Improvements in Electrostatic and other Electrical Measuring Instruments.
Year: 1893

page_text: Date of Application, 3rd Nov., 1893
Complete Specification Left, 3rd Aug., 1894-Accepted, 20th Oct., 1894
PROVISIONAL SPECIFICATION.
Improvements in Electrostatic and other Electrical Measuring
Instruments.
We WILLIAM EDWARD AYRTON Professor of Applied Physics at the City
and Guilds of London Central Institution Exhibition Road London Fellow of the
Royal Society, and THOMAS MATHER Electrician of 19 Agamemnon Road West
Hampstead London N.W. do hereby declare the nature of this invention to be as
follows:-
-
Our invention relates first to safety arrangements for electrical instruments, which
arrangements are particularly applicable to those intended for high pressure circuits
where serious shocks imight be received by persons touching parts electrically
connected with the circuit; secondly to devices for preventing damage by sparking
across, or by internal short circuits in electrostatic instruments some of which
devices also serve for modifying the calibration curves and ranges of the
instruments.
In Specification No. 17093 of 1892 we have described safety terminals, fuse
holders, insulating and short circuiting devices, but the arrangements there shown
do not permit of the fuses being examined or removed without removing the
terminals. This has been found to be sometimes inconvenient and we now,
therefore, employ one or other of the following alternative devices :-We construct
the terminals with movable covers, or we provide the terminals with hinges so that
a portion of the terminal can be turned on on one side and the fuses got at without
removing the terminals or disconnecting the mains. Or we construct each terminal
in two or more pieces so that the fuse can be examined or removed by removing a
portion of the terminal but without disconnecting the mains. Or we arrange the
instrument so that not merely the working parts of the voltmeter, but also the
fuses are disconnected from the terminals by the insulating device, which is alsc
sometimes used for short circuiting the working parts. Or we dispense with the
special insulating device described in Specification No. 17093 and arrange the
fuses in an accessible position in such a way that the withdrawal of the fuses first
disconnects the working parts of the voltmeter from the terminals, and then short
circuits the working parts, the working parts being unshortcircuited and then
connected with the terminals on the insertion of the same or other fuses. So that
with the two latter devices there is no necessity to touch the terminals at all when
it is desired to examine or renew the fuses.
To prevent damage by sparking across or by a short circuit we employ several
devices; one of them is to put a resistance in the circuit which will only allow a
current to pass sufficient to blow the fuses even if a short circuit occurs. Another
method is to put a condenser in circuit with electrostatic instruments whose dielectric
is sufficiently strong to prevent disruption being produced by an electric pressure
largely in excess of that which the instrument is intended to measure. By having.
the capacity of this condenser large relatively to that of the working parts, the
interposition of the condenser does not interfere appreciably with the indications
of the instrument when used on alternate current circuits. On the other hand, by.
having the capacity something like that of the instrument when fully deflected we
are able to open out considerably the beginning part of the scale, for when the
deflection is small the capacity of the working parts is small and they will therefore
take the larger part of the pressure applied to the instrument, whereas for large.
deflections the voltage will be divided more equally between the condeuser and the

page_text: working parts. The range of instruments can be considerably increased in this
way and the calibration curve modified to suit special conditions.
By employing condensers of very small capacity in circuit. with electrostatic
voltmeters we are enabled to construct voltmeters for exceedingly high pressures
without the surfaces of the working parts being very far apart since with such a
condenser interposed only a small fraction of the pressure between the terminals
exists between the working parts.
Dated the Second day of November 1893.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.
COMPLETE SPECIFICATION.
Improvements in Electrostatic and other Electrical Measuring
Instruments.
We, WILLIAM EDWARD AYRTON Professor of Applied Physics at the City
and Guilds of London Central Technical College Exbibition Road London, Fellow
of the Royal Society, and THOMAS MATHER late of 19 Agamemnon Road West
Hampstead London N.W. now of 1 Solent Crescent 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 first to safety arrangements for electrical instruments
which arrangements are particularly applicable to those intended for high pressure
circuits where serious shocks might be received by persons touching parts electrically
connected with the circuit, secondly to devices for preventing damage by sparking
across or by internal short-circuits in electrostatic instruments, some of which
devices also serve for modifying the calibration curves and ranges of the
instruments.
(2.) In Specification No. 17093 of 1892 we have described safety terminals, and
fuse holders and insulating and short-circuiting devices as applied to electrostatic
voltmeters but the arrangements there shown do not permit of the fuses being
examined without removing the terminals from the case. This has been found to
be sometimes inconvenient, since in order to unscrew the combined terminal and
fuse holder it is necessary to first disconnect the wires which join the instrument
with the mains and there is some risk of shock being received unless the pressure
be previously cut off from these wires. In order that the fuses may be examined
& replaced without touching the wires connecting the instrument with the mains &
without it being necessary to first switch off the pressure from these wires we now,
prefer to employ one or other of the following alternative devices or their
equivalent:-We construct the terminals with movable covers or we provide the
terminals with hinges. so that a portion of the terminal can be turned aside & the
fuses got at without removing the terminal or disconnecting the mains; or we
construct each terminal in two or more pieces so that the fuse can be examined or
renewed by taking away a portion of the terminal but without disconnecting the
mains; or we arrange the instrument so that not only the working parts but also
the fuses are disconnected from the terminals by the insulating, device & in addition
the working parts are short circuited by the action of this same device; or we
dispense with any special insulating device such as those just referred to, or that
described in Specification No. 17093, & arrange the fuses in accessible positions
in such a way that the withdrawal of the fuses first disconnects the working parts
of the instrument from the terminals & then short circuits the working parts, the
working parts being unshortcircuited & then connected with the terminals on the
insertion of the same or other fuses, so that with the latter device there is no

page_text: necessity to touch the terminals at all when it is desired to examine or renew the
fuses.
In all
"
(3.) The various devices mentioned in the last paragraph are illustrated in the
accompanying sheet of drawings as applied to an electrostatic voltmeter for
pressures about 2000 volts, the scale being about half full size. In these drawings
metal & other conducting parts shewn in section are crosshatched in full lines,
whereas portions made in insulating material are sectioned in dotted lines.
the figures similar parts are indicated by the same letter; for example the letter F
denotes fuse-tubes which are usually of glass; H indicates fiise-holders; h hinges ;
J the portions which are directly connected with the working parts; K the case;
M the metal parts of the terminals with which the mains are connected; and T the
terminals. Fig. 1 is a plan of an arrangement suitable for "Mordey fuses
& where the back of the instrument is accessible. Figs. 2 & 3 shew respectively
elevation (partly in section) & vertical section of an arrangement adapted for
voltmeters whose fronts are accessible. Fig. 4 is an elevation of a terminal &
fuse-holder made so that the terminal part can be withdrawn without disconnecting
the wire & leaves the fuse tube so that it can be easily taken out & replaced.
Fig. 5 is a section through a b Fig. 4. In Figs. 6 & 7 an elevation (partly in
section) & a side view of a method which permits the terminals & wires to remain
fixed are shewn. Figs. 8 & 9 shew an arrangement similar to that delineated in
Figs. 2 & 3 but provided with a short circuiting device. It will be observed that
in all these figures the arrangements are such as to make it impossible that any
person should accidentally touch any metal in electrical contact with the wires
connecting the instrument with the mains when removing or replacing the fuses.
It will also be readily understood that although the various devices are shewn
applied in a particular manner any of them could be modified to suit instruments with
either back or front connections without departing from the essential nature of the
invention.
(4.) We will now proceed to describe the drawings in greater detail:-
In Fig. the terminals T', T₁, are made of blocks of insulating material of
cubical form, with a metal piece M embedded therein, & are carried by
hinges h', ". A link L connects the two so that both can be turned aside (as
indicated by arrows) simultaneously. This motion disconnects and insulates the
fuses & working parts from the metal M, & the fuse-tube F can then be safely
withdrawn. After replacing the fuses the terminals can be turned into the position
shewn & secured by a screw, catch, or other fastening. In Figs. 2 & 3 the
terminals are shewn formed at opposite ends of a rectangular block of insulating
material. This block is fixed to an arm A hinged at h Fig. 3, and this arm can be
turned through a right angle, or more, in the direction indicated by the arrow in
Fig. 3, thus leaving the ends of the fuse tubes F quite free from the terminals.
When the terminals are in the position shewn the fuse-tubes are pressed against
the metal pieces M by springs V, the upper end of the fuse-tube being lightly
gripped by a split bell-mouthed socket U sufficiently tightly to support the weight
of the fuse-tube, but allowing the latter to be easily pulled out. A screw D serves
to fix the terminals in position.
Figs. 8 & 9 shew a similar arrangement to that depicted in Figs. 2 & 3 with the
addition of a short-circuiting device. A hinged piece ✗ carries a strip Y
(preferably of badly conducting material) and is connected to the point O
in the arm A Fig. 9 by a cord passing through a bush N, the cord being indicated
in both figures by chain-dotted lines. On turning A about the hinge h¹, Fig. 9,
the fuses & working parts are first insulated from the terminals & the working
parts are subsequently short circuited by the strip Y, touching J1 & J" as
indicated in dotted lines in Figs. 8 & 9. A spring on the hinge h brings Y back
to its original position, when the cord is slakened by replacing the terminals in
position. The object of using a piece of badly conducting material for Y is to
avoid the fuses being blown if from any accidental occurrence the short circuit
remains on when the terminals are replaced, any bad conductor such as wood or

page_text: vulcanized fibre being sufficiently conducting to bring the working parts to the
same potential when those parts are disconnected from the terminals. In Figs. 4 & 5
the fuse holder H is carried by a block B fixed to the case. The terminal part is
formed as a socket which fits over H as shewn, & is provided with a bayonet catch
or similar device not shewn.
A spring S keeps T in position. To get at the fuses
the terminal is pushed in a little to release the catch & then withdrawn in the
direction indicated by the arrow in Fig. 4. When the wire from the mains is
properly connected with the terminal T the operation of withdrawing & replacing T
can be done with perfect safety for all the exposed parts of the terminal are of
insulating material. The arrangement shewn in Figs. 6 & 7 is such that both
terminal & fuseholder portions are fixed, the only movable portions being the
fuseholder F and the cover P which is preferably of insulating material. A
catch C serves to keep the cover in position when the instrument is in use.
To take out a fuse tube it is pressed down in the direction indicated by the
arrow in Fig. 6 until the upper end clears the lower face of the terminal
block T T. The tube can be then tilted aside & lifted out of the lower holder H.
The spring V then raises the cupped disc U against the conducting piece Y,
thereby shortcircuiting the working parts of the instrument, (which working parts
are connected with the wires J), when both fuses are withdrawn. Usually we
make Y of badly conducting material for the purpose described above when
referring to Figs. 8 & 9. The fuse-holes in the terminal block T T are made so
as to just fit the caps on the fuse tubes, & thus prevent the possibility of bringing
the top cap into contact with M, unless the lower cap is within the hole in the
holder H. The lengths of the holes in the terminal block & of the fuse caps are
such as to ensure the top of the cap being well away from M, Fig. 6, before the
lower edge of the cap comes below the lower surface of the block. It will be
evident that this last described arrangement of terminals & fuse holders need not
be attached to the instrument which it protects, but may be placed where the
connecting wires leave the mains or in any other convenient position.
(5.) To prevent damage by sparking across or by short circuits in the electro-
static instruments we employ several devices. One of them is to put a resistance
in the circuit which will only allow sufficient current to pass to blow the fuses even
if a short-circuit occurs. In the Specification previously mentioned viz. No. 1709392
we mention the use of resistances to reduce the pressure between the working parts
when a spark begins to cross the gap. We have since found these resistances of
use in limiting the current, for the distance across which a given potential difference
can maintain an arc is less the smaller the current passing. The resistances,
therefore, serve to prevent dangerous arcs & also enables us to use shorter fuses.
We also make our fuses of wire kept taut by a spring so that if a break occurs in
the fuse the ends are well separated & the instrument ceases to indicate.
(6.) Another method of preventing damage by sparking across is to put a
condenser in circuit with electrostatic instruments the dielectric of which condenser
is sufficiently strong to prevent disruption being produced by an electric pressure
largely in access of that which the instrument is intended to measure.
(7.) By having the capacity of this condenser large relatively to that of the
working parts the interposition does not interfere appreciably with the indications
of the instrument when used on alternate current circuits. On the other hand by
having a capacity something like that of the instrument when fully deflected we
are able to open out considerably the beginning part of the scale; for when the
deflection is small the capacity of the working parts is small & they will therefore
take the larger part of the pressure applied to the terminals of the instrument,
whereas for large deflections the voltage will be divided more equally between the
condenser & the working parts. The range of electrostatic instrument can be
considerably increased in this way & the calibration altered to suit special
conditions.
(8.) By employing condensers of very small capacity in circuit with electrostatic
voltmeters we are enabled to construct voltmeters for exceedingly high alternating

page_text: pressures, without the surfaces of the working parts being very far apart―since
with such a condenser interposed only a small fraction of the pressure between the
terminals exists between the working parts.
(9.) In the foregoing Specification we have described our improvements by
reference to electrostatic voltmeters for which they are particularly well adapted,
but we would have it understood that they are applicable to electrostatic watt-
meters, electrometers & other indicating & recording instruments whose action
depends on electrostatic forces.
The arrangements of safety terminals are also applicable to other high pressure
instruments such as electromagnetic & hot wire measuring instruments &
transformers.
Having now particularly described and ascertained the nature of our said iuven-
tion, and in what manner the same is to be performed, we declare that what
we claim is :-
1. The construction of the terminals of electrical apparatus so that the fuses can
be safely examined or removed without disconnecting the wires which connect the
apparatus with the mains substantially as described in §§ 2, 3, & 4 and illustrated
in Figs. 1, 2, 3, 4, 5, 6, 7, 8, and 9.
2. The construction of insulating and shortcircuiting devices as described in
2, 3, & 4 and illustrated in Figs. 6, 7, 8, and 9.
3. The combination of a resistance with a fuse such that in the event of an
internal short circuit occurring the fuses will be blown and the apparatus thereby
disconnected from the mains without the possibility of a dangerous arc being formed
inside the apparatus as described in § 5.
4. The combination of a condenser with an electrostatic voltmeter as a safety
device as and for the purpose described in § 6.
5. The combination of a condenser with an electrostatic voltmeter for the purpose
of altering the law of the instrument as described in § 7.
6. The combination of a condenser with an electrostatic voltmeter for the purpose
of measuring very high pressures without the necessity of having the surfaces of the
working parts far apart as described in § 8.
Dated this 2nd day of August 1894.
WILLIAM EDWARD AYRTON.
MATHER