Improvements in Electrostatic Measuring Instruments.
Year: 1892

page_text: Date of Application, 24th Sept., 1892
Complete Specification Left, 26th June, 1893-Accepted. 2nd Sept., 1893
PROVISIONAL SPECIFICATION.
Improvements in Electrostatic Measuring Instruments.
We, WILLIAM EDWARD AYRTON, Fellow of the Royal Society, Professor of
Applied Physics at the City and Guilds of London Central Institution, Exhibition
Road, London, S. W., and THOMAS MATHER of The Central Institution Exhibition
Road S.W., Electrician, do hereby declare the nature of this invention to be as
follows:-
Our invention relates to electrical measuring instruments whose indications or
records depend on electrostatic attraction or repulsion or on both.
The improvements consist:-First, in making the terminals of such instruments
so that they may be connected with or disconnected from the electric circuit without
danger of shock ;-secondly, in the combination of "safety terminals" as above
described, with fuse holders easily removable from the instruments;-thirdly, in
devices for readily disconnecting all the working parts of such instruments and
other conductors connected with them from the terminals and subsequently bringing
those parts to the same potential;-fourthly, in arranging the capacities of the
parts connected with the mains so that persons touching the outer metal case shall
not receive serious shocks when the instrument is used on alternate current circuits;-
and fifthly, it relates to spark gaps whereby the vital working parts are protected
from injury which might be caused by sparks passing between them.
In carrying out the first part of our invention we enclose the whole of the metal
parts of the terminal, including the binding screw, in insulating material, the
distance between such metal parts and any part of the surface which can be touched
being greater than the sparking distance corresponding with the greatest pressure
the instrument is intended to indicate or record.
In the second part of our invention we form the terminal and safety fuse in one,
the fuse being wholly or partly enclosed in the terminal. The combination is
attached to the instrument in such a way that it can be readily removed for
inspection or renewal of the fuse without interfering with other parts of the
instrument.
The third part is an improvement on the shortcircuiting device described in
Specification No. 1186290 § 21. In addition to disconnecting the cylinders C¹ C¹
from one terminal or contact piece T₁, and connecting them with the other terminal
as there described, we arrange the mechanism and connections so that the cylinders
and also the parts connected to the needle are first disconnected and insulated from
the terminals and subsequently connected metallically together. The metallic
connecting piece may be insulated from the other parts of the instrument. Arrange-
ments are provided which prevent partial connection or disconnection. These
improvements may be used with or without the "set-up devices" referred to in § 21
of Specification No. 1186290.
The fourth part relates to instruments with cases of conducting material insulated
from the working parts of the instruments. Usually the parts connected with the
respective terminals have unequal electrostatic capacities relative to the case,
consequently when the terminals are connected with a source of alternating
potential a person touching the case may receive a shock due to a condenser
current. To obviate this inconvenience we add a contrivance for increasing one or
other capacity (depending on the conditions of the circuit) so as to equalise the
inductive effects on the case, and cause the potential of the case to be always
maintained approximately at zero.
Fifthly, by arranging one or more spark gaps adjusted to allow sparks to pass
when the potential differences between the vital parts, with which the electrodes of

page_text: 27
the gaps are connected, are less than those required to cause sparks to pass between
these vital parts, we protect these parts of the instruments from injury which might
otherwise be caused by "sparking across." In some cases we insert resistance in
the circuit to reduce the pressure between the working parts immediately a spark
begins to cross the gap.
Dated this Twenty-fourth day of September 1892.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.
COMPLETE SPECIFICATION.
Improvements in Electrostatic Measuring Instruments.
We, WILLIAM EDWARD AYRTON 41 Kensington Park Gardens London W.,
Fellow of the Royal Society, Professor of Electrical Engineering, and THOMAS
MATHER 19 Agamemnon Road West Hampstead London N.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 or
records depend on electrostatic attractions or repulsions or on both.
as
(2.) The improvements consist :-First in making the terminals of such instru-
ments so that they may be connected with or disconnected from the electric circuit
without danger of shock; second, in the combinations of "safety terminals
above described with fuse holders easily removable from the instruments; thirdly,
in devices for readily disconnecting all the working parts of such instruments and
other conductors connected with them from the terminals and subsequently bringing
those parts to the same potential; fourthly, in arranging the capacity of the parts
connected with the mains so that persons touching the outer metal case shall not
receive serious shocks when the instruments are used on alternating current circuits,
and fifthly it relates to spark gaps whereby the vital working parts are protected
from injury which might be caused by sparks passing between them.
(3.) Before describing the various parts of our invention in detail we will briefly
refer to the drawings hereto attached. In the first place we would have it under-
stood that where parts are shown in section the metal parts are indicated by
diagonal shading in full lines, whereas parts consisting of insulating material are
sectioned in dotted lines. In all the figures the same parts are indicated by the
and fuse holder and
same letters. Fig. 1 gives a section of a "safety terminal'
shows our improved insulating and short-circuiting device. The section is taken on
the line d et in Fig. 3. Fig. 2 is a section of the fuse holder H and fuse-tube F
taken at a Fig. 1. Fig. 3 illustrates the general arrangement of our improve-
ments 1, 2, 3, and 5, as applied to an electrostatic voltmeter for voltages about 2000,
the scale being about half size. Fig. 4 shows an improved form of fuse-socket, and
in Fig. 5 a modified arrangement for preventing partial connection of the working
parts with the terminals, is delineated.
(4.) In carrying out the first part of our invention we enclose the whole of the
metal parts of the terminal, including the binding screw, in insulating material, the
distance between such metal parts and any part of the surface which can be touched,
being greater than the sparking distance (in air) corresponding with the greatest
pressure the instrument is intended to indicate or register. One of the methods by
which we accomplish this object, is clearly shown in Fig. 1, where the metal parts M
and binding-screw B are shown enclosed in insulating material. In connecting such
safety terminals with high potential mains, the insulating covering of the
connecting wire is removed only from that portion which goes into the hole in
the metal part M Fig. 1, the edge of the covering abutting against the edge of the
hole. Since, in our instruments, the case K Fig. 1, is efficiently insulated from
the working parts and terminals, it will be readily understood that by the use of
66
"

page_text: such "safety terminals," connection with or disconnection from the mains can be
made without danger from shock. It will also be observed that when permanently
connected with high potential mains as above described, no person can accidentally
touch any part of the instrument electrically connected with those mains.
(5.) In the second part of our invention we form the terminal and safety-fuse
holder in one, the fuse being wholly or partly enclosed in the terminal and holder.
The combination is attached to the instrument in such a way that it can be readily
removed for inspection or renewal of the fuse without interfering with any other
part of the instrument. Figs. 1 and 2 show clearly one way in which this com-
bination is effected, the insulating material of the terminal T being in one piece
with the fuse holder H. The fuse is contained in the fuse-tube F retained in
position by a cap J. The combined terminal and fuse holder is shown screwed
into the case K and is readily removable. The attachment to the case may be
made by bayonet joint or other well known means without departing from the
essential nature of our invention, and instead of a screw-cap J, a cap fixed by a
bayonet joint is sometimes employed. In the latter case, part of the cap is usually
formed so as to act as a spring in order to keep the fuse tube tight in its holder.
In Fig. 4, part of a modified fuse holder is shown, the object of which is to avoid
the necessity of having the fuse-tubes F of exactly equal lengths. Here a metal
socket U is surrounded by a spring V fixed to the metal piece M of the terminal.
By this device fuse-tubes differing appreciably in length may be used, the metallic
connection between the terminal and the cap J, Fig. 1 being efficiently maintained.
It will be noticed that the form of combined terminal and fuse-holder shown in
Figs. 1, 2, and 3 is well adapted for maintaining good insulation of the parts from
the case.
(6.) The third part of our invention is an improvement on the insulating and
shortcircuiting device described in Specification 1186200 § 21. In addition to
disconnecting the cylinders C'C' (Fig. 21 Spec. No. 11862%) from one terminal
or contact piece T₂ and connecting them with the other terminal T₁ as there
described, we arrange the mechanism and connections so that the cylinders and also
the parts connected to the needle are first disconnected from the terminals and
subsequently connected metallically together. The metallic connecting piece may
be insulated from the other parts of the instrument. Figs. 1, and 3 show our
Here the
improved insulating and shortcircuiting device as above described.
spring S¹ connected with the part-cylinders C³ rests on the cap J₁ of the fuse-
holder H; whilst S", which is connected with the tube A and needle N, rest on J".
On turning the handle E from the position shown in full lines to that indicated by
dotted lines, the eccentric X (of insulating material) pushes the springs S¹, Sh
away from J and J" into the position indicated by dotted lines in Fig. 1, and on
0 completing the movement the metal piece Y connects S¹ and S" together, thus
shortcircuiting the working parts. In instruments having vertical dials, a weight W
carried by an arm fixed to the rod R prevents the switch being left partially on.
Fig. 5 shows a modified method of preventing partial connection, applicable to a
vertical dial or other form of instrument. An important feature of the insulating
5 and shortcircuiting device illustrated in Figs. 1 and 3 is the fact that when the
instrument is being used, the actuating eccentric X is completely separated from
the springs S, and the insulation of the working parts from the case is in no way
impaired. It will be readily understood that by suitable mechanism the above
described improvements may be used with "set-up" devices such as described
0 in § 21 Specification No. 1186290 previously referred to.
(7.) The fourth part relates to instruments with cases of conducting material
insulated from the working parts. Usually the parts connected with the respective
terminals have unequal electrostatic capacities relative to the case; consequently,
when the terminals are connected with a source of alternating or varying potential,
5 a person touching the case may receive a shock due to a condenser current.
obviate this inconvenience we add a contrivance for increasing one or other capacity
(depending on the conditions of the circuit) so as to equalise the inductive effects
To

page_text: on the case, and cause the potential of the case to be always maintained
approximately at zero. For example, in an instrument such as shown in Figs. 1
and 3 the capacity, relative to the case K, of the parts connected with the
terminal T is usually considerably less than that of the parts connected with T",
and when such an instrument is connected with an alternating circuit in which the
insulation resistances of the two mains from earth are approximately equal, a slight
shock can be felt on touching the case. To prevent this we sometimes arrange a
conductor connected with S Fig. 3 in proximity to, but insulated from the case,
thereby increasing the capacity of those parts so as to balance the capacity of the
parts connected with S". It will be understood that if the insulation resistance of
the mains connected with T¹ be greater than that of the main connected with T
the capacity to be added to equalise the inductive effects would be diminished,
and vice versa.
"
(8.) In the fifth part of our invention we arrange one or more spark gaps
adjusted to allow sparks to pass when the potential differences between the vital
parts, to which the electrodes of the gaps are connected, are less than those required
to cause sparks to pass between these vital parts; we thus protect these parts of
the instruments from injury which might otherwise be caused by "sparking across.'
One arrangement is shown at G Fig 3, the adjustable screw connected with the
spring S¹ and cylinder C¹, and the tube A connected with the needle N, forming
the electrodes. In some cases we insert resistance in the circuits to reduce the
pressure between the working parts immediately a spark begins to cross the gap.
(9.) In the foregoing Specification we have described our improvements by
reference to electrostatic voltmeters, but we would here point out that our invention
is applicable to electrostatic wattmeters and to other indicating and recording
instruments whose actions depend on electrostatic forces.
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. In electrostatic measuring instruments the use of terminals or binding posts
the metal parts of which are enclosed in insulating material as and for the purposes
described in § 4.
2. In electrostatic measuring instruments the combination of "safety" terminals
with fuse holders, the whole being readily detachable, as described in § 5.
3. In electrostatic measuring instruments the improved insulating and short-
circuiting device as described in § 6.
4. In electrostatic measuring instruments the method of balancing the capacities
of the parts of the instrument connected respectively with the different terminals,
relative to the case, as and for the purpose described in § 7.
5. In electrostatic measuring instruments the protection of the working parts
from injury by the use of spark gaps as described in § 8.
Dated this Twenty-fourth day of June 1893.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.