Improvements in the Construction of Instruments for Measuring Electric Currents.
Year: 1894

page_text: Date of Application, 7th Dec., 1894
'Complete Specification Left, 8th Mar., 1895-Accepted, 13th Apr., 1895
PROVISIONAL SPECIFICATION.
Improvements in the Construction of Instruments for Measuring
Electric Currents.
We, FRANCIS HENRY NALDER, CHARLES WILLIAM SCOTT CRAWLEY, and
ALFRED SOAMES, all of 16 Red Lion Street, Clerkenwell, in the City of London,
Electrical Engineers, do hereby declare the nature of this invention to be as
follows:-
Our invention applies especially to Volt meters and Ampère meters and relates
to the structural arrangements of such instruments whereby their manufacture is
simplified and they are better adapted for use with high potentials.
The operating parts we completely enclose in a cast iron case which whilst it
safely encloses them also prevents magnetic interference with the indications of the
instrument.
The case is cylindrical and is closed in front with the exception of a window
through which the index and dial may be seen. The back or cover plate of the
case which is also of cast iron serves as a foundation to carry the mechanism which
however is completely insulated from the case.
For this purpose the back or cover plate is cast with bosses upon it. The
surfaces of the plate are rendered true and flat by grinding and a slab of vulcanised
fibre or vulcanite or other insulating material is fixed by screws against the bosses.
The bobbin on which the coil is wound is bolted to the insulating slab and so also
is a pillar which aids in supporting the dial. The same slab of insulating material
may also carry the binding posts or terminals of the instrument; these projecting
out of the case through an aperture provided in the back or cover.
In every case care is taken that the bolts and metal parts attached to the
insulating slab are sufficiently far removed from the metal of the case.
The
In place of employing a single insulating slab we may divide the insulation
into parts; the bobbin may have its back flange of insulating material and stand
directly upon three or other number of bosses provided on the back of the case
to serve for its support. The binding posts or terminals are in this case mounted
on a piece of insulating material separately fixed to the back or cover.
pillar supporting the dial plate may itself be of insulating material and be directly
fixed to the back or cover plate. The mechanism, which in itself is not novel
consists of two soft iron core pieces lying parallel the one to the other within the
central cavity of the bobbin. One of these core pieces is fixed, the other is carried
by a pivotted frame. When a current passes in the bobbin windings both core
pieces are magnetised and they repel each other, the moveable core picce being
driven away. When the current ceases gravity brings back the moveable core
piece to its original proximity to the fixed core piece. The pivotted frame is
suitably weighted and an indicating finger is attached to it. The bobbins are so
wound that poles are formed in the middle as well as at the ends of the core
pieces. This, in conjunction with the enclosing case of magnetic material, is an
effectual protection from external disturbance.
In order that the indications of the instrument may be easily read it is necessary
that the finger and the dial plate should be close to the window or opening in
the case.
This being so and in order to provide the electrical separation which is
necessary for safety we line the end of the case with a disc of glass and this serves
also to glaze the window or opening through which the dial plate is seen.

page_text: Also to more effectually separate the mechanism from the metal case and to
prevent damage when high potentials are used we apply a coating of vitreous
enamel to the interior of the iron case.
Dated this 7th day of December 1894. ·
FRAS. H. NALDER.
C. W. S. CRAWLEY.
ALFRED SOAMES.
Improvements in the Construction of Instruments for Measuring
Electric Currents.
We, FRANCIS HENRY NALDER, CHARLES WILLIAM SCOTT CRAWLEY and
ALFRED SOAMES, all of 16 Red Lion Street, Clerkenwell, in the City of London,
Electrical Engineers, 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 :-
Our invention applies especially to Volt meters and Ampère meters and relates
to the structural arrangements of such instruments whereby their manufacture is
simplified and they are better adapted for use with high potentials.
The operating parts we completely enclose in a cast iron case which whilst it
safely encloses them also diminishes magnetic interference with the indications of
the instrument.
The case is cylindrical and is closed in front with the exception of a window
through which the index and dial may be seen. The back or cover plate of the
case which is also of cast iron serves as a foundation to carry the mechanism
which however is completely insulated from the case.
For this purpose the back or cover plate is, cast with bosses upon it. The
surfaces of the plate are rendered true and flat by grinding and a slab of vulcanised
fibre or vulcanite or other insulating material is fixed by screws against the bosses.
The bobbin on which the coil is wound is bolted to the insulating slab and so also is a
pillar which aids in supporting the dial. The same slab of insulating material may
also carry the binding posts or terminals of the instrument; these projecting out of
the case through an aperture provided in the back or cover.
In every case care is taken that the bolts and metal parts attached to the
insulating slab are sufficiently far removed from the metal of the case.
In place of employing a single insulating slab we may divide the insulation into
parts, the bobbin may have its back flange of insulating material and stand directly
upon three or other number of bosses provided on the back of the case to serve
for its support. The binding posts or terminals are in this case mounted on a piece
of insulating material separately fixed to the back or cover. The pillar supporting
the dial plate may itself be of insulating material and be directly fixed to the
back or cover plate. The mechanism, which in itself is not novel consists of two
soft iron core pieces lying parallel the one to the other within the central cavity of
the bobbin. One of these core pieces is fixed, the other is carried by a pivotted
frame. When a current passes in the bobbin windings both core pieces are
magnetized and they repel each other, the moveable core piece being driven away.
When the current ceases gravity brings back the moveable core piece to its original
proximity to the fixed core piece. The pivotted frame is suitably weighted and an
indicating finger is attached to it. The bobbins are sometimes so wound that poles
are formed in the middle as well as at the ends of the core pieces. This in con-
junction with the enclosing case of magnetic material, is an effectual protection
from external disturbance.
In order that the indications of the instrument may be easily read it is necessary

page_text: The annexed drawings show Volt or Ampère meters constructed in accordance
with our invention.
Figure 1 is a front elevation of the complete instrument.
Figure 2 is a similar view with the front of the case removed.
Figure 3 is a rear elevation.
Figure 4 is a transverse section.
Figure 5 is also a transverse section and shows a modification.
Figure 6 shows the inner face of the cover plate belonging to this modification.
a and bare the two parts of the cast iron case; the front portion a is cylin-
drical; it is closed at the fore part with the exception of a window or aperture a¹
of sufficient dimensions to admit of the indications of the instrument being easily
read. a is a circular sheet of glass which both glazes the window and lines the
front of the case to separate it sufficiently in an electrical sense from the metallic
parts within. The rear portion 6 of the case is the foundation plate on which the
operative parts of the instrument are erected; it has a raised rim b¹ to which the
fore. part of the case fits; it also has bosses 6 to support an insulating slab
or slabs; and there is an aperture at 63 by which the electric leads can be brought
into the case without approaching too closely to the uninsulated metal. The rim
and the bosses of the cover plate are made true on the face by grinding or other-
wise. The whole of the interior surface of the case is coated with a vitreous glaze
or enamel. The two parts of the case are fastened together with screws.
case so made not only protects the working parts from mechanical violence but
also from magnetic disturbance. The glaze on the interior of the case affords a
safety from electric discharges through the case which would only be attainable
otherwise by increasing the dimensions of the case.
The
c in Figure 4 is a slab of vulcanite or other insulating material supported on the
bosses 6 and fixed by screws upon them. This slab forms a sort of bridge on
which the internal parts stand at a safe distance from the metal of the case; it also
covers and closes the aperture 6³. In Figure 5 the construction is somewhat
different, two insulating slabs being used as will be more fully described later on.
dande are the two leads by which the electric current is taken into the instru-
ment; they are brought through the opening 63 in the cover plate at a suitable
distance apart and by means of screws they are fixed to the slab c.
fis a brass bobbin on which are the windings of insulated wire; for an Ampère
meter a comparatively few turns of thick wire are provided whilst for a Volt meter
numerous turns of fine wire are provided; this however is as usual and is well
understood. The ends of the coil wire are soldered to the leads d and e. For
electrical reasons a slit or saw cut is made in the bobbin at f¹. Within the bobbin
there is a cylindrical cavity to receive the magnetisable parts viz.:-the small
stationary bar g and the smaller bar h which is attached to a pivotted axis
at ik is the indicating needle fixed on the same axis, and is a small weight
carried by the axis and adjustable for the purpose of regulating the instrument.
m is the dial plate with graduations to which the needlepoints; this plate is
attached to the outer flange of the bobbin f and is also supported by a pillar n
which is fixed into or upon the insulating slab c; this pillar may either be of metal
or of insulating material.
Figure 5 shows a somewhat different construction to that indicated in Figure 4.
Two separate slabs or pieces of insulating material care provided; on one of
them the bobbin is set and it forms the back flange of the bobbin, the metal
barrel of the bobbin being screwed into it.

page_text: The metal terminals e and fare fixed into and pass through the other piece of
insulating material c³. The slab c² which covers the aperture 63 in the foundation
plate rests directly on the edges of the plate around this aperture.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
1. A Volt or Ampère meter having a metal case consisting of a back or founda-
tion plate with bosses supporting a slab or slabs of insulating material on which the
operative parts of the instrument are carried at a safe distance from the metal of
the case and consisting further of a body closed at the fore part with the exception
of a window or opening which is glazed with a sheet of glass arranged so as also
to ensure sufficient electrical separation between the fore part of the case and the
internal parts.
2. A Volt or Ampère meter having a metal case and a slab of insulating material
carrying the bobbin and operative parts and supported by bosses within the case 50
as to maintain a sufficient separation between the case and the insulated parts,
such case having also an aperture covered and closed by a slab of insulating
material through which the electric leads are passed.
3. A Volt or Ampère meter having a bobbin the back flange of which consists
of a slab of insulating material and is supported by bosses within a metallic case
in such manner that the metallic parts of the bobbin and also the operative parts of
the instrument are maintained at a sufficient distance from the metal of the case.
4. A Volt or Ampère meter the operative parts of which are carried upon a slab
or slabs of insulating material contained within a metal case the interior of which
is enamelled to aid in maintaining a sufficient electrical separation between the
insulated parts and the metal of the case.
5. A Volt or Ampère meter having a metal case closed at the back by a cover
plate, on which plate two slabs of insulating material are fixed, one carrying the
bobbin and the operative parts and the other closing an aperture in the cover
and carrying the electric leads which pass through the slab to the interior of
the case.
6. A Volt or Ampère meter substantially as described and illustrated by the
annexed drawings.
Dated this 5th den of Mauch 1905
FRAS. H. NALDER.
C. W. S. CRAWLEY.
ALFRED SOAMES.