“On using a sensitive relay in the circuit with the coherer tube,
and an ordinary electric bell in the other circuit of the relay, for
sound signals and as an automatic tapper for the coherer, I obtain
an apparatus which exactly answers every electric wave by a short
ring, and by rhythmical strokes if electric vibrations be excited
continuously.”
“On connecting an electromagnetic recorder in parallel with the bell,
tracing a straight line along the paper band which is moved by a
12-hour clockwork cylinder, I obtain an instrument registering by a
cross line on the moving band every electric wave that reaches the
coherer from across the atmosphere. Such an apparatus was placed at the
Meteorological Observatory at St. Petersburg in July, 1895, one of the
electrodes of the coherer being connected by an insulated wire with an
ordinary lightning conductor, the other electrode of the tube-coherer
being connected with the ground.”
[Illustration: FIG. 42 (Fig. 2 on p. 235 of _The Electrician_, Vol.
XL.).--Method of automatic tapping back by relay current employed for
telegraphy by Prof. Popoff in 1895.]
Prof. Popoff then goes on to say that his apparatus works well as a
lightning recorder, and that he hopes it can be used for signalling to
great distances. He says:--
“I can detect waves at the distance of one kilometre if I employ as
sender a Hertz vibrator with 30 centimetre spheres, and if I use the
ordinary Siemens relay; but with a Bjerknes vibrator 90 centimetres
diameter, and a more sensitive relay, I reach five kilometres of good
working.”
Thus it is plain that Prof. Popoff employed the elevated wire as
receiver in 1895, but did not employ it as sender.
In 1897 Prof. Slaby, of Berlin, published (in German) a book called
“Spark Telegraphy,” in which he described his success in signalling
from 3 to 13 miles across land. From this book we take the following
illustrations of the coherer and its connections:--
Fig. 43 shows the coherer tied on to a glass tube, by which it is
supported.
[Illustration: FIG. 43 (Fig. 7 of Slaby’s book).]
Fig. 44 shows the simplest form of its connection to a one-cell battery
A and a polarised relay B, which switches on another battery of several
cells _a_ operating the Morse instrument or electric bell or sounder
_b_ and also the tapper-back _c_, the hammer of which raps gently on
the coherer tube at every signal.
[Illustration: FIG. 44 (Fig. 8 of Slaby’s book).--Slaby’s arrangement
of Coherer and of tapper-back and relay connections.]
The actual apparatus is depicted in two views, Figs. 45 and 46, where
will be recognised on the left-hand side the coherer and tapper-back;
in the middle the batteries, both for relay and for coherer circuits;
and on the right-hand side a relay and the signalling or calling
instrument, in this case shown as an ordinary electric bell.
[Illustration: FIG. 45 (Fig. 16 of Slaby’s book).--View of Slaby’s
Receiving Apparatus, with call-bell rung by relay, or with Morse
instrument joined on to terminals _M_, and switch to change from
Calling to Signalling. _K K_ are the terminals of elevated wire and
earth, and the Coherer and Tapper-back are close to them.]
[Illustration: FIG. 46 (Fig. 17 of Prof. Slaby’s book).--An elevation
view of Prof. Slaby’s same Apparatus, showing the electromagnet and
hammer of the tapper-back worked by relay current from local battery,
as in Popoff’s plan of 1895.]
A Morse instrument is to be connected to the terminals M, and either it
or the bell can be switched into the circuit at pleasure. The form of
relay depicted is special to Slaby, but the rest of the arrangements
are practically identical with those shown by Marconi at Dover.
Fig. 47 gives a diagram of the actual connections.
Fig. 48 is a picture of one of Slaby’s signalling stations, showing the
way the elevated wire enters the building.
[Illustration: FIG. 47 (Fig. 19 of “Spark Telegraphy”).--Diagram of
Slaby’s connections in the above apparatus. F is the coherer and K the
tapper-back.]
During September, 1899, the Marconi method of signalling to long
distances was demonstrated before the British Association at Dover.
The chief feature of the installation was the elevated wire supported
by a mast, and terminating at the top in a small conductor, which is
usually made of wire netting, and is suspended from an insulating rod.
The lower end of this elevated wire passed into the building through
an aperture, and was connected to one terminal of the usual Ruhmkorff
coil, the other terminal of which was earthed. The signalling key was
of the simplest description, being nothing more than a well-insulated
Morse key worked by hand and causing a make-and-break in the primary
circuit of the coil. The ordinary trembling break of the induction
coil was at work in the usual way, so that while the signalling key
was depressed continuously there was a torrent of sparks between the
knobs of the secondary. This method of signalling was identical with
that employed by everyone since the time of Hertz, except that, instead
of connecting the secondary terminals to two insulated plates, one was
now connected to earth and the other to a small insulated conductor at
considerable elevation.
[Illustration: FIG. 48 (Fig. 11 of Prof. Slaby’s book on “Spark
Telegraphy”).]
[Illustration: FIG. 49 (p. 762, _The Electrician_, Vol.
XLIII.).--Marconi Signalling Mast at Dover Town Hall.]
[Illustration: FIG. 50 (Fig. 1, p. 7, _The Electrician_, Vol.
XLIII.).--Mast at South Foreland; from which Signals went to a similar
Mast at Wimereux, near Boulogne.]
From this mast in the town of Dover (Fig. 49) signals could be sent
to another loftier mast at the South Foreland (Fig. 50), where it is
itself elevated by chalk cliffs far above the sea. From this South
Foreland station, which was similar in all essential respects to the
Dover station, except that its elevation was greater, messages could
be sent and received to and from a station near Boulogne, on the
coast of France, and to and from the East Goodwins lightship. The
signalling was slow, but appeared dependable, and the simplicity of all
the arrangements was remarkable (Fig. 51). Concerning the receiving
apparatus there is little to be said, since it is in essence the same
as that which has already been described. It consists of a coherer of
the plug tube pattern, something like that depicted on page 23, but
excessively reduced in size, the glass tube being the size of a quill,
the two silver plugs close together separated only by a very few nickel
filings. This tube is mounted so that it can be struck after each
signal by a light electric hammer worked by a current from a local
battery switched on by a Siemens’ polarised relay, which is itself
actuated by the coherer current. Whenever the coherer receives a signal
the same current that works the tapper works also the Morse instrument
standing on the table alongside, and records a short or a long signal
on the tape. The coherer with its tapper, the polarised relay, and the
battery (a few dry cells) are all enclosed in one oblong iron box,
through an aperture in which the lower end of the elevated wire can be
inserted and brought into direct connection with the coherer.
To change from transmitting to receiving nothing is needed but the
detachment of this wire from the Ruhmkorff coil terminal and its
insertion through the aperture of the enclosing box so as to touch the
coherer circuit. The object of the box is, of course, the protection of
the coherer from undesired disturbances, exactly as described on page
34, and the collecting wire has the function there described likewise.
The electric tapper-back is also mentioned on page 31, but not as
being operated through a relay by the coherer circuit’s own current.
This last improvement seems to have been devised and employed by Prof.
Popoff at Cronstadt in 1895 (_see_ Fig. 42). No doubt it was arrived
at independently again by Mr. Marconi and the telegraph officials who
assisted him in his early experiments in this country.
The other box shown in Fig. 51 is probably a stand-by in case of
accident.
It is difficult to imagine a simpler contrivance, and it appeared to
work at Dover dependably, the messages coming out slowly in ordinary
dots and dashes, the torrent of sparks being sufficiently rapid not
to necessitate the breaking up of the dash into a series of dots. The
sluggishness of the Morse instrument or the relay, or the circuit as a
whole, enabled this excellent result to be attained with apparent ease.
[Illustration: FIG. 51 (p. 761, _The Electrician_, Vol.
XLIII.).--Apparatus for Sending and Receiving, shown by Prof. Fleming
to the British Association at Dover.]
A diagram of Marconi’s connection of sensitive tube to the relay and
tapper-back and Morse instrument, where W represents the elevated wire,
is given in Fig. 52.
[Illustration: FIG. 52 (Fig. 2, of p. 691, _The Electrician_, Vol.
XLII.).--Diagram of the connection of Relay and Tapper-back and Morse
Instrument, as given in Mr. Marconi’s Paper in the _Journal_ of the
Inst. Elec. Engineers for April, 1899; the relay being an ordinary
Siemens polarised relay.]
The mast at the South Foreland was stated to be 150 ft. high, but the
cliff on which it stands must be at a still greater elevation above
the sea. It was from this station that the real distant signalling was
performed, and probably not from the lower mast at Dover.