IMPROVEMENTS IN Duplex TELEGRAPHY AND IN APPARATUS TO BE USED FOR THIS PURPOSE
Year: 1874

page_text: LETTERS PATENT to Emil Oesterley, of 21, Mincing Lane, in the
City of London, for the Invention of "IMPROVEMENTS IN Duplex
TELEGRAPHY AND IN APPARATUS TO BE USED FOR THIS PURPOSE,"-A com-
munication from abroad by Louis Schwendler, of Garden Reach,
Calcutta, India, Electrician.
PROVISIONAL SPECIFICATION left by the said Emil Oesterley at the
Office of the Commissioners of Patents, with his Petition, on the
27th June 1874.
"
I, EMIL OESTERLEY, of 21, Mincing Lanc, in the City of London, do
hereby declare the nature of the said Invention for "IMPROVEMENTS
IN DUPLEX TELEGRAPHY AND IN APPARATUS TO BE USED FOR THIS PURPOSE,' to be
as follows (reference being had to the Drawings annexed), that is to
say:

page_text: I
Part First. Method.-I secure the safe and efficient working of duplex
telegraphy by employing the method herein called the "double balance
method" (see Figure 1). a, b, d, g, w, and f, arc adjustable resistances;
E is the signalling battery, the internal resistance of which is e; K is
the signalling key of peculiar construction; G is any suitable receiving
instrument placed in the branch g; B is any suitable receiving instru-
ment placed in the branch b; S is any suitable galvanoscope connected
up in the branch b, and which can be short circuited at pleasure by
means of the plug P. The essential characteristic of the double balance
method is that there shall be balance in the branch with respect to
any current arriving along the line, while there shall be balance in the
branch with respect to any current originating in the battery E when
that battery alone is acting, or in other words, of any current received
along the line none shall flow through the branch b, while of any current
sent from the signalling battery in the station, when that station alone
is signalling, none shall pass through the branch g. Thus there are
two different balances in two different branches, and by this means
advantages are secured, not presented by any other duplex method, to
it, firstly, that the signals received from the distant stations can be read
from the receiving instrument G in the branch without their being in
any way interfered with by the signals sent from the home battery,
while the signals sent from the home battery can be read from the
receiving instrument B in the branch without their being in any way
interfered with by the signals received from the distant station; and
secondly, that the condition of "immediate balance " is fulfilled, that is
to say, that when balance in the branch g is at any time disturbed, and
that it has been determined to readjust that balance by altering one
branch only, then that balance can be at once re-established by a single
readjustment made in the branch b, in which branch, moreover, the
approach towards balance is always "accelerated," whereas in any other
branch it would be "retarded," or in other words if balance in the
branch is to be re-established by readjusting bone such readjustment
will be sufficient, whereas if the same balance were to be re-established
by readjusting a or d several successive readjustments would be
necessary in both stations before balance in either station could be
practically established, and theoretically rigid balance could be attained
only after an indefinite number of such successive readjustments.
Part Second. Ratios and Magnitudes of the Resistances.-The safe
and efficient working of duplex telegraphy is secured by fulfilling

page_text: certain definite laws between the ratios and magnitudes of the several
resistances composing any duplex arrangement. The object of observing
the said relations is to place duplex telegraphy, in so far as the particular
duplex method adopted will allow of, on a footing of equality with
respect to the regularity of the signals with single telegraphy, and
further, to work any particular duplex method adopted to the greatest
advantage. The relations to be observed between the resistances will
be different for different duplex methods and obviously the closeness
with which duplex telegraphy will approximate to single telegraphy in
respect to the regularity of signalling will depend on the nature of the
particular duplex method adopted. It will be manifest, however, from
what follows that duplex telegraphy by the double balance method,
when the necessary relations among the resistances are observed, will
absolutely be on a par with single telegraphy in respect to the regularity
of signals. With respect to the ratios of the resistances in the double
balance method (see Figure 1) the resistance a, the resistance d, the
resistance f, the sum of the resistances w and c, and the resistance g
(inclusive of that of the receiving instrument G) are made severally
equal to one another, and with respect to the magnitudes of the
resistances in the same method, the equal resistances a, d, f, &c., are
cach made equal to one-half the observed resistance of the line as
measured from that station for which the magnitudes of the resistances
are to be calculated when its end is put directly to earth at the distant
station and the resistance (which does not include the resistance of
the galvanoscope S, but does include that of the receiving instrument B)
equal to one-sixth of the same measured line resistance. By observing
these relations advantages are gained not presented by any other
arrangement to it, firstly, that the irregularity of the received signals
in the one station are entirely independent of the irregularity of the
received signals in the other station with which it is corresponding;
secondly, that the irregularity of the received signals in each station is
due only to the effects of balance in the branch in that station not
being rigidly established, and to no other influences soever proper to the
method; thirdly, that any disturbance of balance in the branch I will
have the least possible effect on the strength of the received signals;
and fourthly, that the requisite strength of signals is secured in the
most economical manner.
Part Third Regulator The safe and efficient working of duplex

page_text: resistances, when necessary, such that the given laws shall be permanently
adhered to, that is to say, that the conditions of double balance shall be
always fulfilled, and that the several resistances shall invariably have
their best values, no matter how the line may vary. This is effected in
the first place by means of a certain mechanical contrivance that is
herein called a "regulator." In Figure 2 one form of this contrivance
is shewn. The resistances in this diagram are severally identical with
the resistances represented by the same letters in Figure 1, and each of
them consists partly of a fixed resistance (i.e., one which is only adjusted
in the ordinary manner by means of plugs, and whose magnitude is not
altered by the motion of the mechanical contrivance itself), and partly
of an adjustable resistance (i.e., one whose magnitude is altered by the
motion of the mechanical contrivance). If for instance all the six arms
(Figure 2) be turned simultaneously from left to right then all the
resistances increase by certain increments, while on the contrary if all
the six arms be turned simultaneously from right to left all the resistances
will similarly decrease. The increments (or decrements) of all the
resistances with the exception of bare equal, while those of b are one-
third of the increments (or decrements) of the other resistances. The
magnitude of the increments (or decrements) depends solely on the
difference of the sensibility of the receiving instrument G and the so
called "indicator," to be described hereafter. The arm & which adjusts w
is so attached to the axis that it can be moved alone, when desired,
without carrying the axis round with it, in order that the magnitude of w
may be readjusted when necessary without altering the other resistances.
The separate readjustment of w is occasionally required in order to keep
the sum of w and e constantly equal to f, for e the internal resistance of
the battery will be liable to vary in the course of time (although but
slowly and steadily if a suitable battery, such as Minotto's, be employed).
The simultaneous readjustment of all the resistances by means of the
"regulator" may be effected either by hand or automatically, but in
both cases an "indicator" is required, which in the former case when
any disturbance occurs in the branch g will guide the operator by
shewing him in which direction to turn the regulator, and in the latter
case will by its own motion determine the simultaneous readjustment of
the resistances,, for example, by means of an electro mechanical arrange-
ment that supplies the place of the operator.
Part Fourth. Readjustment in Battery Branch.-The sum of the
resistances e and w has to be kept equal to the resistance f; but since e

page_text: represents the resistance of a battery it may and generally will alter in
the course of time, and it is therefore necessary from time to time to
readjust w in order to maintain the above equality. This is effected as
follows (see Figure 1):-Throw the galvanoscope S into circuit in the
branch and send a current from the battery (the distant station
remaining at rest) and note the deflection produced thereby in the
galvanoscope S, or better reduce by the aid of permanent magnets the
deflection produced thereby in the galvanoscope S so as to bring
the needle back to zero. This being done direct the operator at the
distant station to send a current from his battery by pressing down his
key, and at the same time again send a current from your own battery;
if under these circumstances the needle of the galvanoscope S remains
undisturbed then the sum of the resistances e and wis equal to the
resistance f, but if by the fact of the distant station also sending a
current the needle of the galvanoscope S is disturbed then readjust the
resistance w until the needle has regained its original position, when
again the sum of the resistance e and we will be equal to the resistance f.
Part Fifth. Indicator.-The indicator N is an aperiodic instrument,
that is, so sensitive in comparison with the receiving instrument G. that
it indicates any disturbance of balance in the branch g long before the
receiving instrument G is sensible of it.
с
it
may
The indicator may be of several different kinds, that is
to say,
be worked clectro-statically, electro-magnetically, electro-dynamically,
&c., and it may consequently consist of various forms of clectrometer,
galvanometer, electro-dynamometer, &c.
Again there are different methods of connecting up the indicator N,
as for example:--
Firstly. Figure 3 gives the general arrangement when the receiving
instrument G is itself used as the contact breaker; m is a large
resistance as compared with any resistance in the duplex arrange-
ment; J is a condenser of a certain capacity; N is the indicator. This
arrangement fulfills the condition that the indicator N is not worked
by the signalling currents on account of the retarding influence of the
resistance m and the condenser J which allows the contact at n to be
opened before a current can establish itself through N, but when no
signals are passing, i.c., when the home station alone is sending and
balance is somewhat disturbed, then a part of the slight force disturbing
balance will move the indicator N (but will not work the receiving

page_text: instrument G) on account of there being now sufficient time for it to
act on the indicator, which will either shew the operator in which
direction to turn the "regulator" in order to restore balance, or if the
readjustment be effected automatically will set a mechanical train (the
"operator") in motion, which will alter the resistances to the requisito
extent by means of the "regulator." The indicator in this case consists
of a suspended aperiodic swinging body which is turned about the axis
of its suspension whenever a force acts on the indicator. The moveable
suspended body may be either a system of magnets suspended in a
metallic tube by a single fibre and kept at rest by the earth's magnetism
or by fixed magnets, or a system of magnets suspended bifilarly in a
metal tube; or a wire coil surrounded by a metallic tube (aluminum by
preference) attached to it and free to move with it, and suspended
bifilarly in the presence of powerful permanent or electro-magnets. If
the indicator be an electrometer of any convenient form the connections
remain exactly the same as in Fig. 3, only in this case the magnitudo
of the resistance m may be reduced.
Secondly. Figure 4 shows the arrangement of the indicator when a
special contact breaker is employed. The special contact breaker may
consist of a polarized receiver P connected up between the line wire and
the duplex arrangement (1), in such a manner that "single signals"
arriving from the distant station (i.c, the distant station alone signalling)
do not (and do not tend to) work it; and (2), so adjusted that the feeble
current that there may be in the line during "duplex signals" (i.e, both
stations sending simultaneously) is not sufficiently strong (although it
tends) to work it, but (3), adjusted sufficiently sensitively that the strong
current which is in the line when the home station alone is sending
works it well. In this case therefore when the home station alone is
sending the receiver P will be worked and will close the contact at n,
thus completing the circuit through the indicator. If then balance be
disturbed the indicator will be brought into play and will restore it in
the manner already described. This arrangement requires that both
stations shall work with the same pole of the signalling battery to line,
and it is therefore less general in application than the previous method
which is subjected to no such restriction. This however cannot be
regarded as a disadvantage for it is better always to work with copper
to line (as imperfect insulation has then less effect), and in practice I
prefer this arrangement.

page_text: Part Sixth. Operator.-In case the "regulator" (Fig. 2) is to be
worked automatically, then between it and the "indicator" N (already
described) is placed an "operator." Figure 5 shews generally how the
connections are made. When the "indicator" N makes contact at v;
the current from the local battery E' passes through the coil v, attracts
the armature t, and opens the contact at u. So soon as the contact at u
is opened the local current through the coil v is interrupted and the
armature is released. So long then as the contact at persists the
armature will be alternately attracted and released. The first move-
ment of releases an escapement and sets a train of clockwork in motion-
which turns the axis carrying the arms of the "regulator." The motion
of the clockwork is step by step, so that the momentary release of the
escapement allows the clockwork to move through one step, and thus by
means of the arms of the "regulator" which are thereby turned through
one step to increase (say) all the resistances by a certain increment.
So soon as the force acting on the "indicator" changes in sign, then
contact will be made at v' instead of v, and the armature will come
into play instead of t, and the arms of the "regulator" will be turned
in the reverse direction, and all the resistances will be diminished by a
certain decrement. When balance is established the "indicator" is in
equilibrio, and contact will be made neither at nor at ¹; but so soon
as balance is disturbed then a force acts on the "indicator" and contact
is made either at v or at v¹, according to the direction of the disturbance,
and persists until balance is restored.
Part Seventh. Coustant Resistance Key.-The safe and efficient
working of duplex telegraphy is secured by employing a key, or its
equivalent, of peculiar construction which I call the "constant resistance
key." Whatever may be the special form given to such keys they are
all based on one and the same principle, viz., that during the first move-
ment (up or down stroke) a force is stored up in a spring before the
contacts are changed, which force finally causes the change in these
contacts; for this reason the two principal contacts 1 and 2 of the key
co-exist only for an almost infinitesimal time, the length of which is
morcover independent of the signalling speed..
In Fig. 6 one of the many possible forms of the key is given; in (a)
the key is at rest; in (b) the key is in its intermediate position; in (c)
the key is in the position of sending. S is the spring that stores up

page_text: energy. One of its ends is rigidly fixed to a point o independant of the
key, and the other end to the point t of the key.
Part Eighth. Translation.-In duplex telegraphy translation at any
intermediate station is effected when necessary by adopting at the trans-
lating station arrangements precisely similar to those already described
for the case of terminal stations. As when translating the armature of
the signalling instrument acts as an automatic key to retransmit the
received signals; these armatures are arranged and constructed in exactly
the same manner as has been described in the case of the "constant
resistance key."
Part Ninth. Automatic Adjustment in the Branch b.-Under certain
circumstances it may be sufficient and convenient to readjust balance
when disturbed in one branch only. In this case the readjustment is to
be made invariably in the branch lying opposite the line, that is to say,
in the branch b (see Figure 1). The "indicator" and the "operator'
as already described remain unaltered, but the "regulator" becomes
simpler in form as only one arm, namely for b, is now required.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said Emil Oesterley in the Great Seal Patent Office on
the 23rd December 1874.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, EMIL
OESTERLEY, of 21, Mincing Lane, in the City of London, send
greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Twenty-seventh day of June, in the year of our
Lord One thousand eight hundred and seventy-four, in the thirty-
eighth year of Her reign, did, for Herself, Her heirs and successors, give
and grant unto me, the said Emil Oesterley, Her special license that
I, the said Emil Oesterley, my executors, administrators, and assigns, or
such others as I, the said Emil Oesterley, my executors, administrators,
and assigns, should at any time agree with, and no others, from time
to time and at all times thereafter during the term therein expressed,
should and lawfully might make, use, exercise, and vend, within the

page_text: United Kingdom of Great Britain and Ireland, the Channel Islands,
and Isle of Man, an Invention for "IMPROVEMENTS IN DUPLEX TELEGRAPHY
AND IN APPARATUS TO BE USED FOR THIS PURPOSE," (a communication to me
from abroad by Louis Schwendler, of Garden Reach, Calcutta, India,
Electrician,) upon the condition (amongst others) that I, the said
Emil Oesterley, my exccutors or administrators, by an instrument
in writing under my, or their, or one of their hands and scals, should
particularly describe and ascertain the nature of the said Invention, and
in what manner the same was to be performed, and cause the same to
be filed in the Great Scal Patent Office within six calendar months
next and immediately after the date of the said Letters Patent.
NOW KNOW YE, that I, the said Emil Oesterley, do hereby declare
the nature of the said Invention, and in what manner the same is to
be performed, to be particularly described and ascertained in and by
the following statement (reference being had to the Drawings annexed),
that is to say:
-
For the sake of distinctness I have divided the description of the said
Invention into several parts.
Part First. Method. The safe and efficient working of duplex
telegraphy is secured by employing the method herein called the
"double balance method" (see Figure 1). a, b, d, g, w, and f, arc
adjustable resistances; E is the signalling battery, the internal resistance
of which is e; K is the signalling key of peculiar construction; G is
any suitable receiving instrument placed in the branch g; B is any
suitable receiving instrument placed in the branch 6; S is any suitable
galvanoscope connected up in the branch 6, and which can be short
circuited at pleasure by means of the plug p. The essential characteristic
of the double balance method is that there shall be balance in the branch
b with respect to any current arriving along the line, while there shall
be balance in the branch g with respect to any current originating in the
battery E when that battery alone is acting, or in other words of any
current received along the line none shall flow through the branch b,
while of any current sent from the signalling battery in the station when
that station alone is signalling none shall pass through the branch g.
Thus there are two different balances on two different branches, and by
this means advantages are secured not presented by any other duplex
method, to wit, firstly, that the signals received from the distant station

page_text: can be read from the receiving instrument G in the branch g without
their being in any way interfered with by the signals sent from the home
battery, while the signals sent from the home battery can be read from
the receiving instrument B in the branch b, without their being in any
way interfered with by the signals received from the distant station; and
secondly, that the condition of "immediate balance" is fulfilled, that is
to say, that when balance in the branch is at any time disturbed and
that it has been determined to readjust that balance by altering one
branch only, then that balance can be at once re-established by a single
readjustment made in the branch b, in which branch morcover the
approach towards balance is always "accelerated," whereas in any other
branch it would be "retarded" or in other words, if balance in the
branch is to be re-established by readjusting one such readjustment
will be sufficient, whereas if the same balance were to be re-established
by readjusting a ord several successive readjustments would be necessary
in both stations before balance in either station could be practically
established, and theoretically rigid balance could be attained only after
an indefinite number of such successive readjustments. The receiving
instrument connected up in the branch of the arrangement for the
purpose of recording the out-going signals, should be shunted by means
of a wire equal in resistance to the resistance of the instrument itself, in
order to prevent the disturbing effects of induction.
Part Second. Ratios and Magnitudes of the Resistances.-The safe
and efficient working of duplex telegraphy is secured by fulfilling
certain definite laws between the ratios and magnitudes of the several
resistances composing any duplex arrangement. The object of observing
the said relations is to place duplex telegraphy, in so far as the par-
ticular duplex method adopted will allow of, on a footing of equality
with respect to the regularity of the signals with single telegraphy;
and further, to work any particular duplex method adopted to the
greatest advantage. The relations to be observed between the resistances
will be different for different duplex methods, and obviously, the closc-
ness with which duplex telegraphy will approximate to single telegraphy
in respect to the regularity of signalling will depend on the nature of
the particular duplex method adopted. It will be manifest however
from what follows that duplex telegraphy, by the double balance method
when the necessary relations among the resistances are observed, will
absolutely be on a par with single telegraphy in respect to the regularity

page_text: of signals. With respect to the ratios of the resistances in the double
balance method (see Figure 1) the resistance a, the resistance d, the
resistance f, the sum of the resistances w and e, and the resistance g
(inclusive of that of the receiving instrument G) are made severally
equal to one another, and with respect to the magnitudes of the
resistances in the same method the equal resistances a, d, f, &c., are
cach made equal to one half the observed resistance of the line, as
measured from that station for which the magnitudes of the resistances
are to be calculated when its cnd is put directly to earth at the distant
station, and the resistance b (which does not include the resistance of
the galvanoscope S, but does include that of the receiving instrument B).
equal to one-sixth of the same measured line resistance. By observing
these relations advantages are gained not presented by any other arrange-
ment, to wit, firstly, that the irregularity of the received signals in
the one station are entirely independent of the irregularity of the
received signals in the other station with which it is corresponding;
secondly, that the irregularity of the received signals in each station
is due only to the effects of balance in the branch in that station
not being rigidly established, and to no other influences soever proper
to the method; thirdly, that any disturbance of balance in the branch
will have the least possible effect on the strength of the received signals;
and fourthly, that the requisite strength of signals is secured in the most.
economical manner.
Part Third. Regulator.-The safe and efficient working of dulplex
telegraphy is secured by adopting a mode of readjusting the several
resistances when necessary, such that the given laws shall be
permanently adhered to, that is to say that the conditions of double
balance shall be always fulfilled, and that the several resistances shall
invariably have their best values, no matter how the line may vary.
This is effected in the first place, by means of a certain mechanical
contrivance that is herein called a "regulator." In Figure 2, one form
of this contrivance is slicwn. The resistances in this diagram are
severally identical with the resistances represented by the same letters
in Figure 1, and each of them consists partly of a fixed resistance (i.c.,
one which is only adjusted in the ordinary manner by means of plugs,
and whose magnitude is not altered by the motion of the mechanical
contrivance itself), and partly of an adjustable resistance (i.c., one whose
magnitude is altered by the motion of the mechanical contrivance). If,

page_text: for instance, all the six arms (Figure 2) be turned simultaneously from
left to right, then all the resistances increase by certain increments; while
on the contrary, if all the six arms be turned simultaneously from right
to left, all the resistances will similarly decrease. The increments (or
decrements) of all the resistances, with the exception of b, are equal, while
those of bare one-third of the increments (or decrements) of the other re-
sistances. The magnitude of the increments (or decrements) depends solely
on the difference of the sensibility of the receiving instrument G, and the
so called "indicator ".to be described hereafter. The arm (h) which adjusts
w is so attached to the axis that it can be moved alone when desired
without carrying the axis round with it, in order that the magnitude
of w may be readjusted when necessary without altering the other
resistances. The separate readjustment of w is occasionally required in
order to keep the sum of w and e constantly equal to f, for e the
internal resistance of the battery will be liable to vary in the course of
time (although but slowly and steadily if a suitable battery, such as
Minotto's, be employed). The simultaneous readjustment of all the
resistances by means of the "regulator" may be effected either by hand
or automatically, but in both cases an "indicator" is required, which
in the former case, when any disturbance occurs in the branch y will
guide the operator by showing him in which direction to turn the
regulator, and in the latter case will by its own motion determine
the simultaneous readjustment of the resistances, for example, by means
of an electro-mechanical arrangment that supplies the place of the
operator.
Part Fourth. Readjustment in Battery Branch.-The sum of the
resistances e and w has to be kept equal to the resistance f, but since e
represents the resistance of a battery it may and generally will altor
in the course of time, and it is therefore necessary from time to
time to readjust w in order to maintain the above equality. This is
effected as follows (see Figure 1):-Throw the galvanoscope S into
circuit in the branch and send a current from the battery (the distant
station remaining at rest) and note the deflection produced thereby in
the galvanoscope S, or better reduce by the aid of permanent magnets
the deflection produced thereby in the galvanoscope S so as to bring the
needle back to zero. This being done direct the operator at the distant
station to send a current from his battery by pressing down his key, and
at the same time again send a current from your own battery; if under
these circumstances the needle of the galvanoscope S remains undisturbed

page_text: then the sum of the resistances e and wis equal to the resistance, but
if by the fact of the distant station also sending a current the needle of
the galvanoscope S is disturbed, then readjust the resistance w until the
needle has regained its original position, when again the sum of the
resistances e and we will be equal to the resistance f.
Part Fifth. Indicator.-The indicator N is an aperiodic instrument,
that is, so sensitive in comparison with the receiving instrument G that
it indicates any disturbance of balance in the branch a long before the
receiving instrument G is sensible of it.
The indicator may be of several different kinds, that is to say, it may
be worked electro-statically, electro-magnetically, electro-dynamically,
&c., and it may consequently consist of various forms of electrometer,
galvanometer, electo-dynamometer, &c.
Again, there are different methods of connecting up the indicator N, as
for example:-
Firstly. Figure 3 gives the general arrangement when the receiving
instrument G is itself used as the contact breaker; M is a large
resistance as compared with any resistance in the duplex arrange-
ment; J is a condenser of a certain capacity; N is the indicator. This
arrangement fulfils the condition that the indicator N is not worked by
the signalling currents on account of the retarding influence of the
resistance M and the condenser J, which allows the contact at n to be
opened before a current can establish itself through N; but when no
signals are passing, i.c., when the home station alone is sending and
balance is somewhat disturbed, then a part of the slight force disturbing
balance will move the indicator N (but will not work the receiving
instrument G) on account of there being now sufficient time for it to act
on the indicator, which will either shew the operator in which direction
to turn the "regulator" in order to restore balance, or, if the readjust-
ment be effected automatically, will set a mechanical train (tho
"operator") in motion, which will alter the resistances to the requisite
extent by means of the "regulator." The indicator in this case consists
of a suspended aperiodic swinging body, which is turned about the axis
of its suspension whenever a force acts on the indicator. The movable
suspended body may be either a system of magnets suspended in a
metallic tube by a single fibre and kept at rest by the earth's magnetism
or by fixed magnets, or a system of magnets suspended bifilarly in a

page_text: metal tube, or a wire coil surrounded by a metallic tube (aluminium by
preference) attached to it, and free to move with it, and suspended
bifilarly in the presence of powerful permanent or electro-magnets. If
the indicator be an electrometer of any convenient form the connections
remain exactly the same as in Figure 3, only in this case the magnitude
of the resistance M. may be reduced.
Secondly. Figure 4 shews the arrangement of the indicator when a
special contact breaker is employed. The special contact breaker may
consist of a polarized receiver P connected up between the line wire and
the duplex arrangement (1) in such a manner that "single signals"
arriving from the distant station (i. c., the distant station alone signalling)
do not (and do not tend to) work it, and (2) so adjusted that the feeble
current that there may be in the line during "duplex signals" (i. c., both
stations sending simultaneously) is not sufficiently strong (although it
tends) to work it, but (3) adjusted sufficiently sensitively that the strong
current which is in the line when the home station alone is sending
works it well. In this case therefore when the home station alone is sending
the receiver P will be worked, and will close the contact at n, thus com-
pleting the circuit through the indicator. If then balance be disturbed
the indicator will be brought into play and will restore it in the manner
already described. This arrangement requires that both stations shall
work with the same pole of the signalling battery to line, and it is there-
fore less general in application than the previous method, which is
subjected to no such restriction. This however cannot be regarded as
a disadvantage, for it is better always to work with copper to line (as
imperfect insulation has then less effect), and in practice I prefer this
arrangement. I also employ for the purpose of indicating disturbances
before they can be felt by the receiving instrument an "external duplex
indicator," which may consist of a small compass with an exceedingly
light needle and index placed on the top of the receiving relay, and the
indications of which I further employ for adjusting balance for transient
and permanent currents in the system.
Part Sixth. Operator. In case the "regulator" (Figure 2) is to be
worked automatically, then between it and the "indicator" N (already
described) is placed an "operator." Figure 5 shows generally how the
connections are made. When the "indicator" N makes contact at V
the current from the local battery E passes through the coil v, attracts
the armature t, and opens the contact at u. So soon as the contact at u

page_text: is opened the local current through the coil is interrupted and the
armature is released. So long then as the contact at persists the
armature will be alternately attracted and released. The first move-
ment of t releases an escapement and sets a train of clockwork in
motion, which turns the axis carrying the arms of the " "regulator."
The motion of the clockwork is step by step, so that the momentary
release of the escapement allows the clockwork to move through one
step, and thus by means of the arms of the "regulator," which are
thereby turned through one step to increase (say) all the resistances by
a certain increment. So soon as the force acting on the "indicator'
changes in sign then contact will be made at v¹ instead of v, and the
armature will come into play instead of t, and the arms of the " regu-
lator" will be turned in the reverse direction, and all the resistances will
be diminished by a certain decrement. When balance is established the
"indicator" is in equilibrio, and contact will be made neither at v nor
at v, but so soon as balance is disturbed then a force acts on the
"indicator," and contact is made either at v or at v³, according to the
direction of the disturbance, and persists until balance is restored.
Part Seventh. Constant Resistance Key.-The safe and efficient
working of duplex telegraphy is secured by employing a key or its
equivalent of peculiar construction, which I call the "constant resistance
key." Whatever may be the special form given to such keys they are
all based on one and the same principle, viz., that during the first
movement (up or down stroke) a force is stored up in a spring before
the contacts are changed, which force finally causes the change in these
contacts. For this reason the two principal contacts 1 and 2 of the key
co-exist only for an almost infinitesimal time, the length of which is
moreover independent of the signalling speed.
In Figures 6 one of the many possible forms of the key is given.
In (a) the key is at rest; in (b) the key is in its intermediate position;
in (c) the key is in the position of sending. S is the spring that stores
up energy. One of its ends is rigidly fixed to a point o independent of
the key, and the other end to the point of the key. With the constant
resistance key I employ, if requisite, two batteries, the one for producing
the signals, and the other, which is much less intense, for rendering
more constant the proportion of the true currents, thus maintaining
balance more rigidly when inductive action takes place in any of the
branches of the arrangement, .

page_text: Part Eighth. Translation.-In duplex telegraphy translation at any
intermediate station is effected, when necessary, by adopting at the
translating station arrangements precisely similar to those already
described for the case of terminal stations, as when translating, the
armature of the signalling instrument acts as an automatic key to re-
transmit the received signals. These armatures are arranged and con-
structed in exactly the same manner as has been described in the case of
the "constant resistance key."
Part Ninth. Automatic Adjustment in the Branch 6.-Under certain
circumstances it may be sufficient and convenient to readjust balance
when disturbed in one branch only. In this case the readjustment is to
be made invariably in the branch lying opposite the line, that is to say,
in the branch b (see Figure 1). The "indicator" and the "operator"
as already described remain unaltered, but the "regulator" becomes
simpler in form, as only one arm, namely for b, is now required. Under
certain circumstances, and notably in the case of long submarine cables
or underground circuits, it may be preferable to transmit the requisite
impulses which produce the signals by means of induced currents instead
of direct currents, and that consequently in any or all of the branches
composing the duplex arrangement inductive resistances, whether in the
form of the so-called accumulators or condensers, or of induction bobbins
with primary and secondary coils, may be employed in the place of or in
addition to the electrical resistances. Also in connection with the
d branch I employ when requisite a piece of submarine cable of any
convenient form, whose length bears a fixed ratio to the length of the
cable to be worked by the arrangement.