mproved Ammeters and
Year: 1887

page_text: Date of Application, 28th May, 1887
Complete Left, 27th Feb., 1888
Complete Accepted, 4th May, 1888
mproved Ammeters and
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, Professor of Mechanics at the
City and Guilds of London Technical College Finsbury, Doctor of Science do hereby
declare the nature of this invention to be as follows :-
:-
One form of our instrument consists of one or more short straight fine parallel wires
made of conducting, or semi conducting, material fastened at both ends to supports,
one or more of which is insulated, fixed inside a short, horizontal tube. The wire, or
wires, are attached at their middles by means of a stirrup and a fine wire connection
to one end of one of our magnifying springs under tension described in our patent
specification number 2156 of 1883. The other end of the spring is fixed so that its
axis is at right angles to that of the wire, or wires. On passing a current through the
wire, or the wires joined electrically in series or in parallel, they stretch slightly, causing
a pointer attached to the end of the magnifying spring to rotate, and the amount of
rotation measures the current passing through the instrument without the employment
of levers, wheels and pinions, or other multiplying gearing. When the instrument.is
used as an Ammeter we attach a shunt to the wires, the shunt consisting of a inetallic
rod or strip which is longer than the wires, and so does not heat. Sometimes in such
cases, and generally when our instrument is required to give a maximum reading for
a small potential difference maintained at the terminals, the main electrodes of the
instrument; instead of being attached simply to the ends of the stretched parallel wires,
are each attached alternately to two or more points along the length of the wire, or
wires, by means of very flexible connections so that as much current may pass through
each section of each wire as is required to heat it sufficiently. When the instrument
is used as a Voltmeter the material we employ for the wires depends on the range
desired, for instruments of low range we use the better conducting materials, and for
instruments of high range inferior conductors such as filaments of carbon.
In another form of our instrument we replace the short straight parallel wires by
short spirals of fine wire arranged with their axes parallel to one another and attached
at their middles by means of a stirrup and a fine wire connection to one end of one of
our magnifying springs as above described.
CM1007
"

page_text: COMPLETE SPECIFICATION.
Improved Ammeters and Voltmeters.
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 :-
One form of our instrument consists of one, or more, short straight parallel wires W,
figure 1, made of conducting, or semi conducting, material fastened at both ends to
supports A, B, one or more of which is insulated, fixed inside a short horizontal
metallic tube T T, figures. 1 and 2. The wire or wires are attached at their middles
by means of a stirrup X to one end of one of our magnifying springs described in our
English patent specification No. 2156 of 1883, more especially from lines 17 to 52
page 8. The other end of the spring is fixed by means of a fine wire G K, or other
flexible connection, to a support S. An initial tension is given to the magnifying
spring, causing the two halves of each wire to be slightly pulled out of the straight
line. On passing a current through the wire, or wires joined electrically in parallel or
in series the wire or wires are slightly warmed, and therefore stretch slightly, causing
the pointer P attached to the end of the magnifying spring to rotate through a large
angle without the employment of levers, wheel and pinion, wheel and axle, or other
ordinary multiplying gearing, and this rotation of of the pointer measures the current
passing through the instrument.
When many wires are employed it is desirable, for the purpose of facilitating the
cooling, to put pieces of metal between the various wires, but not touching them, the
pieces of metal being attached to the outer framework of the apparatus. Or the wires
may be put two or more in separate metallic tubes, the centres of all the wires being
connected by means of a stirrup with one end of one of our magnifying springs. As
this, however, leads to the stirrup being very wide when many wires are employed, it
is better, instead of putting all the tubes parallel to one another, to fix them so that the
two halves of each tube form the two opposite spokes of a wheel. In that case the
wires may be conveniently attached to the stirrup where they cross one another. Or
the wires W may be arranged like the spokes of a bicycle wheel, figure 3, passing
round pins A, B, C, D, &c., some or all of which are insulated, the sides of the various
tubes now forming the stationary pieces Y. Y. Fig. 3. The stirrup in that case takes
the form of the hub H of the wheel, and figure 4 shows the complete instrument with
the wires arranged in this way. The expansion of the wires forming the spokes of the
wheel, which takes place on their becoming warm, when the current passes through
them, allows the spring to draw the hub H out of the plane of the rim R of the wheel,
and so produces a rotation of the pointer attached to the end of the spring, and this
rotation of the pointer measures the current passing through the wires.

page_text: In figure 5 two such bicycle wheels A B C, A¹ B¹ C¹, are employed, the hub H
of the one being attached to one end of the magnifying spring the other end of the
spring being attached, by means of the fine wire, or other flexible connection, to the
support S which is itself carried by the hub H of the other bicycle wheel. And the
rotation of the pointer will be caused by the expansion of all the wires of both wheels.
U U is a metallic screen to assist the cooling, and projections V. V. which aro
seen in figure 3, but for the sake of clearness are omitted in figure 5 pass between
each pair of wires.
Sometimes the attachment of the wires to the rim of the wheel is made by a flexible
strip of metal, or other spring, so that in constructing the instrument there shall be an
indication of two much tension in any wire, and these tensions may be equalised in all
the wires by obvious adjustments.
But whatever arrangement of attachment of the wires be employed, the gist of our
invention consists in measuring, by means of one of our magnifying springs attached
to the centres of the wires as previously described, the increase of say of the wires
when the current passes through them.
With any of these forms of instruments we may employ a commutator to enable
various numbers of the wires to be joined electrically in series or in parallel for the
purpose of varying the sensibility of the instrument in a known ratio. And as the
direction of the current through any one of the wires is unimportant as it is the
expansion of the wires by rise of temperature that is utilised it is not necessary that
the current should pass through any particular wire in the same direction when the
wires are in parallel and in series. Hence the commutator attached to the forms of
instrument described in this specification may be of much simpler construction
than that employed in our well known commutator ammeters and voltmeters where
the magnetic action of a current in deflecting a magnetised needle is the property
employed.
When the instrument is used as a voltmeter the material we employ for the wires
depends on the range desired for the instrument; for instruments of low range we use
the better conducting materials, while for instruments of high range we sometimes
use carbon filaments.
When the instrument is used as an ammeter we attach a shunt to the wires, the
shunt consisting of a rod, or strip, or sheet which is longer than the wires, and so does
not heat. Sometimes in such cases, and generally when our instrument is required to
give a maximum reading for a small potential difference maintained at the terminals,
the main electrodes of the instrument, instead of being attached simply to the ends of
the stretched wires, are each attached permanently to two or more points along the
length of the wire or wires, by means of very flexible connections so that as much
current may pass through each section of each wire as is required to heat it
sufficiently.
When the temperature of the instrument is varied as a whole either by changes in
the atmospheric temperature, or by a prolonged passage of the current through the
instrument both the zero and the sensibility of the instrument may vary. But by a
proper selection of the material or alloy with which to construct the framework carrying
the stretched wire, or wires, the former temperature error can be made to balance the
latter for a definite part of the scales. We, therefore, construct the framework of different
materials, or alloys, in different cases. When it is desired that the framework shall
have the same coefficient of expansion as the stretched wires, and when this would add
greatly to the cost of the instrument in consequence of the stretched wire or wires
being composed of an expensive material, we sometimes construct the framework in
the following way :-To two opposite parts of the caso or other convenient part of the
instrument are attached two fairly strong springs tending to draw two pieces of slate
or other material away from one another; these pieces of slate are held together
against the tension of the springs by two wires of the same material as the stretched
wires but stronger. And the stretched wire or wires which are attached at their
middle, or middles, to the end of the magnifying spring are also attached at their ends
to the pieces of slate, so that the distance between the supports carrying the ends of

page_text: the latter wire, or wires, simply depends on the length of the former. If then a current
be sent through the latter wire or wires, but not through the former, there will be a
deflection of the pointer, whereas if both sets of wires be of the same length an
alteration of temperature of the instrument as a whole will not vary the length of the
wire, or wires, attached to the magnifying spring relatively to the distance apart of the
supports to which the ends of the wire, or wires, are attached.
Having now particularly described and ascertained the nature of our said
Invention, and in what manner the same is to be performed, we declare that what
we claim is :
1. The combination of our magnifying spring with a wire, or wires, made of
conducting or semi conducting material for the purpose of measuring currents, or
potential differences, by the extension of the wire, or wires, produced by the passage
of the current.
2. The combination of a commutator with an instrument composed of many wires
employed for the measurement of currents, or potential differences, by the extension of
the wires produced by the passage of the current.
3. The forms of instrument substantially as herein described.