some multiple of 10
and thereby make the instrument direct reading on the tangent scale.
This form of the instrument is best made with the needle large and its magnetic
moment multiplied by the sine of its inclination to the vertical a little less than
that of the "swinging coil" when carrying its maximum current. In this case the
value of that part of the "constant" which depends on the linear dimensions of the
instrument must be found by comparison with a standard galvanometer as it
cannot be accurately calculated because the positions of the poles of the needle are
not known. The needle should be made of a bundle of steel wires in order that its
magnetic axis may not alter its position with regard to its geometrical axis.
An instrument constructed according to the first modification herein described
will by itself measure currents of electricity absolutely, that is, without reference to
any standard quantities other than those of length, mass, and time. Also it will
measure the strength of the magnetic field in which it is placed and can therefore
be used as a magnetometer.
An instrument constructed according to the second modification is independent of
the strength of the magnetic field in which it is placed.
In either modification the values of current given by the instrument are
accurately proportional to the currents they represent; there is no error from
twisting of the suspending fibres, for they are not twisted; and, although depending
on the strength of a spring for the absolute value of the constant, this spring need
only be used at one particular twist, in which position its torsional rigidity can be
accurately measured.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is
1. The use for measuring currents of electricity of a galvanometer wherein the
ratio and the product of the strength of the current and the strength of the magnetic
field are measured simultaneously by the deflection of a needle, and by the moment
of a swinging coil substantially as described.
2. In a galvanometer the use in combination of a magnetic needle, a swinging
coil which is always brought back to zero in making an observation, and stationary
coils by which a part of the influence of the current upon the needle is neutralised,
substantially as described.
3. In a galvanometer wherein the effect of the current upon the needle is
neutralized by a balancing coil, forming the said balancing coil in two parts, one of
which can be tilted or inclined to effect a perfect balancing, substantially as described.
4. In a galvanometer comprising a magnetic needle and a swinging coil, the use
of a second magnet to destroy the reaction of the needle upon the swinging coil
substantially as described.
5. The use for measuring currents of electricity of a galvanometer wherein the
strength of the current is measured by the deflection of a magnetic needle and a
swinging coil clamped together and by the moment exerted by the coil on the
needle substantially as described.
6. In a galvanometer the combination with a magnetic needle of a swinging coil
the two being connected substantially in the manner described.
7. The galvanometer, constructed, arranged and operating substantially as
described with reference to the drawings.
Dated this 10th Jan of Dambau 1001.