Improvements in or appertaining to Working Quadruplex and Duplex Telegraph Signal Receiving Instruments.
Year: 1894

page_text: Date of Application, 10th Jan., 1894
Complete Specification Left, 26th Sept., 1894-Accepted, 10th Jan., 1895
PROVISIONAL SPECIFICATION.
Improvements in or appertaining to Working Quadruplex and
Duplex Telegraph Signal Receiving Instruments.
A communication by WILLIAM FRANCIS MELHUISH, of Calcutta, India, Chief
Superintendent in the Indian Government Telegraph Department.
I, WILLIAM PHILLIPS THOMPSON, F.C.S., M.I.M.E. of The Agency for
Foreign Patent Solicitors, 6 Lord Street, Liverpool and 6 Bank Street, Manchester,
both in the County of Lancaster and 323 High Holborn in the County of
Middlesex, Civil Engineer, do hereby declare the nature of this invention to be as
follows:-
The legibility of signals received at the end of a long telegraph line or cable,
whether they be written (recorded) or fleeting (audible or visual) signals, depends
largely upon the abruptness of the current flow and current extinguishment within
the instrument which announces or indicates the signals.
Causes exist, the principal ones among them being electro-magnetic inertia or
the flow of inverse extra current and electro-magnetic momentum or the flow of
direct extra current; which, when taken collectively, are called the impedance
of the circuit, which at one time obstruct the flow of current and at another time
retard its cessation, in either case injuriously affecting the working rate of the
receiving instrument by sometimes causing siguals either to completely fail or to
run into each other.
This injurious property of impedance may pervade the entire circuit from end to
end including the receiving instruments, and, therefore, any device which can
be applied to that part of the circuit which contains the receiving instruments,
having for its aim the mitigation, counteraction or nullification of the retarding
effect of inertia and momentum, must result in a clearer and more distinct enunciation
of signals and consequently an accelerated speed of signalling.
"
My correspondent claims to have succeeded in mitigating, if not completely
nullifying, so much of this injurious impedance effect as may be operative over the
length of wire contained within and constituting the signal receiving instruments
placed in the circuit. For this purpose, an electro-magnetic adjunct or
"twin-
transformer is employed, purposely designed to act in combination with, and to
form an essential component part of, the quadruplex or duplex receiving
instruments included in any circuit. As a consequence of this combined action
between transformer and signal receiving instruments, impedance effects within
the receiving instruments are diminished, if not completely counteracted, by the
emission of two transient currents, one an extra and the other an induced current,
which are generated in the adjunctive twin-transformer by the passage through it
of the primary signalling current. These transient currents flow round each
receiving instrument, in separate paths of their own, simultaneously.with, if not
slightly in advance of, the primary signalling current, but at the moment the latter
is reversed by the signalling key the twin-transformer instantly emits reversed
extra and induced currents which flow round the receiving instruments in a
direction opposite to that of the inverse and direct extra or impedance currents
that may still be flowing round the same instrument in another path, and, therefore,
the retardation effects of the latter are, by the restitutive effects of the former,
either extinguished or very considerably diminished, the result being greater
abruptness and smartness in signal formation and a faster rate of signalling.

page_text: The currents resulting from this system at the receiving end consist of an extra
transient current and an induced transient current in addition to the original
primary or steady current and though each of these currents circulates about the
receiving instruments in separate paths yet they combine and act together jointly
in the formation in each receiving instrument of one common polarity or magneto-
motive force. The concentration of the forces of three magnetising currents on
he movable indicator of each receiving instrument, coupled with instantaneous
reversals of current, admits of the adjustable parts of the receiving instruments
used in quadruplex and duplex telegraphy being given a better working adjustment
and consequently of longer distances being worked over than can be accomplished
by the use of quadruplex and duplex receiving instruments of the ordinary type
unconnected with twin-transformer action.
This invention causes primary signalling current generated by the battery at
the sending end to reach the receiving instrument at the other end in the triple
form of primary, extra and induced current, each current combining with the other
in working the receiving instrument.
The "twin-transformer" interpolated, one twin being placed in the principal
circuit and the other twin in the balancing circuit, is always in the path of the
received primary current which passes through one or other of the twins according
to the controlling or directing movements of the signalling keys and in its passage
generates transiently within the twin-transformer extra and induced currents which
are separately conducted, into and around the receiving instrument by the twin
transformer..
Dated this 9th day of January 1894
WM. P. THOMPSON & Co.,
Agents for the Applicant.
.
Improvements in or appertaining to Working Quadruplex and
Duplex Telegraph Signal Receiving Instruments.
A communication from WILLIAM FRANCIS MELHUISH, of Calcutta, India, Chief
Superintendent in the Indian Government Telegraph Department.
I, WILLIAM PHILLIPS THOMPSON, F.C.S., M.I.M.E., of The Agency for Foreign
Patent Solicitors, 6 Lord Street,, Liverpool, and 6 Bank Street, Manchester,
both in the County of Lancaster and 323 High Holborn in the County of
Middlesex, Civil Engineer, 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:—
Part 1. Treating of the defects which the invention is designed to remedy.
Part 2. Describing the twin-transformer.
Part 3. On the action of the twin-transformer.
Part 4. Describing the receiving instruments.
Part 5. Treating of the action of the receiving instruments and twin-transformer
when combined together..
PART I.
The legibility of signals received at the end of a long telegraph line or cable

page_text: 1
or the flow of inverse extra-current and electro-magnetic momentum or the flow of
direct extra-current, which, when taken collectively are called the impedance of the
circuit, which at one time obstruct the flow of current, and at another time retard
its cessation, in either case injuriously affecting the working rate of the receiving
instrument by causing signals, sometimes, either to completely fail, or to run into
each other. This obstructive effect is especially felt on wires having quadruplex
instruments in circuit and causes signals to split when the length of the circuit is
considerable.
This injurious property of impedance may pervade the entire circuit from end to
end including the receiving instruments and therefore, any device which can be
applied to that part of the circuit which contains the receiving instruments, having
for its aim the mitigation, counteraction, or nullification of the retarding effect of
inertia and momentum must result in a clearer and more distinct enunciation of
signals, consequently an accelerated speed of signalling.
My correspondent claims to have succeeded in mitigating, if not in completely
nullifying so much of this injurious impedance effect as may be operative over the
length of wire contained within and constituting the signal receiving instruments
placed in the circuit. For this purpose he employs an electro-magnetic adjunct or
twin-transformer purposely designed to act in combination with. and to form an
essential component part of, the quadruplex or duplex receiving instruments
included in any circuit. As a consequence of this combined action between twin-
transformer and signal-receiving instruments, impedance effects within the receiving
instruments are diminished if not completely counteracted by the emission of two
transient currents, one an extra and the other an induced current which are
generated in the adjunctive twin-transformer by the passage through it of primary
signalling current. These transient currents flow round each receiving instrument
in separate paths of their own simultaneously with, if not slightly in advance of,
the primary signalling current as at, the moment the latter is reversed by the
signalling key, the twin-transformer instantly emits reversed extra and induced
currents in advance of the reversed primary which currents flow round the receiving
instruments in a direction opposite to that of the inverse and direct extra or
impedance currents that may still be flowing round the same instrument in another
path and, therefore, the retardation effects of the latter are by the restitutive
effects of the former either extinguished or very considerably diminished, the result
being greater abruptness and sinartness in signal formation, the prevention of signal
splitting and, altogether, a faster rate of signalling.
The currents resulting from this system at the receiving end consist of an extra
transient current & an induced transient current in addition to the original primary
or steady current and though each of these currents circulates about the receiving
instruments in separate paths, yet they combine and act together jointly in the
formation in each receiving instrument of one common polarity or magneto-motive
force. The concentration of the forces of three magnetising currents on the
movable indicator of each receiving instrument coupled with instantaneous reversals
of current at the desired moment admits of the adjustable parts of the receiving
instruments used in quadruplex and duplex telegraphy being given a better
working adjustment and consequently of longer distances being worked over than
can be accomplished by the use of quadruplex and duplex receiving instruments
of the ordinary type unconnected with twin-transformer action in which the
initial magneto-motive force set up in the receiving instrument by the steady
current is maintained as long as the signal under formation lasts, whereas with
twin-transformer action the initial magneto-motive force is curbed down from the
maximum force needed to drive the indicator across from its resting to its working
position or vice versa to just that degree of strength which is needed to maintain
the movement undisturbed. The latter strength will always be found to be less
than the former and on telegraph circuits of great lengths, the speed of signalling
will be accelerated by curbing the magneto-motive force acting on the receiving
instrument in the manner effected by the twin-transformer combination.

page_text: THE TWIN-TRANSFORMER.
The electro-magnetic adjunct or twin-transformer is illustrated in plan-in
Plate I. It consists fundamentally of two partially intertwined Christie quadri-
laterals or extra-current bridges each quadrilateral or twin being made in every
respect like the other quadrilateral or twin, and both twins are united or linked
together by a continuous iron ring bond. In view to elucidate what is not perhaps
obvious from a general inspection of Plate I the twin-transformer is diagram-
matically reproduced in Figures I, II, III, Plate II which figures are intended to
serve as a key to Plate I.
In Plate the convolutions of wire contained within the terminals of the
twin marked (K) and (1) are represented in the quadrilaterals depicted as
Figures II, III, in Plate II, by the convolutions or arm marked with the
letters a, a, beginning at (K)' :
Between (¹) and (W)' by d¹, d³.
The coils of wire forming two of the arms, namely, a, a, and d, d, of one twin
and two of the arms, namely a¹, a', and di, d', of the other twin are wound
inductively and side by side round about the same iron core.
The coils of wire forming the remaining two arms, namely b², 61, and c, c', of the
other twin are each wound bifilarly or non-inductively. All these branches or
arms, hereafter referred to by the single letters a, b, c, d, and a¹, b¹, c¹, d¹, are
obtained preferably by taking two exactly similar silk covered copper wires and
coiling them together side by side on a bobbin the two ends of each wire being
brought out to terminal screws which are numbered 1, 2, for the first wire, and
3, 4, for the second wire as illustrated in Plate I. Each of these two wires must
consist either of an entire branch or of a definite fractional length of the branch of
which it forma a part as tortor and so on. In the latter case by means of
four terminal screws on each bobbin and the use of short external connecting
wires a variety of coil combinations can readily be formed and thus the magnetism
set up in the core by the coils, can be concentrated on any particular section or
sections of the iron core or it can be distributed evenly over it as may be desired.
This plan of multiple coil construction gives better insulation between the two
wires and admits of a fault, should one occur, in the branch, being more readily
localised and removed. Complete differentiation or nullification of the magnetic
effects of outgoing or sending current results from passing the outgoing current
round the core covered by the arms a, d, a', d', so that the magnetic flux, if any
set up in the core by a, d, tends to meet that set up in the core by the arms a¹, d¹,
the collision resulting in mutual extinction of each other's magnetic effects and the
non-production of transient current in either bridge. The dimensions of the
inductor coils a, d, a', d', and the iron magnetic circuit passing through them are
made relatively to each other so as to produce the required output of transient
current energy with the least increase to the time-constant of the whole circuit on
which the twin-transformer is to be used. To this end the iron magnetic circuit
is made of the best soft iron either in solid or laminated form, cylindrical or

page_text: angular in cross section either rectangular, ellipsoidal, or annular in plan and the
copper wire forming each coil or arm is preferably equal in resistance to the
resistance of the instrument ordinarily used on the circuit and is of that gauge
which throughout the length of the core gives a depth of winding not less than
half nor greater than the diameter of the iron core. The diameter of the iron
core represented in Plate I is 14 inches, diameter of core plus coil about 3 inches,
length of each coil about 1 inch, number of coils 8, number of coils in each magnetic
arm, four sequential quarter coils resistance of each arm 500 ohms, gauge of
wire No. 34 S.W.G., shape of magnetic circuit ellipsoidal extra-current generating
efficiency 75 per cent. and its induced-current generating efficiency is not inferior
to that figure.
1 ne characteristic property of an extra-current bridge is its production of self-
induced current when primary current is sent through it. Primary current sent
into one of the bridges of the twin-transformer, say, in at (K) emerging at (L)',
Figure 1, Plate II generates extra current in the inductive arm a, and in the
inductive arm d the respective courses taken by these extra-currents are:-First,
that which emerges backwards from the arm a passes through arm then across
the path from junction k to junction and back to a. Second, that which emerges
backwards from the arm d passes across the path from junction k to junction then
through the arm c and back to d, so that the path froin k to 1 is common to both
extra-currents, which may therefore be said to meet at the junction & and cross
over to junction together, where they part company. Similarly primary current
sent into the other bridge from (K¹) towards (W) likewise generates extra-
current in the inductive arm a', and in the inductive arm d'. The respective
courses taken by these extra-currents are:-
First, that which emerges backwards from the arm a¹ passes through arm & then
across the path from junction ¹ to junction w and back to a¹.
Second, that which emerges backwards from the arm d' passes across the path
from junction ' to junction w then through the arm c¹ and back to d', so that the
path from to w is common to both extra-currents which may therefore be said to
meet at the junction and cross over to junction wo together where they part
company. In the Figure I (taken one bridge at a time it must be remembered)
primary current and its direction of flow are represented by long arrows. It will
be noticed that the directions of current flow through the inductive arms ale
opposite and it, therefore, follows that if the primary current be allowed to divide
and flow simultaneously into both twins in the directions (K) to (L) and (K')*
to (W) the current-inducing tendencies nullify each other and no extra-current is
produced, the twin-transformer being in this case conductively and inductively
differentiated. In Figure II primary current is shewn as flowing into the shaded
convolutions of the twin-transformer at (L) leaving it at (K) and the short
feathered arrows show the courses of the resulting extra-current. But this is not
all. As the inductive arms a, d, of one twin and d³, a', of the other twin are wound
side by side over the same iron core, strong currents will be induced in the unshaded
convolutions a' and d¹ by the contiguous primary current the direction of flow of
which is the same as that of the extra-current in the primary twin. The currents
thus induced are represented by the short unfeathered arrows. In Figure III
prinary current is shown as flowing into the unshaded convolutions of the twin-
transformer at (K')' leaving it at (W) and the short feathered and unfeathered
arrows indicate the courses of the resulting extra and induced currents.
It follows from the foregoing illustrations that the twin-transformer acts the part
of a duplex current-transformer. For in the first place the current that is self
induced in the primary conductor by the primary current is extracted from that
conductor, that is to say, it is diverted into an adjacent conductor having no primary

page_text: current in it, in which conductor it becomes available for being turned to a useful
purpose. In the second place current is simultaneously induced in the contiguous
secondary conductor by the primary current and this induced current is in the same
manner diverted into a separate adjacent conductor where it, too, can be applied to
a useful purpose.
It likewise follows from the foregoing description that the twin
which generates extra-current acts as a primary inductor, the other twin becoming
the secondary inductor to it. Moreover, as primaries and secondaries the twins are
interchangeable without affecting the strengths or impairing the efficiencies of the
currents induced.
Each coil or bobbin of the particular receiving instrument of whatever
description which is to work in combination with the twin-transformer must be
double wound and each of these two wires should by preference though, this is not
absolutely essential, consist of wire of exactly the same length and exactly the
same gauge and resistance and the ends of each coil must be terminated to separate
binding screws. In Figure I, Plate III one method of double wire winding is
shown the two wires on each bobbin being coiled on together side by side. The
wire shown in black beginning at (K) and ending at (L) forms one complete coil.
Its opposite or differentiating coil also shown in black begins at (K¹) and ends
at (W). The other wire shown in white beginning at (k) and ending at (1) forms
another complete coil. Its opposite or differentiating coil also shown in white
begins at (k) and ends at (w). Another method of double coil winding is shewn
in Fig. II Plate III. In this method instead of the two wires on each bobbin
being coiled on together side by side in contiguous parallelism, one wire is
completely coiled on the bobbin first as shewn and then the other wire is coiled on
over the first coil completely superposing it. In this plan of winding if both wires
are of the same gauge, length and resistance they may be interchanged at pleasure
without affecting the balance conditions of the circuit. This latter method of
winding has been found to give better results than the former method when
applied to the B. or non-polarised relay used in quadruplex working. It admits
of either the upper or lower coils being interpolated with the twin-transformer
to suit the requirements of particular lines but it has been generally found that
more stable signals are obtained when the lower coils are used for the steady,
and the upper coils for the transient currents instead of the converse.
might not always be the case however. In the case of the polarised relay used
in hand-signalling duplex, either method of winding gives satisfactory results.
But for very fast signalling or for circuits having considerable inductive capacity
or impedance the superposed system of coiling can be used to produce a greater
curbed effect within the receiving instrument than the parallel system of coiling
produces by using the outer coil for steady currents. In this case the strong
transient currents come next to the iron core and do most of the work in driving
the indicator to and fro while the steady current though operative at some distance
from the iron core, is still strong enough to hold the indicator under control by
neutralising any tendency which foreign and incursive currents from neighbouring
wires may have to set up a disturbing effect.
This

page_text: interpolated respectively between the corresponding terminals (k), (!) and (k¹), (w)
of the twin-transformer F and when so interpolated they provide bridges or
separate paths for the passage of the dual transient currents induced by the twin-
transformer. The same plate also shows the manner in which two stations are
connected up for differential duplex working. It will be seen that one of the
transformer twins is interpolated in the line wire circuit, and the other twin is
interpolated in the artificial or balancing wire circuit the same procedure being
followed at each end of the line.
Now when station A sends signals to station B, its own receiving instrument is
magnetically unaffected by either outgoing steady or by induced current, because
both receiving instruments and twin-transformer are electrically and magnetically
differentiated for all sending currents.
The courses of the outgoing currents are parallel namely-from battery through
signalling key to the points of bifurcation K K₁ of the receiving instrument,
thence the divided current passes through the divided coils of the receiving
instrument and out at its terminals L and W and outwards respectively to K
and Kit of the transformer then through the divided coils of the transformer and
out at its terminals L' and W' proceeding thence in one case from L' to the distant
station and completing its circuit through the earth and in the other case from W
to the rheostat and back to the battery, and as the direction of current through
the coils K to L and K to L is opposite to that of the current in the coils K¹
to W and K to W, complete nullification of outgoing current effects is the
result.
At the distant station B however there is a triple effect produced. The course
of the primary current arriving from station A is: From line to L' and K* of
transformer thence to L and K of receiving instrument and to earth.
This arriving current in passing through the transformer from Lt to K' generates
an extra current which in the course of its circuit flows across between the
terminals (1) and (k) of the transformer and it also induces another current which
in the course of its circuit flows across between the other terminals (k¹) and (W).
The courses of these latter currents are 1st extra current. From transformer
terminal () to receiving instrument (1). From receiving instrument (k) to trans-
former (k). 2nd induced current. From transformer terminal (k¹) to receiving
instrument (¹) from receiving instrument (w) to transformer (w).
Collecting these currents there results a triple magnetising effect within the
receiving instrument thus:-
First, primary or steady current from L to K
Second, extra
Third, induced
l to k
"
"
of receiving instrument.
¹ to w
"
"
At the very instant when the sending key in the course of signalling reaches its
zero or midway point and before the sending current is actually reversed, the
transformer begins a reverse action sending an extra-current through the receiving
instrument from (k) to (l) and an induced-current from (w) to (¹) completely
washing out the previous magnetic polarity and driving the indicator of the
instrument back, if the latter be a polarised one, to its position of rest in readiness
to instantly respond to the next influx of working current and thus accelerating
the speed of working. But, if the circuit be a quadruplex one, and the receiving
instrument be the B or non-polarised instrument, then the effect of the transient
reversed currents emitted by the transformer when the direction of the primary
current is changed during the time the signal is undergoing formation, is to
momentarily check and retard the return of the indicator to its position of rest.
In the first case accelerated speed of signalling is obtained by driving back the
indicator, and in the second case accelerated speed of signalling is obtained by
checking, at the right moment, the return of the indicator.
When the sending keys at each station go down together, causing the signals

page_text: produced to be true duplex signals, then the course of the steady current at each
station (A and B) is :-From key to (K¹) and (W) of receiving instrument, thence
to (K) and (W) of transformer and on to rheostat and earth.
The primary current in passing through the transformer at each station,
generates an extra-current which flows across between the terminals (k¹) and (w)
and induces another current which flows across between (1) and (k).
The course of these latter currents are:-1st. The extra current, from trans-
former (¹) to receiving instrument (k¹), from receiving instrument (w) to
transformer (10). 2nd. The induced current, from transformer (1) to receiving
instrument (1) From receiving instrument (k) to transformer (k). Collecting
these currents, there results as before a triple magnetising effect within each
receiving instrument thus :-
First steady current from K' to W
Second extra
Third induced
""
"
"
"
k¹ to to of receiving instrument,
to K
whether both sending keys are released simultaneously or in succession, the release
is in either case attended by an instantaneous reversal of the extra and induced
currents as before. This triple current effect taken in conjunction with the
instantaneous reversals of the induced currents admits of duplex being worked
over distances considerably greater than can be worked over by the ordinary type'
When there are more
of instruments unconnected with twin-transformer action.
than one receiving instrument in the circuit, as is the case in quadruplex working
for instance, then the auxiliary effect of the transformer may be apportioned
suitably to each instrument by a joint interpolation of them; see Plate III, which
shows diagrammatically two methods of interpolation, first, with the coils of each
relay A, and B, joined in series; Figure III. Second, with the same coils joined
in parallel; Figure IV. But if it be desired to obtain maximum transformer
auxiliary effect, then, in the case of quadruplex working only the B or non-polarised
receiving instrument or that which is independent of current direction for its action
should be combined with the transformer, the other instrument A, or that which is
dependent on current direction for its action being of the ordinary type and joined
up in the circuit in the ordinary manner. The effect on the B or non-polarised
receiving instrument of such combination is most marked. It completely bridges
over within the receiving instrument the disturbing effect of those momentary
periods of no current which are caused by reversals of the A or pole-changing key
and even over distances unusually long, the combination produces signals as firm
and stable on the B side as the signals are on the A side of the quadruplex
circuit. Thus the combination of the transformer with the B receiving instrument
admits of quadruplex being worked over distances considerably greater than can
be worked over without such combination and probably with a less expenditure
of current than is required for the ordinary system of quadruplex working because
it has been found possible to work, direct, quadruplex over a distance of 1,000 miles
with received currents in the proportion of 2 for the B side to 1 for the A side,
whereas without the trausformer combination it was necessary over the same
circuit to alter the current strengths making them 3 for the B side to 1 for the
A side. It is unnecessary to illustrate by a separate diagram a complete
quadruplex circuit, because the receiving instrument shewn in Plate IV may be
taken to represent the non-polarised relay in a quadruplex circuit as well as the
polarised relay in a duplex circuit, the manner of interpolation being identical in
both cases.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, as communicated to
me by my foreign correspondent 1 declare that what I claim as novel is as
follows, that is to say :-
1st. The twin-transformer and method of constructing the same described in

page_text: Part II and illustrated in Plate I for producing or generating and distributing
duplex or dual transient currents. Such method of construction supplying a local
primary induction circuit for the generation of extra-currents and a local secondary
induction circuit for the generation of induced-current each of which local circuits
consists of a quadrilateral inductor, converter or transformer and each of these
transformers similar in all respects to the other transformer and are magnetically
or inductively united or linked together by means of a closed iron ring or core
threaded through and therefore common to the two inductive arms or branches of
each quadrilateral so that the primary induction circuit and secondary induction
circuit become by means of the iron ring threaded through and uniting them
automatically convertible, interchangeable or the reciprocal of each other in
response to the controlling primary current flowing in the principal circuit.
2nd. The receiving instrument or relay in regard to the method pursued in
winding the wire on the bobbins intended for it so as to bring a simultaneous.
triple magnetising effect to bear on the indicator by the provision of a separate
path for primary or steady current and a separate path for induced-current as
described in Part IV and illustrated by Figures I and II of Plate III. The paths
provided for primary current and those provided for transient currents are inter-
changeable and such interchangeability can be caused to produce a graduated
magneto-motive force.
3rd. The combination of the twin-transformer and the receiving instrument, the
latter specially designed and made to function in conjunction with the twin
transformer to form when acting together and jointly interpolated in any quadruplex
or duplex circuit. "The triple current acting and instantaneous reversing
receiving apparatus," two of these triple currents acting together within those coils
of the receiving instrument that are on one side of the circuit and the third current
assisting them within one of the coils of the receiving instrument on the opposite
side of the circuit the latter having been transformed and translated across to that
side by means of the closed iron core which locally links the line and artificial sides
of the circuit together. Thus the magneto-motive forces pertaining to the three
currents, steady, extra, and induced are concentrated or united and caused to act
together severally and collectively on the armature lever or indicator of the
receiving instrument. But though magnetically acting together these triple currents
steady, extra and induced are, by the manner pursued in the combination of their
paths with the twin-transformer electrically insulated from each other and therefore
each current circulates round the core of the receiving instrument or relay in
different conductors, spirals or convolutions the conductor carrying one kind of
current being electrically closed by the balance conditions of the system against
the direct entrance into it of current of either of the other kinds.
Dated this 26th day of September 1894
WM. P. THOMPSON & Co.,
Agents for the Applicant.