Improvements in Electrostatic Measuring Instruments.
Year: 1890

page_text: Date of Application, 29th July, 1890.
Complete Specification Left, 29th Apr., 1891-Accepted, 27th June, 1891
PROVISIONAL SPECIFICATION.
Improvements in Electrostatic Measuring Instruments.
We, WILLIAM EDWARD AYRTON, Fellow of the Royal Society, Professor at
the City and Guilds of London Central Institution Exhibition Road London and
THOMAS MATHER of 5 Melrose Terrace Lillie Road Fulham London S.W.
Electrician, do hereby declare the nature of this invention to be as follows:-
Our invention relates to electrical measuring instruments whose indications
depend on electrostatic attraction or repulsion or on both. The improvements
consist first in making the fixed and movable parts of such instruments of certain
definite forms and placing them in certain definite relative positions so as to obtain
an instrument having great sensibility, or a long range with fair proportionality
between the deflection and the electrical magnitude measured, or so as to obtain an
instrument with a short or with a long range, but which is especially sensitive at
that part of the scale about which it is intended to be used.
Our invention relates secondly to improved methods of making and fitting
together the parts of the movable system of such instruments so as to obtain good
electrical contact between these parts, as well as when required between the movable
system and some portion of the fixed system, and thirdly to the combination of such
electrostatic measuring instruments with resistances and capacities for the purposes
hereafter specified.
We will refer to the movable system as the needle and to the fixed part or parts
which attract or repel the needle as the cylinders, but we do not confine ourselves
to the employment of the cylindrical shape for the fixed system since portions of
cylinders or parts of other shapes may be used for the fixed system without
departing from the essential features of our invention.
It has been hitherto customary to construct the aluminium needle of a cylinder
electrometer out of several pieces rivetted or cemented together but such methods
do not always ensure good electrical contact between the parts so attached and thus
a serious error is introduced when the instrument is employed with alternating
potential differences. In order to overcome this defect we sometimes make the
entire needle out of one piece of metal, bent, moulded, or otherwise made to the
required shape. A few examples of such needles, each constructed from one piece
of metal are shown in Figures 1, 5 and 7. That in Figure 1 is made by bending
or moulding a rectangular sheet of metal to a Z shape there shown in full fines; the
dotted lines indicate the outer edges of stays E which may be inserted to keep the
needle in shape. The form shown in Figure 5 is made from a sheet cut to the shape
indicated in Figure 11; this is moulded to the required form by bending it about a
line A B and the projections ttt are turned over at right angles so as to meet in
the centre as seen at C Figure 5. Similarly that in Figure 7 is made from a piece
of the shape shown in Figure 9. The projections t thus form spokes, and these are
secured together or to an axial wire or tube by screws and nuts or other well known
means.
Figures 2 and 4 show needles made respectively from two pieces of the shapes
seen in Figures 8 and 10 needles of the latter type may however be made in one
piece.
Figure 6 shows a double one-sided needle made in two parts D and D' of the
shape shown in Figure 9 but of different sizes, and in Figure 3 a double two-sided
needle made from 4 pieces D, D, D₁, D', is illustrated. By suitable modification
single, double, triple, etc. needles with one, two or more sides may be constructed.
The methods of construction indicated in Figures 2, 3, 4, and 6, or modifications

page_text: of them, are especially suitable for instruments which require the needle to be in
two or more parts insulated from one another. Needles inade in any of the ways
shown in Figures 2, 3, 4, 5, 6, 7, or in modifications thereof may have both their
inner and outer surfaces acted on by the electrified fixed portions and thus the
sensitiveness of the instrument may be made large or a long range conveniently
obtained.
In some cases we turn over the edges of the needles so as to form a kind of bead
round them or make the spokes of channel section; this strengthens the needles
and also prevents brush discharges from sharp points or edges which would otherwise
be exposed.
If the needle of a cylinder electrometer be suspended near the fixed parts its
equilibrium may become unstable when the potential difference between the parts
reaches a certain value.. This value depends on the distance of the needle from
the cylinders, on the extent of the attracting and repelling surfaces, on the weight
of the needle and on the distance of the centre of gravity below the point of
suspension. To prevent this instability we make the needle heavy but arrange the
mass so that the moment of inertia is small. We also keep the distance between
the centre of gravity and the point of support as small as convenient further, those
parts of the fixed system whose potentials differ most from that of the needle we
sometimes place at a greater distance from the latter. In the case of quarter
cylinder electrometers, for example, arranged to be used idiostatically and therefore
having one pair of quarter cylinders connected electrically with the needle, this
pair of quarter cylinders may be placed close to the needle but the other pair we
put at a greater distance and by this arrangement we not only secure greater
stability but also greater freedom from sparking. In another case we arrange part
or all the quarter cylinders so that they can be separated from or brought near to
the needle, this enables us both to prevent instability and vary the sensitiveness.
To obtain a calibration curve of a desired shape we may place the surfaces of the
cylinders or of the needle, or of both, eccentric to the axis of rotation whereas to
cheapen the construction we sometimes employ only one pair of quarter cylinders
either to attract or repel the needle and in these or in other cases we may replace
onc pair of quarter cylinders by a complete cylinder or tube which may, or may
not, form a cover for the instrument.
To obtain a range of about 90° we place the needle so as to be wholly or almost
wholly outside the attracting cylinders when it is in its zero position. By making
the needle narrow and by placing the cylinders eccentrically the range may be
further increased. To obtain still greater ranges we prefer to use one-sided needles
such as are shown in Figures 5, 6 and 7, and to suspend or pivot them so that the
parts of the needles remote from the axis pass between portions of the fixed system
so arranged that the capacity of the neeille relative to the fixed system varies as
the needle turns. This result we attain in several ways two of which are as follows:
First, set the cylinders eccentrically with respect to the needle and to each other
so that the radial distance separating the needle from the fixed system varies as
the needle turns. Second, arrange the parts concentrically or approximately
concentrically but shape the fixed parts so that their length parallel to the axis is
different in different parts. A combination of these or of equivalent methods for
causing the capacity of the needle relatively to the fixed system to vary as the
needle turns may be used, and we find that by the employment of such devices
electrostatic measuring instruments can be constructed in which the maximum
deflection is 270° or more.
When the controlling force is gravity, or is magnetic, and when it is desired that
thic instrument shall be very sensitive about one part of the scale we arrange that
this part of the scale shall be about 90° from the zero position. For since the
control due to gravity about 90° from the zero position is nearly constant for a
considerable angle it follows that a small variation in the electric attractions or
repulsions may produce a considerable change in the deflection of the needle.
Since also the controlling moment becomes less when the deflection exceeds 90° we

page_text: can by suitably arranging the shapes of the parts, attain great sensitiveness
bordering on instability.
When the controlling force is other than gravity or magnetism we obtain great
sensibility about one particular part of the scale by shaping the parts so that the
capacity of the needle relative to the fixed parts varies rapidly about that part of
its range.
. In some cases we give the needle of our instruments a "set up" by an eccentric
or otherwise and this "set up" device may be so arranged as to break the connection
with the main or mains and to short circuit the instrument when the needle is freed
to take the zero.
We also, in some cases, arrange that the pointers of our instruments shall be either
horizontal or vertical at the working part of the scale so that an attendant may see
from a considerable distance whether the volts, watts or whatever the electrostatic
instrument be measuring are of the required magnitude.
In instruments intended to measure comparatively small electrical quantities we
may support the needle axis on antifriction wheels.
We sometimes arrange our electrostatic instruments to measure the watts given
to any circuit, machine or other apparatus and this may be done in several ways.
For example, one method is to combine with the instrument a non-inductive
resistance, or a resistance compensated by capacity, of suitable proportions and to
join this in series with the circuit in which the power is to be measured as in the
well known method of measuring watts by an electrometer. But instead of
changing the connections of the needle or quadrants and determining the watts from
the difference of two readings we first connect the middle of the noninductive
resistance to the case of the instrument and then one end of it to the case; the
watts may be determined from the mean of the two readings. Usually the difference
between them is small and where great accuracy is not required either may be
taken as correct.
Moreover when considerable accuracy is desirable this is more
readily obtained from the means than from the differences. In another method we
employ needles constructed in one or other of the ways above described in which
the parts are insulated from each other and in this way we are able to make direct-
reading wattmeters.
In order to increase the sensitiveness of our instruments or to enable a stronger
control to be used, to prevent sparking across and a so to damp the oscillations of
the needle we sometimes arrange them so that the needle is immersed in an
insulating medium of high specific inductive capacity and large dielectric strength.
We may also adapt our instruments to give records either by photography or by
tracing curves on moving surfaces by means of the pointer, or the pointer may be
formed as a tube and put in connection with an ink or other fluid reservoir in a
manner analogous to that used in Thomson's syphon recorder.
Dated this Twenty-ninth day of July 1890.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.
COMPLETE SPECIFICATION.
Improvements in Electrostatic Measuring Instruments.
We, WILLIAM EDWARD AYRTON of The Central Institution Exhibition Road
London S. W., Professor of Applied Physics and THOMAS MATHER of The Central
Institution Exhibition Road S.W. Electrician 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:-
(1). Our invention relates to electrical measuring instruments whose indications
depend on electrostatic attractions and repulsions or on both.
The improvements consist, first, in making the fixed and movable parts of such
instruments of certain definite forms, and placing them in certain relative positions

page_text: so as to obtain an instrument having either great sensibility or a long range with
fair proportionality between the deflection and the electrical magnitude measured,
or so as to obtain an instrument with a short or with a long range but which is
especially sensitive about that part of the scale at which the instrument is intended
to be used. Secondly, our invention relates to improvements in the methods of
making and fixing together the parts of the movable system of such instruments so
as to obtain good electrical contact between these parts, as well as, when required,
between the movable system and some portion of the fixed system.
2. We will refer to the movable system as the needle, and to the fixed parts
which attract or repel the needle as the cylinders, but we do not confine ourselves
to the employment of the cylindrical shape for the fixed system since portions of
cylinders, or parts of other shapes, may be used for the fixed system without
departing from the essential features of our invention. For the sake of convenience
we will first describe the second part of our invention. We may here point out
that Figures 1 to 11 (inclusive) hereafter referred to are shown on the sheet of
drawings which accompanied the Provisional Specification.
3. It has hitherto been customary to construct the aluminium needles of cylinder
electrometers out of several pieces rivetted or cemented together, but such methods
do not always ensure good electrical contact between the parts so attached, and
thus a serious error may be introduced when the instrument is employed with
alternating potential differences. In order to overcome this defect we sometimes
make the entire needle out of one piece of metal, bent or moulded to the required
shape. A few examples of such needles are shown in Figures 1, 5, and 7 of the
drawings left with the Provisional Specification. That in Fig. 1, is made by
bending or moulding a rectangular sheet of metal to a Z shape there shown in full
lines; the dotted lines indicate the outer edges of stays E which may be inserted
to keep the needle in shape. The form shown in Figure 5 is made from a sheet cut
to the shape indicated in Fig. 11; this is moulded to the required shape by bending
it about a line A. B, and the projections are turned over at right angles so as
to overlap in the centre as shown at C Fig. 5. Similarly that in Fig. 7 is made
from a piece of the shape seen in Fig. 9. The projections thus form spokes and
these are secured together or to an axial wire or rod by screws and nuts or by other
known means.
4. Figures 2 and 4 show needles made respectively from pieces of the shapes
shown in Figures 8 and 10. Needles of the latter type may, however be made out
of a single piece. A complete needle resembling Fig. 2 excepting that it has
three spokes instead of two, is shown in Figures 12 and 13 accompanying this
the Complete Specification.
5. Figure 6 of the drawings accompanying the Provisional Specification shows
a double one-sided needle made in two parts D and D' of shapes shown in Fig. 9
but of different sizes, and in Fig. 3 a double two sided needle made from four
pieces D, D, D', D', is illustrated. By suitable modifications, single, double,
triple &c. needles with one, two or more sides may be constructed.
6. The methods of construction indicated in Figs. 2, 3, 4, and 6 or modifications
thereof, are specially suited for instruments which require the needle to be in two
or more parts insulated from one another. It will also be evident that one of the
many modifications in the construction of needles such as illustrated in Figures 2,
3, 4, 5, 6, and 7, is that instead of bending sheet metal as above described, they
may be cut or stamped out of metal tubes of appropriate diameters.
.
7. Needles constructed in any of the ways shown in Figures 2, 3, 4, 5, 6, and 7,
or in modifications thereof, may have both their inner and outer surfaces acted on
by the electrified fixed systems, and thus either the sensitiveness of the instrument
'may be made large, or a long range conveniently obtained.
8. In some cases. we turn over the edges of the needle so as to form a kind
of bead or a Hange round then, or make the spokes of a channel section; this
strengthens the needles and prevents brush discharges from sharp points which may
otherwise be exposed.

page_text: 9. We will now refer to the other parts of our invention :-
If the needle of a cylinder electrometer be suspended so that it is near the fixed
parts, its equilibrium may become unstable when the potential difference between
the needle and the fixed parts reaches a certain value. This value depends on the
distance of the needle from the cylinders, on the extent of the attracting and
repelling surfaces, on the weight of the needle, and on the distance of the centre of
gravity of the needle from the point of suspension. To prevent this instability, we
make the needle heavy, but in order that the time of oscillation should not be
unduly long, we arrange the mass so that its moment of inertia about the axis of
the needle is small. We also keep the suspension wire or fibre as short as con-
venient. An example of a needle such as described above is shown in Figures 12
and 13, from which it will be seen that the central part is formed of a rod or wire so
as to give considerable weight but not add greatly to the moment of inertia about the
axis. To this end it is preferable to make the rod of a material having a large
specific gravity.
10. An electrostatic voltmeter for which a needle such as shown in Figures 12
and 13 would be suitable is shown in Figures 14 and 15 drawn to a smaller scale.
The latter figure represents a plan of the instrument and the former shows a
sectional elevation taken on the zig-zag line F, G, H, I, J, K, Fig. 15; from the
figures it will be seen that both the inside and the outside surfaces of the needle N
are exposed to the action of the attracting cylinders C C¹. These cylinders are
carried by an insulating ring E and the needle is suspended by a fine wire W,
Fig. 14.
In this instrument great sensibility is obtained by employing a long
needle and having both its sides fairly close to the cylinders C and C. This
arrangement we are able to use without producing instability by employing a
weighted needle such as described in paragraph 9. Stops and guides (not all
shown) are provided to prevent the needle N touching the cylinders C under
any ordinary circumstances.
11. We will here mention that in Figure 12 and in all subsequent figures the
needle is denoted by the letter N those cylinders (if any) which are electrically.
connected with the needle are denoted by C whilst those intended to be at a different
potential to that of the needle are denoted by C¹.
12. In some cases we place the parts of the fixed system whose potential differs
most from that of the needle at a greater distance from the latter than those parts
which are at or nearer to, the potential of the needle. In the case of a quarter
cylinder electrometer, for example, arranged to be used idiostatically as an electro-
static voltmeter and therefore having one pair of quarter cylinders connected
electrically with the needle, this pair of quarter cylinders may be placed
close to the needle, but the other pair put at a greater distance from it. This
disposition is illustrated in Figures 19 and 20 (Sheet 2) which shows the needles
and cylinders of an electrostatic voltmeter reading up to about 1500 volts. By
this arrangement we not only secure the greater stability of a suspended needle,
but greater freedom from sparking. The close proximity of the quarter
cylinders C to the needle assists in screening it, and also helps to damp its
vibrations by the air friction. In the instrument shown, the needle is pivotted,
and is controlled by a spiral spring Figs. 19 and 20. The spring also puts
the needle in metallic connection with the cylinders CC. The edges of the
needle are turned over as and for the purposes described in paragraph 8.
13. To obtain a calibration curve-i.e. the law connecting the angular deflec-
tion and the electrical quantity to which the deflection corresponds of a desired
shape, we sometimes place the surfaces of the cylinders, or of the needle, or
of both, eccentric to the axis of rotation, whereas to cheapen construction we
sometimes employ only one pair of quarter cylinders either to attract or repel
the needle; and in these and in other cases we replace the other pair of quarter
cylinders by a complete cylinder, or tube, which may or may not form a cover for
the instrument.

page_text: 14. One arrangement of eccentrically placed cylinders is shown mig. 25,
N being the needle and CC the attracting cylinders.
15. Examples of using only one pair of quarter cylinders are illustrated in
Figures 14, 15, and 21; in both cases the quarter cylinders are double as shown so
as to act on both surfaces of the needle.
The outer cases of these instruments are electrically connected to the needle so
that the latter is screened from external electric forces.
16. The shape and disposition of the needle and cylinders depend on the
maximum deflection desired, on the character of the calibration curve required and
on the magnitude of the electrical quantities to be measured.
17. We will first refer to the question of range:-
To obtain a range of about 90 degrees we sometimes place the needle so as to be
wholly or almost wholly without the attracting quarter cylinders when it is in its
zero position. Such arrangements are illustrated in Figures 14, 15, 19, 20, and 21.
To obtain a still greater range we prefer to employ one-sided needles such as are
shown in Figures 5, 6, and 7 of the drawings left with the Provisional Specification,
and to suspend or pivot them so that the part of the needle remote from the axis
passes between portions of the fixed system so arranged that the capacity of the
system varies as the needle turns. This result we obtain in various ways, two of
which are as follows:-First set the cylinders eccentrically with respect to the
needle and to each other so that the radial distance separating the needle from
the fixed parts alters as the needle is turned round; this device is illustrated in
Figure 23. Second, arrange the parts concentrically or nearly so, but shape the
fixed parts so that their length parallel to the axis is different in different parts;
an arrangement of this kind is shown in Figures 16, 17, and 18. The latter figure
shows the development of the cylinder CC and the letter X in that figure and in
Fig. 17 is placed at the end where the axial length of the cylinders is greatest.
The wavy line in Figure 16 shows approximately the projection of the outer of the
two cylinders CC, the inner, to prevent confusion not being shown in the elevation.
In some cases where the pressures to be measured are not very large, the inner of
the two cylinders may be dispensed with. From the foregoing it will be evident
that a combination of the two methods of causing the capacity of the system to
vary as the needle turns, may be employed. By either of the two devices illustrated
in Figures 16, 17, 18, and in Fig. 23, a maximum deflection of about 270° degrees
is possible, and by making the needle narrower a still greater deflection may be
obtained.
18. The nature of the calibration curve of an electrostatic instrument depends on
the character of the controlling force and on the rate at which the electric capacity
of the system varies as the needle turns, the latter being determined by the shapes
of the needles and cylinders and their proximity; and by suitably arranging these
parts we are able to obtain either an instrument having fair proportionality, or one
that is very sensitive about a particular part of its scale.
19. We will first take the case of a controlling force such as is exerted by a
spiral spring or other equivalent means of obtaining a uniformly increasing force.
To obtain a fairly proportional calibration with such a control we sometimes
employ the arrangement shown in Figs. 16, 17, and 18, and which is partly
described in paragraph 17. This is intended to measure high pressures. The
pivotted needle N passes between the two cylinders C C shaped as shown in the
development Fig. 18; these screen the needle more or less from the inductive
action of the cylinder C¹. The needle is connected to C by a spiral spring (not
shown) and as the potential difference between C and C¹ rises the needle moves in
a counter-clockwise direction. The shape of the cylinders C C is so arranged that
equal increments of pressure. produce approximately equal increments in the
deflection over a considerable portion of the complete range. It will also be under-
stood that by having the needle insulated from C and placing it at the end remote
from Y when at zero, the arrangement could be made more sensitive as a voltmeter,

page_text: but in order to get proportionately the shape of these cylinders would be quite
different to that shown in Fig. 18, for in that case the axial length would require
to change most rapidly at the shorter end remote from Y. Figure 23 shows
diagrammatically an arrangement of eccentric or spiral cylinders the axial length
of which is constant. Such an instrument is most sensitive at the upper part of its
scale. Again, the instrument shown in Figs. 19 and 20 is most sensitive when the
deflection is between 40 and 50 degrees.
20. When the controlling force is gravity, or is magnetic, so that the controlling
moment attains a maximum and then diminishes with further increase of deflection,
the shapes of the parts required to give calibration curves of a certain character
will require to be different to those in instruments having uniformly increasing
controlling moments. For example when the controlling force is gravity and it is
desired that the instrument shall be very sensitive about one part of its scale we
arrange that this part of the scale shall be nearly 90 degrees from the zero position.
Since the controlling moment due to gravity at about 90 degrees from the zero
is nearly constant for a considerable change of angle, it follows that a small
variation in the electrostatic attraction or repulsion may produce a considerable
change in the deflection of the needle. Since also, the controlling moment becomes
less when the deflection exceeds 90°, we can, by suitably arranging the shapes of
the parts attain great sensitiveness bordering on instability. A diagrammatic
illustration of a gravity instrument is shown in Fig. 21 so arranged as to be
sensitive at the higher readings. In this instrument the capacity of the needle
relative to the cylinders C¹ increases nearly uniformly over a considerable angle,
but as the deflecting moment varies as the square of the potential difference whilst
the controlling moment increases slowly say from 45° to 90°, it will be evident that
the instrument will be very sensitive when the deflection approaches 80 or
90 degrees. It may here be pointed out that to bring the instrument, having a
needle made as shown, into action, some method of moving the needle nearer the
open side of the double cylinders C'C' such as is described in the next paragraph
would be necessary.
21. In some cases we give the needles of our instruments a "set up" by an
eccentric or otherwise, and this "set up" device may be so arranged as to break
connection with the main or mains and to shortcircuit the instrument when the
needle is freed to take the zero. An example of such a device is shown in Figures 21
and 22, the former of which roughly represents a gravity control instrument and
the latter a diagrammatic view of the set up and shortcircuiting apparatus. Here
the attracting cylinders C C are electrically connected with the metal rod R,
Fig. 21 and 22, which is carried by an insulating spindle O.
This latter can be turned by a milled head M.
The contact pieces T₁ and T2 are in connection with the points between which
the pressure is to be measured, T, being also connected to the needle and the
outer case of the instrument. When the rod R is in the position shown in full
lines the instrument is shortcircuited and the arm A is clear of the pointer P.
The adjustment of the zero can then be tested. By turning the milled head M
in a clockwise direction to bring the rod R into the position shown in dotted
lines the cylinders C₁ are first insulated and then put in connection with the
contact piece T₁; the same motion causes the arm R, which projects through a
slot X in the dial plate, to give the needle a set up by moving the pointer from the
position shown at U Fig. 21 to that shown at V. An initial control is therefore
given to the needle and it does not begin to move until the pressure exceeds a certain
value after this value has been passed the range is completed for a comparatively
small increase in the pressure.
We do not confine ourselves to the particular set
up and shortcircuiting device shown in Figs. 21 and 22 for it will be evident that
the same results can be effected in various ways without departing from the essential
features of our invention.
22. We may also in some cases arrange that the pointer shall be either horizontal

page_text: or vertical at the working part of the scale so that an attendant may see from a
distance when the quantity measured is of the required magnitude.
23. In order to increase the sensitiveness of electrostatic instruments, or to
prevent sparking across, or to enable a stronger control to be used and also to damp
the oscillations, we in some instances arrange them so that the needle is immersed
in an insulating medium of high specific inductive capacity and of large dielectric
strength.
Having thus particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed and carried into effect, we
declare that what we claim is
1. The improvements in the methods of constructing the movable parts of
electrostatic measuring instruments so as to obtain good electrical contact between
these parts as well as, when required, good electrical contact between the moving
and the fixed parts, as described in paragraphs 3 to 8 inclusive and illustrated in
Figures 1 to 11.
2. The improvements in constructing the suspended needles of electrostatic
measuring instruments so as to obtain stability as described in paragraph 9 and
illustrated in Figures 12 and 13.
3. The improved methods of shaping the fixed parts of electrostatic measuring
instruments so as to obtain long range, or great sensibility or a particular calibra-
tion curve, as illustrated in Figures 16, 17, 18, 19, 20, 21 and 23 and described in
paragraphs 13 to 21 inclusive.
4. The improvements in "set up" and short circuiting devices for electrostatic
measuring instruments as described in paragraph 21 and illustrated in Figures 21
and 22.
5. The improved method of increasing the sensibility or preventing sparking in
clectrostatic measuring instruments by filling or partially filling the interior with a
medium of high specific inductive capacity or of great dielectric strength.
Dated this Twenty-ninth day of April 1891.
W. E. AYRTON.
T. MATHER.