Improvements in Electricity Meters.
Year: 1891

page_text: Date of Application, 1st Jan., 1891
Complete Specification Left, 30th Sept., 1891-Accepted, 7th Nov., 1891
PROVISIONAL SPECIFICATION.
Improvements in Electricity Meters.
I, GEORGE HOOKHAM of Nos. 7 & 8 New Bartholomew Street, Birmingham
Manufacturer do hereby declare the nature of this invention to be as follows:-
Referring to the Complete Specification of Letters Patent granted to me and
numbered 4225/87 I there describe a disc or cylinder or other figure of revolution
rotating as the armature of a motor in a magnetic field and also having Foucault
currents generated in it in such a way that the rotations of the disc or cylinder are
proportional to the electrical current passing through it. If contact for introducing
the current to be metered be made at the edge of the disc by means of mercury, in
time, even though the mercury may be as pure as it is possible in practice to obtain
it, so much mechanical resistance is set up that the disc will not start with small
currents, unless a very powerful field is employed. In order to reduce this resistance
I entirely immerse the disc in mercury. I may place the field-magnet poles on one
side only of the spindle of the disc, and introduce the current by means of contact
with the mercury at the rim of the disc, & draw it off near the axis in such a
way that almost all the current flows between the iron pole-pieces. The pole-
pieces should be insulated from the mercury; the rim & central portion of the
disc should be amalgamated, preferably by previously plating these parts with silver,
and the layers of mercury between the disc & the pole-pieces should be relatively
thin. If then the disc be of copper by far the greater part of the current will
pass through the copper, and only a percentage through the mercury. In order
to secure proportionality in the meter, the retardation due to the fluid friction of
the mercury should be negligible compared with that due to Foucault currents in
the disc. The field of the brake should be as strong, and the speed of the disc
as slow as possible. I may conveniently form the vessel for the mercury of plaster-
of-paris, the disc being coated with wax to the required thickness, or enclosed in
a removable casing, and the plaster cast round it. The casing is afterwards
removed, & a chamber of the proper dimensions is the result.
If I employ a shunt-electro-magnet, either this must be practically saturated, or
I employ separate constant magnets for the brake, the edges of the pole-pieces of
the field-magnet where Foucault currents are generated being bevelled, tapered,
or otherwise shaped so as to reduce such currents as much as possible: or I extend
the pole-pieces, nearly cutting through the extended portions from behind, so that
the magnetism is choked at the necks so formed. Then, although the intensity
of the central portion of the poles may vary with the E.M.F. of the circuit, the
edges, where the Foucault currents are principally formed, remain in a constant
state, and the meter becomes approximately a Watt-meter. This plan of choking
the magnetism so as to obtain constancy in part of a variable magnet is applicable
to other forms of meter besides that herein described. The pole-pieces of the
electro-magnet may form complete circles, and the brake may be formed with
constant magnets and a separate disc on the same spindle but outside the mercury.
The instrument may then be made, within its range, an accurate Watt-meter.
I may compensate for error introduced by the fluid friction by compound-winding
an electro-magnet, or by winding series-coils on the short-circuiting bar of a
permanent magnet, or by passing the main current through a resistance of platinoid
& shunting a portion of the current through a resistance of platinum, which is
heated by it.
Dated this 21st day of December 1800

page_text: DE BOITIATION.
Improvements in Electricity Meters.
I GEORGE HOOKHAм of Nos. 7 and 8 New Bartholomew Street Birmingham in
the County of Warwick Manufacturer do hereby declare the nature of my inven
tion and in what manner the same is to be performed to be particularly described
and ascertained in and by the following statement :-
In the Complete Specification of Letters Patent granted to me dated March 21st,
1887 No. 4225 I describe a disc cylinder or other figure of revolution rotating as
the armature of an electro motor in a magnetic field. The said armature has
Foucault currents generated on it in such a way that the rotations of the armature
are proportional to the electrical current passing through it. If contact for
introducing the current to be metered is made at the edge of the disc by means of
mercury after a time even though the mercury may be as pure as it is in practice
possible to obtain it so much mechanical resistance is set up that the disc
will not start with feeble currents unless a very powerful magnetic field is
employed. In order to reduce this resistance I immerse the disc wholly in
mercury. The field magnet poles may be placed on one side only of the spindle
of the disc and the current may be introduced by means of contact with the
mercury at the rim or edge of the disc and drawn off near the axis in such a way
that almost all the current flows between the iron pole pieces. The pole pieces
are insulated from the mercury and the rim or edge and central portion of the
disc are amalgamated and the layers of mercury between the disc and pole pieces
should be thin relative to the thickness of the disc. If the disc be of copper
by far the greater portion of the current will pass through the copper disc and
only a percentage through the mercury. In order to secure proportionality in the
meter the retardation due to fluid friction of the mercury should be negligible
compared with that due to Foucault currents in the disc. The field of the brake
should be as strong and the speed of the disc as slow as possible.
If I employ a shunt electro-magnet either this must be practically saturated or
separate constant magnets must be employed for the brake the edges of the pole
pieces of the field magnet where Foucault currents are generated being bevilled
tapered or otherwise shaped so as to reduce such currents as much as possible.
Or the pole pieces may be extended the extended parts being cut nearly through
from behind so as to choke the magnetism at the necks or contracted parts so
formed. In this case the intensity of the central portion of the poles varies with
the electro motive force of the circuit the edges where the Foucault currents are
principally formed remain in a constant state and the meter becomes approximately
a Watt-meter. This method of choking the magnetism so as to obtain constancy
in part of a variable magnet is applicable to other forms of meters besides that
herein described.
The pole pieces of the electro magnet may form complete circles and the brake
may be formed with constant magnets and a separate disc on the same spindle but
outside the mercury. The instrument may then be made within its range an
accurate Watt-meter.
The error introduced by the fluid friction may be compensated by compound
winding an electro magnet or by winding series turns on the short-circuiting bar of
a permanent magnet of the kind described in the Specification of the Patent
hereinbefore referred to.
I will now proceed to describe with reference to the accompanying drawings the
manner in which my invention is to be performed.
Figure 1 represents in elevation an electricity meter constructed according to my
invention and Figure 2 represents the same in plan. a is the rotating disc of
copper immersed in the mercury b in the circular trough c. The spindle d of
the rotating disc a turns upon centres at ee ; f is a case containing a permanent

page_text: compound magnet of the kind represented and described in the Specification
hereinbefore referred to. The pole pieces of the said magnet are marked f' ƒ'.
One of these pole pieces is immersed in the mercury immediately above the
disc a the other is situated underneath the trough e as represented. The part
of the disc a situated between the pole pieces is in a powerful magnetic
field. Both faces of the disc a are varnished excepting the part enclosed by a
small circle around the centre and the edge of the disc which parts are
amalgamated for the purpose of ensuring perfect metallic continuity for the
passage of the currents; g g are the conducting wires by which the electric
current enters and passes out of the trough c the said current passing mainly
through that part of the disc a situated between the pole pieces the pole
piece dipping in the mercury is insulated by being varnished or otherwise insulated.
Instead of permanent magnets in the case a shunt electro magnet may be
employed. The electric current to be measured being passed through the instrument
by the conductors g g the disc a commences rotating the permanent rate of rotation
of the said disc being directly proportional to the strength of the current neglecting
frictional error. The motion of the disc is transmitted through the worm wheel i
on the spindle d to an ordinary indicating apparatus.
Although in the arrangement shown in Figure 1 it is possible to make the
magnetic field so powerful that the effect of the Foucault currents practically masks
that of fluid friction yet this can only be done at an expenditure of magnetizing
force which in the case of permanent magnets is almost prohibitory and even with
an electro magnet is objectionable. Accordingly in the latter case I compound-
wind the shunt magnet with a certain number of series turns (to be found by trial)
tending to weaken the magnetic field and so increase the speed of the armature for
the larger currents. With permanent magnets this method is inadmissible as it
might interfere with the constancy of the steel magnets. In this case I use a
short-circuiting bar n of soft iron magnetically isolated from the permanent magnet
by a stratum of non-magnetic metal p such for example as brass at one or both
ends as shown in Figure 2. This bar is wound with series turns in a manner to
be determined by trial and the same effect may be produced thereby as in the
former case and the meter made proportional throughout its range. The same
general methods are applicable to the arrangement hereinafter described and
represented in Figures 5 and 6 but in this case the series turns are wound in an
opposite sense and when a short-circuiting bar is used the isolation of the bar
from the permanent magnet is not necessary. When the series turns q represented
in Figures 1 and 2 are employed one of the terminals g is connected with one
of the terminals ' the current entering and passing out by the other terminals g q'.
When I employ a shunt electro-magnet for the production of the magnetic field
in which the disc rotates I may adopt the arrangement shown in elevation in
Figure 3 and in plan in Figure 4.
72 represent the pole pieces of the electro magnet which in this case I prefer
to make curved and having a configuration of about three fourths of a circle as
best seen in the plan Figure 4. At the points ³ ³ that is near the ends of the
pole pieces f² I make a depression by which the sectional area of the said
pole pieces at that point so reduced that the portions of the pole pieces beyond
those points are of uniform magnetic strength although the other portions of the
said pole pieces may have a varying strength. The portions of the pole pieces
marked ff in the drawing produce a magnetic field which may vary in strength
by which magnetic field the rotation of the disc a is effected but the magnetic field
between the parts that is beyond the points f³ ƒ³ is constant and constitutes
he brake of the meter. The parts not described resemble those of the meter
Figures 1 and 2 and are marked with the same letters of reference.
3
In Figures 5 and 6 I have represented in elevation and plan respectively a form
f my invention in which the brake is external to the mercury trough c.
In this modification each pole piece of the magnet f is forked or divided the
parts of the divided pole pieces are marked respectively k l. The parts k of the

page_text: pole pieces are continuous or unbroken rings by which no Foucault currents are
produced. The rotating disc a immersed in the mercury in the trough cis
situated in and put in motion by the magnetic field between the parts k k of the
pole pieces the other parts of the pole pieces by virtue of the depressions
produce a constant magnetic field in which the brake works. This brake consists
of a flat ring m carried by arms fixed on the spindle d of the disc a.
In this modification of my invention I prefer to make the parts k k of the pole
pieces circular and of the same size and concentric with the disc a which rotates
between them. I prefer to make the conductor g in metallic connection with a
ring in the mercury trough c so as to furnish a passage for the current all round
the edge of the disc. The arrangement Figures 5 and 6 differs in no essential
respect from the other forms of my invention.
Having now particularly described and ascertained the nature of my invention
and in what manner the same is to be performed I declare that I claim as my
invention of "Improvements in electricity meters
First In an electricity meter actuated by an electro-motor and in which the
armature is a disc cylinder or other figure of revolution the use of contracted pole
pieces of the kind represented and the method of directing the current flowing
between them in the manner herein before described and illustrated in the accom-
panying drawings.
Secondly-In an electricity meter actuated by an electre-motor and in which
the armature whether a disc cylinder or other figure of revolution is entirely
immersed in mercury the amalgamation of those portions of the armature at which
the current is introduced and is drawn off for the purpose of maintaining the
contact resistances constant in the manner hereinbefore described and illustrated in
the accompanying drawings.
Thirdly- -In an electricity meter actuated by an electro-motor the partial cutting
through of the pole pieces in order to maintain constant magnetic strength at the
extremities of the pole pieces in the manner herein before described and illustrated
in the accompanying drawings.
―
Fourthly In an electricity meter actuated by an electro-motor and in which
the armature whether a disc cylinder or other figure of revolution is entirely
immersed in mercury compensating for the fluid friction error by compound-winding
the magnets or winding series turns on a bar of iron partially short-circuiting the
magnets in the manner hereinbefore described and illustrated in the accompanying
drawings.
Fifthly The arrangement or combination of parts of an electricity meter
herein before described and illustrated in Figures 5 and 6 of the accompanying
drawings combining a constant magnetic field in which the brake works with a
variable magnetic field in which the armature works both fields being obtained
from the same magnet.
Dated the 30th day of September 1891.
W. T. WHITEMAN,
Agent for the Applicant.