Improvements in Electric Measuring Instruments.
Year: 1893

page_text: Date of Application, 7th Oct., 1893
Complete Specification Left, 9th July, 1894-Accepted, 11th Aug., 1894
PROVISIONAL SPECIFICATION.
Improvements in Electric Measuring Instruments.
I, BENJAMIN DAVIES of University College, Liverpool in the County of
Lancaster Engineer, do hereby declare the nature of this invention to be as
follows:-
This invention has for its object an electric measuring instrument, and is so
designed that it may be used as an ammeter, a volt-meter, or a watt-meter.
The instrument may be considered as a simple bar magnet, with a coil capable
of rotating around one pole of it, the axis of the coil being in the plane of rotation,
and at right angles to the axis of the magnet. The current to be measured flows
through the movable coil, the latter carrying a pointer by means of which the
motion is observed. The magnet may be either a permanent steel one, or an
electro-magnet, and the entire magnetic circuit may be composed of any magnetic
substance and air combined, or air only. The instrument may be so constructed
as to produce a uniform field of force always at right angles to the direction of
motion of the coil, whatever the deflection may be. This construction makes the
angle of deflection proportional to the ampères, the volts, or the watts, depending
on the nature of the instrument. By my arrangement there is a wide range of
action, the scale extending from degrees to about 340 degrees of arc.
nature of the scale may be varied to suit any special work.
The
One form of the instrument-that for measuring small voltages-consists of a
permanent magnet having a large circular pole piece of magnetic material all
enclosed in a circular vessel of a similar material. One end of the magnet is fixed
eccentrically to the pole piece, the other end fixed in a similar manner to the
bottom of the circular vessel. The diameter of the pole piece is made rather less
than the inner diameter of the vessel, thus leaving an annular air gap wherein the
coil may move. The coil is so suspended or pivoted as to cut the lines of force in
one direction only.
Dated thin esh dan of
tabo 1002
WM. P. THOMPSON & Co.,
Of 6, Lord Street, Liverpool, Patent Agents for the Applicant.
COMPLETE SPECIFICATION.
Improvements in Electric Measuring Instruments.
I, BENJAMIN DAVIES of University College, Liverpool in the County of
Lancaster, Engineer, 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 :-
This invention relates to an electric instrument which may be used for measuring
electric power, electric current or electric difference of potential.
While it might be possible to arrange the apparatus with two scales so as to
indicate either volts or ampères, I prefer to construct each apparatus for one of
these uses only.
The apparatus is best described by aid of the accompanying drawings, in these,
Figure 1 shews a vertical section through the centre of the instrument.
Figure 2 is a section through X Y and also showing the wooden base into which
are fixed the terminals of the instrument.

page_text: Figure shows the art and pointer.
In these-
BCD and B'C' D' are two halves of a cylindrical iron box fastened together
at X Y so as to form an enclosure. To each end is firmly fixed a magnet S N.
At the centre of the enclosure is the pole piece A which is firmly held in position
by one or the other of the magnets.
The box and
The steel
Along the
The magnetic circuit therefore consists of two parallel paths, namely, NA BCS N
and NA B'C' S N. This circuit is all iron except the air gap G.
the pole piece may be made of any convenient magnetic material.
magnets N S are magnetized in position by means of the coil Z Z.
axis of the box is placed the spindle which carries the moving coil H.
The only force urging this coil to move exists in the annular air-gap. This force
is proportional to the product of the intensity of magnetic induction in the gap
and the number of times the current traverses the gap. The intensity of induction
may be made to vary from point to point or be made quite uniform by simply
giving the pole-piece A the desired shape. In this way, a scale of any nature may
be obtained.
Figure 1 represents the instrument as a voltmeter. The current enters by the
terminal T and is conveyed along a wire to L', from which it passes through
the spiral spring R', the spiral spring being fixed to a nave J' mounted on the
insulator I. From the nave, the current enters the coil H by a wire. It leaves
the coil by a wire placed alongside the upper portion of the spindle and is then
conveyed to the nave J and the spiral spring R. From here it is conveyed to the
resistance coil V of few thousand ohms at the base of the instrument, and thence
to the other terminal.
The rest of the instrument is evident from the figures. K is the brass case
enclosing all, and which carries on top the scale and pointer and glass cover.
V is the pointer. M M' are insulators. Q Q' are small holes in the iron box
through which conductors may be placed in connection with the coil Z Z' for the
purpose of magnetising the system. N is the balancing device.
Since the magnetic circuit is nearly a "closed one, the sensitiveness of the
instrument is not liable to vary with time, on account of a decline in the strength
of the magnets.
From Figure 3 it is seen that deflections may be got up to about 300°. So a
very open scale is possible with a pointer of very moderate length. This also
means a long range of action; for instance, one volt and 140 volts may be easily
read on the same instrument.
In an ampère meter the coil H is to be placed in "parallel" with the main
current, and the auxiliary resistance U discarded.
For a power meter the magnetic circuit must be all of wrought iron and the air
gap G made wider. Then with the main current (or a portion of it) passing
through the coil Z Z and the shunt current passing through the coil H or vice versa,
the readings of the instrument will be proportional to electrical power. Since the
moving coil is entirely inside an iron box, the instrument is unaffected by any
ordinary external magnetic disturbances.
Further it will be observed that, by placing the zero exactly opposite the
magnet, positive and negative currents can be measured, each over an arc of 150°.
The coil, instead of being pivoted as above described, may be in many cases
more conveniently suspended by a metallic thread or ribbon.
The instrument may also be constructed without the upper half of the iron box
and the magnet which it carries.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
1. An electric measuring device in which a coil conveying a part or the whole
of the current is moved by variations of that current around one vole of one or

page_text: more magnets in combination with a pointer and scale, one of them being stationary
and the other connected with the coil and moving with it.
2. An instrument for measuring the voltage ampèrage or power of an electric
current, consisting of a coil moving round a single magnetic pole and surrounded by
an iron box in close contiguity, forming with the magnet and pole a magnetic
circuit all iron except the air gap necessary for the coil to pass.
3. The instrument for measuring electricity substantially as described.
D
LT11001
WM. P. THOMPSON & Co.,
Of Liverpool, Birmingham, Manchester, Nottingham, and London,
Patent Agents.