Improvements in Electrical Measuring Instruments and Apparatus for Use with Alternating Currents.
Year: 1893

page_text: Date of Application, 16th Dec., 1893
Complete Specification Left, 17th Sept., 1894-Accepted, 20th Oct., 1894
TRUVIDIONAL SPECIFICATION.
Improvements in Electrical Measuring Instruments and Apparatus
for Use with Alternating Currents.
We, WILLIAM EDWARD AYRTON of 41 Kensington Park Gardens London W.
Fellow of the Royal Society Professor of Electrical Engineering, and THOMAS
MATHER of 19 Agamemnon Road West Hampstead London N.W. Electrician do
hereby declare the nature of this invention to be as follows:-
Our invention relates, firstly to methods of applying voltmeters to indicate
the pressure between mains at a distance from the instrument without the use of
pilot wires; and secondly, to apparatus for showing the current flowing through
a circuit or circuits which may or may not be at a distance from the measuring
instrument.
To effect our first purpose we put a coil in series with the voltmeter and connect
the free end of the coil with one main and the free terminal of the voltmeter
with the other main. The coil is placed so as to be acted upon inductively by ·
the current in the main or mains, the connections being made so that the induced
electromotive force is in series with but opposed to the potential difference
between the mains, the vector difference between the two pressures being what
we directly measure with a voltmeter. This opposing electromotive force increases
with the main current and by suitable adjustment of the positions of the coil,
or of the number of convolutions, or both, or of the position of an iron core,
the combination can be arranged to compensate, or allow for, the loss of
in any given main or feeder. One method applicable to small "drops" is to put
a long coil between and in the plane of the mains, another method capable of
more general application is to wind the coil on a laminated iron ring or frame
and thread the ring on one of the mains, preferably the one with which the coil
is directly connected. For circuits in which the maximum current is small we
sometimes thread the main (which will then be flexible) through the ring several
times so that the number of convolutions on the secondary coil need not be
excessive.
pressure
To measure the current in a main or service wire, (which may or may not be at
a distance from the measuring instrument,) we thread a laminated iron ring or
frame over that main or wire and connect the ends of a coil wound on the ring
with a voltmeter; the reading of this instrument will then depend on the current
flowing through the main, consequently it may be graduated to read ampères.
Since the laminated ring or frame is merely threaded on the main, there is no need
to cut the wire and thereby damage its insulation, and the circuit of the measuring
instrument is kept entirely insulated from the mains. This is of great importance
on high pressure circuits. When the main or service wire conveys only a small
current, as a maximum, we sometimes thread the wire through the iron ring several
times, and in this way avoid making the number of convolutions on the secondary
circuit very large.
To make an instrument of very long range we wind the
secondary coil so that different numbers of convolutions are available and by means
of a switch connect the measuring instrument to the terminals of a large or of a
small number of convolutions according as the current to be measured is small or
large. By suitable adjustment of the number of convolutions the constant of the
instrument may be changed in any convenient ratio, (say one to ten); without
interfering with the main circuit.

page_text: Our invention is particularly adapted for use with electrostatic voltmeters for
the currents passing through these instruments are so very small that the impedance
introduced by the secondary coil is not serious.
To reduce the alteration in reading of instruments, arranged as above described,
caused by changes in the frequency of alternation of the current, we sometimes
employ inductive coils or condensers or combinations of coils and condensers to
counteract these effects of such changes.
Dated this Fifteenth day of December 1893.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.
COMPLETE
SPECIFICATION.
Improvements in Electrical Measuring Instruments and Apparatus
for Use with Alternating Currents.
We, WILLIAM EDWARD AYRTON of 41 Kensingtou Park Gardens London W.
Fellow of the Royal Society, Professor of Applied Physics, and THOMAS
MATHER late of 19 Agamemnon Road W. Hampstead, 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 firstly to methods of applying voltmeters to indicate
the pressure between mains at a distance from the instrument without the use of
pilot wires, and secondly to apparatus for showing the current flowing through a
circuit or circuits which may or may not be at a distance from the measuring
instruments.
(2.) In both parts of our invention we use a coil or coils (usually with iron
cores) acted on inductively by the current flowing in the mains or circuits, the
connections in the two parts of our invention being shown symbolically in Figs. 1
and 3 respectively of the accompanying drawings. In all the drawings Figs. 1,
2, 3, and 4, the mains, conductors, or service wires are indicated by the letter C,
the iron parts by F, the coils wound over the iron by w, and air gaps in the iron
circuits by g, whilst the letter M indicates the measuring instruments.
(3.) To effect our first purpose we put a coil in series with a voltmeter and
connect the free end of the coil with one main and the free terminal of the
voltmeter with the other main. The coil is placed so as to be acted on inductively
by the current in the main or mains, the connections being made so that the induced
electromotive force is in series with but opposed to the potential difference between
the mains, the vector difference being what we directly measure with the voltmeter.
For example, a coil may be wound on a laminated iron ring or frame slipped over
the alternate current conductor and having its ends connected with the voltmeter
and mains as symbolically shown in Fig. 1. The iron then acts as the core of a
transformer whose primary is the main which is embraced by the iron and whose
secondary is the coil wound on the iron. Such an iron core with its coil we call
for brevity a ring," but it will be understood that we do not limit ourselves to a
circular shape or to a closed iron circuit, for the iron may have various shapes, or
form complete or part of incomplete magnetic circuits without departing from the
essential nature of our invention.
*
(4.) The opposing electromotive force induced in the secondary coil increases
with the main current, and by suitable adjustment of the core or of the number of
convolutions, or both, the ring can be arranged to compensate for the loss of
pressure in any given main or feeder, and thus cause the voltmeter to indicate the
pressure between distant points on these mains or feeders without the use of pilot
wires to connect those distant points with the instrument.
(5.) The adjustment of the positions of the laminated iron core may be effected

page_text: in various ways, some of which are indicated in Figs. 2, 3, and 4. In Fig. 2 the
coil wo is shown wound on a U shaped core F and the armature F being separable,
can be moved towards or away from the core F so as to make the magnetic circuit
more or less complete. By this means the adjustment can be made so that the
compensation is correct. Or the coil may also be partly wound on the armature F
as indicated in Fig. 3, the parts F, F1 being supported in any suitable manner; or
they may be hinged together as illustrated in Fig. 4. In Fig. 3 the air gaps are
shown large for the sake of clearness, whilst in Fig. 4 the winding is, for the same
reason, not shown.
(6.) Any of the forms of "ring" shown in the drawings can be threaded on the
main when an end is accessible, but in addition, those indicated in Figs. 2, 3, & 4,
can be clipped over any exposed part of the main without the necessity of cutting
the main or wire or its insulating covering.
(7.) The form of core illustrated in Fig. 4 is that which we usually prefer.
Here the parts F & F¹ are hinged together at h and can be readily opened out as
indicated by dotted lines and clipped over the conductor C. The construction also
allows of the ready adjustment of the air gap g, and therefore the electromotive
force induced in the secondary circuit can be varied between wide limits.
(8.) For use as a compensator we find it advantageous to use an air gap, because
the induced electromotive force is then more nearly proportional to the current
through the mains and the compensation can be arranged to be correct over a
wider range of current.
66
(9.) For circuits in which the maximum current is small we sometimes thread
the main (which will then be flexible) through the "ring" several times, or the
return wire may be also passed through in the opposite direction, so that the
number of convolutions of wire on the secondary winding need not be successive.
(10.) To measure the current in a main or service wire which may or may not
be at a considerable distance from the measuring instrument we thread or clip
& ring," over that main and connect the ends of the coil wound on the iron
with a measuring instrument as indicated symbolically in Fig. 3. The reading of
this instrument will then depend on the current flowing through the main C,
consequently the instrument M may be graduated to read the current flowing
through C in amperes. Since the "ring" is merely threaded ou or clipped over
the wire there is no need to cut the wire and thereby damage the insulation, and
the circuit of the measuring instrument is kept entirely insulated from the inains.
This is of considerable importance on high pressure circuits.
(11.) When the main or service wire only conveys a small current we sometimes
thread the wire several times through the "ring" and in this way avoid making
the number of turns on the secondary circuit of the "ring" very large; when,
however, neither end of such a main or service wire is at hand but the main or
service wire has sufficient slack we coil up this slack and clip the ring over several
convolutions of the coiled main or service wire. It will also be readily understood
that the run of the currents in two or more branch circuits can be measured by
leading all these circuits through the ring in the proper direction.
(12.) To make an instrument of very long range we wind the secondary coil so
that different numbers of convolutions are available, and by means of a switch or
its equivalent connect the measuring instrument with the terminal of a large or of a
small number of convolutions according as the current to be measured is small or
large. By suitable adjustment of the number of convolutions the constant of the
instrument may be changed in any convenient ratio say one to ten &c. without
interfering with the main circuit. An air gap is also advantageous in this case for
the induced electromotive force is then more nearly proportional to the current in
the wire C; and by this means the necessity of changing the scale on the
instrument when the number of convolutions is altered may be sometimes avoided.
(13.) Our invention is particularly adapted for use with electrostatic instruments
for the current passing through this type are so small that the impedance
introduced by the secondary coil is not serious.

page_text: Having thus 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. The combination of an electrostatic voltmeter with a "ring" threaded on or
clipped over one of a pair of alternate current conductors, one terminal of the
voltmeter being connected with one of the conductors, the other terminal of the
voltmeter being connected through. the secondary coil of the ring with the other
conductor as described.
2. The combination of an electrostatic measuring instrument with a "ring"
threaded or clipped over an alternate current conductor, the terminals of the
measuring instrument being joined respectively with the ends of the secondary
coil as described.
3. The construction of a "ring" arranged so as to be readily clipped over an
alternate current conductor, one form of which is shown in Fig. 4.
Dated this 15th day of September 1894.
WILLIAM EDWARD AYRTON.
THOMAS MATHER.