For the last eighteen months a system has been in active operation in
Belgium whereby the ordinary telegraph wires are used to convey
telephonic communications at the same time that they are being
employed in their ordinary work of transmitting telegraphic messages.
This system, the invention of M. Van Rysselberghe, whose previous
devices for diminishing the evil effects of induction in the telephone
service will be remembered, has lately been described in the _Journal
Telegraphique_ of Berne, by M.J. Banneux of the Belgian Telegraph
Department. Our information is derived from this article and from
others by M. Hospitalier.
The method previously adopted by Van Rysselberghe, to prevent
induction from taking place between the telegraph wires and those
running parallel to them used for telephone work, was briefly as
follows: The system of sending the dots and dashes of the
code--usually done by depressing and raising a key which suddenly
turns on the current and then suddenly turns it off--was modified so
that the current should rise gradually and fall gradually in its
strength by the introduction of suitable resistances. These were
introduced into the circuit at the moment of closing or opening by a
simple automatic arrangement worked exactly as before by a key. The
result, of the gradual opening and gradual closing of the circuit was
that the current attained its full strength gradually instead of
suddenly, and died away also gradually. And as induction from one wire
to another depends not on the strength of the current, but on the rate
at which the strength changes, this very simple modification had the
effect of suppressing induction. Later Van Rysselberghe changed these
arrangements for the still simpler device of introducing permanently
into the circuit either condensers or else electro-magnets having a
high coefficient of self-induction. These, as is well known to all
telegraphic engineers, retard the rise or fall of an electric current;
they fulfill the conditions required for the working of Van
Rysselberghe's method better than any other device.
Having got thus far in his devices for destroying induction from one
line to another, Van Rysselberghe saw that, as an immediate
consequence, it might be concluded that, if the telegraph currents
were thus modified and graduated so that they produced no induction in
a neighboring telephone line, they would produce no sound in the
telephone if that instrument were itself joined up in the telegraph
line. And such was found to be case. Why this is so will be more
readily comprehended if it be remembered that a telephone is sensitive
to the changes in the strength of the current if those changes occur
with a frequency of some hundreds or in some cases thousands of times
_per second_. On the other hand, currents vibrating with such rapidity
as this are utterly incompetent to affect the moving parts of
telegraphic instruments, which cannot at the most be worked so as to
give more than 200 to 800 separate signals _per minute_.
[Illustration: Fig. 1]
[Illustration: Fig. 2]
The simplest arrangement for carrying out this method is shown in Fig.
1, which illustrates the arrangements at one end of a line. M is the
Morse key for sending messages, and is shown as in its position of
rest for receiving. The currents arriving from the line pass first
through a "graduating" electromagnet, E2, of about 500 ohms
resistance, then through the key, thence through the electromagnet, R,
of the receiving Morse instrument, and so to the earth. A condenser,
C, of 2 microfarads capacity is also introduced between the key and
earth. There is a second "graduating" electromagnet, E1, of 500 ohms
resistance introduced between the sending battery, B, and the key.
When the key, M, is depressed in order to send a signal, the current
from the battery must charge the condenser, C, and must magnetize the
cores of the two electromagnets, E1 and E2, and is thereby retarded in
rising to its full strength. Consequently no sound is heard in a
telephone, T, inserted in the line-circuit. Neither the currents which
start from one end nor those which start from the other will affect
the telephones inserted in the line. And, if these currents do not
affect telephones in the actual line, it is clear that they will not
affect telephones in neighboring lines. Also the telephones so
inserted in the main line might be used for speaking to one another,
though the arrangement of the telephones in the same actual line would
be inconvenient. Accordingly M. Van Rysselberghe has devised a further
modification in which a separate branch taken from the telegraph line
is made available for the telephone service. To understand this
matter, one other fact must be explained. Telephonic conversation can
be carried on, even though the actual metallic communication be
severed by the insertion of a condenser. Indeed, in quite the early
days of the Bell telephone, an operator in the States used a condenser
in the telegraph line to enable him to talk through the wire. If a
telephonic set at T1 (Fig. 2) communicate through the line to a
distant station, T2, through a condenser, C, of a capacity of half a
microfarad, conversation is still perfectly audible, provided the
telephonic system is one that acts by induction currents. And since in
this case the interposition of the condenser prevents any continuous
flow of current through the line, no perceptible weakening will be
felt if a shunt S, of as high a resistance as 500 ohms and of great
electromagnetic rigidity, that is to say, having a high coefficient of
self-induction, be placed across the circuit from line to earth. In
this, as well as in the other figures, the telephones indicated are of
the Bell pattern, and if set up as shown in Fig. 2, without any
battery, would be used both as transmitter and receiver on Bell's
original plan. But as a matter of fact any ordinary telephone might be
used. In practice the Bell telephone is not advantageous as a
transmitter, and has been abandoned except for receiving; the Blake,
Ader, or some other modification of the microphone being used in
conjunction with a separate battery. To avoid complication in the
drawings, however, the simplest case is taken. And it must be
understood that instead of the single instrument shown at T1 or T2, a
complete set of telephonic instruments, including transmitter,
battery, induction-coil, and receiver or receivers, may be
substituted. And if a shunt, S, of 500 ohms placed across the circuit
makes no difference to the talking in the telephones because of the
interposition of the separating condenser, C, it will readily be
understood that a telegraphic system properly "graduated," and having
also a resistance of 500 ohms, will not affect the telephones if
interposed in the place of S. This arrangement is shown in Fig. 3,
where the "graduated" telegraph-set from Fig. 1 is intercalated into
the telephonic system of Fig. 2, so that both work simultaneously, but
independently, through a single line. The combined system at each end
of the line will then consist of the telephone-set, T1, the telegraph
instruments (comprising battery, B1, key, M1 and Morse receiver, R1),
the "graduating" electromagnets, E1, and E2, the "graduating"
condenser, C1, and the "separating" condenser, C2. It was found by
actual experiments that the same arrangement was good for lines
varying from 28 to 200 miles in length. A single wire between
Brussels, Ghent, and Ostend is now regularly employed for transmission
by telegraph of the ordinary messages and of the telemeteorographic
signals between the two observatories at those places, and by
telephone of verbal simultaneous correspondence, for one of the Ghent
newspapers. A still more interesting arrangement is possible, and is
indicated in Fig. 4. Here a separating condenser is introduced at the
intermediate station at Ghent between earth and the line, which is
thereby cut into two independent sections for telephonic purposes,
while remaining for telegraphic purposes a single undivided line
between Brussels and Ostend. Brussels can telegraph to Ostend, or
Ostend to Brussels, and at the same time the wire can be used to
telephone between Ghent and Ostend, or between Ghent and Brussels, or
both sections may be simultaneously used.
[Illustration: Fig. 3]
[Illustration: Fig. 4]
It would appear, then, that M. Van Rysselberghe has made an advance of
very extraordinary merit in devising these combinations. We have seen
in recent years how duplex telegraphy superseded single working, only
to be in turn superseded by the quadruplex system. Multiplex
telegraphy of various kinds has been actively pursued, but chiefly on
the other side of the Atlantic rather than in this country, where our
fast-speed automatic system has proved quite adequate hitherto.
Whether we shall see the adoption in the United Kingdom of Van
Rysselberghe's system is, however, by no means certain. The essence of
it consists in retarding the telegraphic signals to a degree quite
incompatible with the fast-speed automatic transmission of telegraphic
messages in which our Post Office system excels. We are not likely to
spoil our telegraphic system for the sake of simultaneous telephony,
unless there is something to be gained of much greater advantage than
as yet appears.--_Nature._
* * * * *