Improvements in Electric Measuring Instruments.
Year: 1895

page_text: Date of Application, 15th Mar., 1895
Complete Specification Left, 13th Dec., 1895-Accepted, 7th Mar., 1896
PROVISIONAL SPECIFICATION.
Improvements in Electric Measuring Instruments.
I ROOKES EVELYN BELL CROMPTON, Engineer, of Arc Works, Chelmsford
in the County of Essex do hereby declare the nature of this invention to be as
follows:-
My invention relates to that class of instruments which are used for the
measurements of differences of electrical potential, known as electrostatic voltmeters,
in which the movement caused by the attraction or repulsion between two sets of
metal plates or sheets which are electrically charged, is used to indicate the
difference of potential between them..
A base plate is provided of any suitable material conducting or non
conducting.
If made of conducting material the portions of the instrument which are charged
electrically are insulated from it.
On this base plate stand the fixed metal plates or sheets which are electrically
connected together in two sets insulated from one another and charged from the
two points the difference of potential between which it is desired to measure.
These fixed metal plates, which I shall hereinafter call cells, form portions of
two concentric cylinders.
The movable part of the instrument consists of a pointer attached to movable
metal plates or sheets hereinafter called vanes. These vanes are capable of rotating
through a portion of a circle on the same axis as that of the concentric cylinders
forming the cells and the vanes are set at such a distance from that axis that they
move between the cells in a suitable manner and without touching the side of the
cells.
The vanes and pointer are mounted in any suitable manner on an axle provided
with pivots, which pivots work in suitable bearings.
A scale over which the pointer moves and on which the indications of the
instrument are read is provided. Two metal plates nearly circular in form are
placed one on each side of the pointer parallel with the plane in which the pointer
moves, and electrically connected to the pointer but insulated from the case which
contains the whole instrument. Two fuses of any suitable form, being preferably
glass tubes, contain a fine wire. The terminals of the instruments are connected to
the ends of the fuses.
A stop pin arrangement is worked from the milled bead outside the case of the
instrument in such a manner that when the pointer is swung to the same end of the
scale as the stop pin, this pin can be raised by means of the milled head to confine
the pointer to that end of the scale and so prevent it from swinging violently about
when it is desired to move the instrument from one place to another.
The cells of the instrument are electrically connected in two groups every other
cell being connected in to one group and the alternate ones into the other
group.
The vanes, pointer and the metal plates are electrically connected to one group
of cells which are electrically connected by means of a fuse to one terminal. From
the other terminal there is electrical connection through the other fuse to the three
alternate cells forming the other group.

page_text: On connecting the terminals to any two points having a difference of potential
between them it will be seen that there will be the same difference of potential
between one set of cells and the vanes together with the other set of cells which
are connected with the vanes as between the two points whose difference of potential
it is desired to measure. Therefore the vanes will be attracted by those cells
which are at a different potential from themselves and repelled by those which are
at the same potential as themselves. There will therefore be a movement of
rotation of the vanes together with the pointer on their pivots. This movement is
resisted by the weight of the pointer itself, the effect of which may, if too great,
be reduced to any desired extent by an adjustable counter balance. The pointer
will therefore take up a position such that the two forces are in equilibrium which
act on it viz. that of gravity and of the electrical attraction and repulsion between
the vanes and cells.
To avoid false indications of the pointer due to the existence of an electric charge
on the glass window or on the case (the effect of which is to attract or repel the
charged pointer away from its proper position) either from some external cause or
by slow electric leakage from one or other of the terminals, the case of the
instrument may be electrically connected to one of the terminals preferably the
same terminal as that to which the pointer is connected, thus placing the case and
pointer in a fixed and definite electrical relation with one another. It is advisable
also in this case to cover the glass with a metallic foil electrically connected to the
case leaving only a small opening through which the scale may be observed; or
instead of the glass a metal front with a small opening for observing the scale may
be used. A preferable plan however is to screen the pointer and it is with this
object that I place it between the two metal plates, these two plates being as above
described in electrical connection with the pointer. The two plates should be of
such dimensions that as much of the pointer as possible is enclosed between them
leaving only a small portion of the end exposed to enable its position on the scale
to be observed. I prefer this plan of screening the pointer because it enables me
to dispense with the electrical connection of the case of the instrument to one of
the terminals and therefore the case may be handled with safety; or it may even
for further security be connected to earth.
I have found that by fitting pieces of insulating material such as ebonite or mica
within the cells I can modify the range of movement of the vanes and pointer to
get a scale, the divisions of which shall be more regular.
This regularity of scale divisions may also be varied by altering the shape of the
vanes or cells, or both. The cells aud vanes also need not form parts of cylinders
but may be flat so long as they are arranged at such a distance apart from one
another that the vanes in their motion do not thouch the cells.
It will be obvious that the number of cells and vanes may vary. For example
I may use 8 or 10 cells with vanes to correspond. Also two or more concentric
sets of cells may be used with concentric sets of vanes to correspond mounted on a
single axle.
Two or more sets of cells and vanes may be used set beside one another on one
axle.
I find it sometimes preferable to carry the vanes and pointer on knife edges
instead of on pivots.
When my instruments are used to measure high electro motive forces I prefer to
cover the terminals with ebonite or some other insulating material so that no part
of the instrument which is "in circuit" can be touched. A spirit level may be
provided by which the instrument can be accurately set up before using it.
Dated the 15th day of March 1895.

page_text: Improvements in Electric Measuring Instruments.
I ROOKES EVELYN BELL CROMPTON, Engineer, of Arc Works, Chelmsford,
in the County of Essex, 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 :-
My invention relates to that class of instruments which are used for the
measurements of differences of electrical potential, known as electrostatic voltmeters,
in which the movement caused by the attraction or repulsion between two sets of
metal plates or sheets which are electrically charged, is used to indicate the
difference of potential between them.
My invention will be made clear by the following figures:-
Fig. 1 is the front view of my electric measuring instrument (hereinafter called
"the instrument") with the dials removed.
Fig. 2 is a cross section of the same on the line X. Y of Fig. 1.
Fig. 3 is a front view with the dials in position.
Fig. 4 shows the attracting and repelling metal plates or sheets to a larger
scale.
Figs. 5 and 6 show (to an enlarged scale) one method of supporting the axis
which carries the indicating pointer.
Figs. 7, 8, 9 and 10 shew modifications in the form of attracting or repelling
plates or sheets.
Referring to Figures 1, 2, 3 and 4. A is a base plate of any suitable material
conducting or non-conducting. If made of conducting material the portions of the
instrument which are charged electrically are insulated from it.
On this base plate A stand the fixed metal plates or sheets which are electrically
connected together in two sets insulated from one another and charged from the
two points the difference of potential between which it is desired to measure.
These fixed metal plates, which I shall hereinafter call cells, form portions of
two concentric cylinders and are shewn at B.
The moveable part of the instrument consists of a pointer C attached to the
movable metal plates or sheets hereinafter called vanes D. These vanes are able
to rotate through a portion of a circle on the same axis as that of the concentric
cylinders forming the cells B and the vanes are set at such a distance from that
axis that they move between the cells in a suitable manner shown in Figs. 2 and 4,
and without touching the sides of the cells. The vanes and pointer are mounted
in any suitable manner on an axle provided with pivots, at E F which pivots work
in suitable bearings.
The scale over which the pointer C moves and on which the indications of the
instrument are read is provided shewn at G. H and J are two metal plates nearly
circular in forin placed one on each side of the pointer C parallel with the plane
in which the pointer moves, and electrically connected to the pointer but insulated
from the case K which contains the whole instrument. L, L are two fuses of
any suitable form, being preferably glass tubes as shewn in the drawing, containing
a fine wire.
The terminals of the instrument are shewn at M, and are electrically connected
to the ends of the fuses..
N is a stop pin arrangement to be worked from the milled head O outside the
case of the instrument in such a manner that when the pointer C is swung to the
same end of the scale as the stop pin this pin can be raised by means of the milled
head to confine the pointer to that end of the scale and so prevent it from
swinging violently about when it is desired to move the instrument from one place
to another.

page_text: The cells B of the instrument are electrically connected in two groups every
other cell being connected into one group and the alternate ones one into the other
group. For example if there are six cells as shown in Figs. 1, 2, and 4 Nos. 1, 3
and 5 will be electrically connected, forming one group, and 2, 4 and 6 will be
electrically connected forming the other group. The vanes D, pointer C and the
metal plates H and J are electrically connected to one group of cells which are
electrically connected by means of a fuse L to one terminal.
From the other terminal there is electrical connection through the other fuse L
to the three alternate cells forming the other group.
On connecting the terminals M to any two points having a difference of potential
between them it will be seen that there will be the same difference of potential
between one set of cells, and the vanes together with the other set of cells which
are connected with the vanes as between the two points whose difference of
potential it is desired to measure. Therefore the vanes will be attracted by those
cells which are at a different potential from themselves and repelled by those
which are at the same potential as themselves. There will therefore be a move-
ment of rotation of the vanes together with the pointer on the pivots E F. This
movement is resisted by the weight of the pointer itself, the effect of which
may, if too great, be reduced to any desired extent by the adjustable counter-
balance P.
The pointer will therefore take up a position such that the two forces are in
equilibrium which act on it viz. :-that of gravity and of the electrical attraction
and repulsion between the vanes and cells.
If the weight of the pointer be insufficient to balance the electrical attraction
and repulsion between the vanes and cells it will be obvious that more weight
may be added either to the pointer or in the form of a separate weight on the
same axis.
To avoid false indications of the pointer C due to the existence of an electric
charge on the glass Q or case K (the effect of which is to attract or repel the
charged pointer away from its proper position) either from some external cause or
by slow electric leakage from one or other of the terminals, the case K of the
instrument may be electrically connected to one of the terminals, preferably the
same terminal as that to which the pointer C is connected, thus placing the case
and pointer in a fixed and definite electrical relation with one another. It is
advisable also in this case to cover the glass Q with a metallic foil electrically
connected to the case leaving only a small opening through which the scale may be
observed, or instead of the glaɛs Qa metal front with a small opening for observing
the scale may be used.
A preferable plan however is to screen the pointer C and it is with this object
that I place it between two metal plates H and J, these two plates being as above
described in electrical connection with the pointer. The two plates H and J
should be of such dimensions that as much of the pointer as possible is enclosed
between them leaving only a small portion of the end exposed, to enable its position
on the scale to be observed.
I prefer this plan of screening the pointer C because it enables me to dispense
with the electrical connection of the case of the instrument to one of the terminals
and therefore the case may be handled with safety; or it may even for further
security be connected to earth.
.
I have found that by fitting pieces of insulating material such as ebonite or mica
within the cells B as shown at R in Fig. 4, I can modify the range of movement
of the vanes and pointer so as to get a scale, the divisions of which shall be more
regular. The regularity or irregularity of scale divisions may also be varied by
altering the shape of the vanes or cells, or both.
For example, I may shape them as shown in Fig. 8 or in other suitable ways.
In Fig. 8 the cells are marked B and the vanes D and both are portions of
cylinders cut to approximately a diamond shape. The cells and vanes also need
not form parts of cylinders as in Fig. 4, but may be flat as in Fig. 7 so long as

page_text: they are arranged at such a distance apart from one another that the vanes in their
motion do not touch the cells.
It will be obvious that the number of cells and vanes may vary from that shown
in my figs. For example I may use 8 or 10 cells with vanes to correspond. Also
two or more concentric sets of cells may be used with concentric sets of vanes to
correspond mounted on a single axle. This is shown in Fig. 10 where two such
concentric sets are shown, the cells being marked B and the vanes D as before..
Two or more sets of cells and vanes may be used in the manner shown in Fig. 9 in
which they are shown set beside one another on one axle. The cells and vanes of
one set being marked B and D respectively those of the other set being marked B1
and D' respectively.
I find it sometimes preferable to carry the vanes and pointer on knife edges.
instead of on pivots as shown in Fig. 4 such an arrangement is shown to a much
enlarged scale in Figs. 5 and 6 in which S is the axle carrying both vanes and
pointer, T T are the knife edges rocking on polished surfaces U U.
When my instruments are used to measure high electro motive forces I prefer
to cover the terminals M with ebonite or some other insulating material in the
manner shown in Fig. 2 so that no part of the instrument which is "in circuit"
can be touched. In Fig. 3 V is a spirit level by which the instrument can be
accurately set up before using it.
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. In an electrostatic voltmeter the use of a fixed set of cells and a movable-
set of vanes placed and shaped so as to form parts of concentric cylinders and
arranged substantially as hereinabove described with reference to Figs. 1, 2 and 4.
2. The modifications in shape and number of the said cells and vanes as herein--
above described with reference to Figs. 7, 8, 9 and 10.
3. The use of pieces of insulating material attached to the cells for the purpose
of varying the irregularity of the scale divisions as hereinabove described with
reference to Fig. 4.
4. In an electric measuring instrument of the class used for measurements of
differences of electrical potential-known as electrostatic voltmeters-in which
the movement caused by the attraction or repulsion between two sets of metal
plates or sheets which are charged is used to indicate the differences of potential
between them, the combinations or arrangements and use of fixed cells, movable
vanes and parts co-operating, substantially as and for the purposes set forth and
shewn by the accompanying drawings.
Dated the 13th day of December 1895.
WM. BROOKES & SON,
55 & 56 Chancery Lane, London, Agents for the Applicant..