It may be well to explain that in my Royal Institution lecture I made
no reference to the transmission of waves along _wires_. I regard the
transmission of waves in _free space_ as the special discovery of
Hertz; though undoubtedly he got them on wires too. Their transmission
along wires is, however, a much older thing. Von Bezold saw them in
1870, and I myself got quantitative evidence of nodes and loops in
wires when working with Mr. Chattock in the session 1887-8 (_see_,
for instance, contemporary reports of the Bath Meeting of the British
Association, 1888, in _The Electrician_), and I exhibited them some
time afterwards to the Physical Society, the wires themselves becoming
momentarily luminous at every discharge except at the nodes, thus
enabling the waves to be actually seen, having been made stationary by
reflexion as in the corresponding acoustic experiment of Melde. This
experiment does not appear to have been properly known (p. 78).
[Illustration: FIG. 23.]
It may be worth mentioning that the arrangement frequently referred to
in Germany by the name of Lecher (viz., that shown in Fig. 23), and
on which a great number of experiments have been made, is nothing but
a pair of Leyden jars with long wires leading from their outer coats,
such as I constantly employed in these experiments. The wires from the
outer coat in my experiment were very long, sometimes going five or six
times round a large hall, like telegraph wires. And many measurements
of wave length were thus made by me at the same period as that in which
Hertz was working at Carlsruhe. The use of air dielectric instead of
glass permits the capacity to be adjusted, and also readily enables the
capacity to be small, and the frequency, therefore, high; but otherwise
the arrangement is the same in principle as had frequently been used by
myself in the series of experiments called “the recoil kick” (_Proc._
Roy. Soc., June 1891, Vol. 50, pp. 23-39). For these and other reasons
no reference has been made in my lecture to the work done on wires by
Sarasin and De la Rive; nor to other excellent work done by Lecher,
Rubens, Arons, Paalzow, Ritter, Blondlot, Curie, D. E. Jones, Yule,
Barton, and other experimenters.
APPLICATION OF THIS METHOD OF SIGNALLING AT A DISTANCE TO ACTUAL
TELEGRAPHY.
Although the method of signalling to a moderate distance through walls
or other non-conducting obstructions by means of Hertz waves emitted
from one station and detected by Branly filing tubes at another station
was practised by the author and by several other persons in this
country, it was not applied by them to actual telegraphy. The idea
of replacing a galvanometer, which was preferably a well-damped or
speaking galvanometer, by a relay working an ordinary sounder or Morse
was an obvious one, but so far as the present author was concerned he
did not realise that there would be any particular practical advantage
in thus with difficulty telegraphing across space instead of with
ease by the highly developed and simple telegraphic and telephonic
methods rendered possible by the use of a connecting wire. In this
non-perception of the practical uses of wireless telegraphy he
undoubtedly erred. But others were not so blind, though equally busy;
and notably Dr. Alexander Muirhead foresaw the telegraphic importance
of this method of signalling immediately after hearing the author’s
lecture on June 1st, 1894, and arranged a siphon recorder for the
purpose. Captain Jackson also, at Devonport, made experiments for the
Admiralty, and succeeded in telegraphing between ships in 1895 (or
1896). Prof. Popoff’s telegraphic application in 1895 is mentioned on
page 62.
By some chance a knowledge of the coherer method of detecting electric
waves did not spread fast in Germany, the many German workers in
Hertz waves continuing, for some time after 1894, the older and less
efficient, though for metrical purposes often more convenient, mode of
detecting them. But, in Italy, the work described in the preceding
lecture became well known, and the subject was developed largely,
especially by Prof. Righi, of Bologna, in the optical direction. It
was also developed in the same direction with many most interesting
results by Prof. Bose, of Calcutta, as mentioned in the text. Prof.
Righi made a large number of experiments, which he has since described
in an Italian treatise, “Opticé Elettrica,” and it appears that it was
from him that Signor Marconi learned about the subject, and immediately
conceived the idea of applying it to commercial telegraphy. He appears
to have worked at the subject for a short time in Italy, aiming at
getting the receiver to be more satisfactory and dependable, and
improving the early form of Branly filings tube depicted on page 23 by
greatly diminishing its size, bringing the terminals closer together,
and replacing the coarse borings by fine filings. He also sealed them
up in a vacuum, just as the author did, as related on page 34. The only
differences, indeed, between his procedure and the author’s during this
time were that Signor Marconi preferred nickel filings with a little
mercury and a low vacuum, whereas the author adhered chiefly to iron
and brass filings and to high vacua. At last he brought it over to
Dublin, where he was advised to take it to the Chief of the Government
Telegraphs, Mr. Preece, and accordingly he took his, at that time,
crude apparatus to the Post Office in a sealed box. There was no point
of novelty in it at this stage.
With the powerful aid of the Post Office Signor Marconi proceeded to
develop his system of telegraphy on a large scale; and, sometimes
failing, sometimes succeeding, gradually increased the distance over
which signalling was possible, and especially began to develop from
unpromising beginnings his special method for long-distance, viz., the
employment of a sending and receiving conducting plate or other small
surface, at the top of a lofty pole, connected through what was at
that time supposed apparently to be the real radiator, with the earth.
This elevated plate, connected as it now is through a simple spark gap
with the earth, is an obvious modification of a Hertz vibrator; for it
may be regarded simply as a Hertz vibrator with its axis vertical, as
Hertz often used it, and with its lower plate replaced by the earth,
so as to double the available capacity; but the action of a pair of
such elevated plates, connected through the earth conductively and
through the air inductively, as now used by Marconi for sender and
receiver respectively, is not quite like that of a Hertz vibrator and
a Hertz receiver acting on one another by emitted radiation in the
ordinary way. If it were not the same earth to which the plates were
connected, they would have to act ordinarily by radiation, but since
it is the _same_ earth, and that earth conducting (possibly, indeed,
with a submerged cable sheath connecting favourably-chosen stations),
then the two elevated plates are partially like the greatly separated
terminals of a _single_ Hertz vibrator.
Only one of the plates is charged during a sending operation, the other
is at zero potential, but some trace of the electrostatic lines from
one plate may extend in curved lines to the other, just as they extend
to every elevated conductor within hail of the sender in any direction.
Then comes the snap of the spark gap and the sudden discharge,
equivalent to the rush of an opposite charge of electricity suddenly
into the sending plate, disturbing the electric equilibrium at a
distance--at any distance to which any trace of electrostatic field had
been able to reach--and giving a kind of what is called in lightning
a “_return stroke_.” The effect of this on the distant plate and
conductor must be almost infinitesimal; nevertheless, separating it
from the earth is the most sensitive detector to a minute sudden rush
or jerk of electricity that can be imagined, or that has hitherto
been invented,--the coherer. Accordingly, absurdly minute though
the disturbance is, the coherer feels it, instantly increases in
conductivity, works the relay, and gives the signal. Every spark at
the distant spark gap causes a similar rush in or out of the distant
elevated plate, and the receiving plate collects such a fraction of
this disturbance as to stimulate the coherer and give a signal every
time. Not that it is to be supposed never to miss fire. At the present
time a coherer is not a rough instrument that can be left free from
expert attention with safety for a long time. There are times when it
goes on working for days or even weeks, but there are other times when
it gives trouble and needs some form of attention. Let us hope that
these latter times will become less frequent, and that the whole thing
will become quite dependable before long. The pertinacious way in which
Mr. Marconi and his able co-operators have, at great expense, gradually
worked the method up from its early difficult and capricious stage to
its present great distances and comparative dependableness is worthy of
all praise.
Telegraphy by means of Hertz waves, though perhaps chiefly developed
in this country, has also been pursued successfully by Prof. Slaby in
Germany, who has attained considerable distance over land, with its
numerous obstacles, and has published an account of his researches in
a book called “Funkentelegraphie”; while like success over land has
been attained by M. E. Ducretet, M. Blondel and others in France. M.
Ducretet has, indeed, put on the market a compact apparatus whereby
beginners can readily try their hands at this mode of signalling; as
well as a large-scale apparatus like that employed by Lieutenant Tissot
for lighthouse signalling on the coast of Brittany.
The filings tube now chiefly employed by the author is of the following
form:--It is a sealed glass tube containing carefully selected iron
filings, and exhausted to the highest vacuum. Close together are two
little silver globes melted each on its own platinum wire terminal,
which are connected with convenient screws on an ebonite stand. The
filings are adjusted so as just to cover the two silver globes, and no
more; a pocket, or reservoir, however, is sometimes provided whereby
more or fewer filings can be easily introduced into the working
compartment for experimental purposes. This pocket serves to fix the
whole tube to its ebonite body, which is provided with a clamp to
attach it to the stiff spring, or movable lever, or other form of
support, through which it is to receive the mechanical shocks necessary
to restore or decohere it after an electrical stimulus.
The usual plan is to employ an electrical hammer to rap strongly on a
stiff brass spring to which the ebonite is clamped, but another plan is
to attach the coherer to a lever tilted strongly by an electromagnet
after the fashion of a sounder. A rapid succession of gentle taps is
often better than one violent one, but experience is the best test
of the kind of restoration wanted, for it depends a good deal on the
strength of the electrical stimulus. There are methods of dispensing
with this decohering operation altogether.
After a fairly strong electric stimulus all the filings are stuck
together into a sort of mat, and nothing but a thorough shaking up will
pull them asunder again. A still more violent electric shock may indeed
have a decohering effect, but it is not a plan to be recommended, for
it seems to be a heat effect, akin to the blowing of a fuse.
For protecting a coherer from undesired stimuli, _e.g._, from the
radiator at its own station, the general method is described on page
35, &c., and the details of it, with the necessary switch for
changing over from sending to receiving, are mentioned further on (page
60). But by referring to page 106 it will be seen that M. Branly had
already employed such a protecting case, and had worked details out
admirably.
Recently Signor Tommasina has shown that, if one end of a short rod
or wire be dipped into filings while sparks are occurring in the
neighbourhood, the filings adhere to it and to each other, and with
care a long string of them can be picked up. The author has examined
the behaviour of filings under electrical influence on a glass plate in
a microscope, and their movements towards the formation of a complete
conducting bridge between the tinfoil terminals together with their
disjunctive behaviour when the electrical stimulus is too strong, the
thorough cohesion set up by a succession of electrical stimuli, and the
partial or complete disruption by an appropriate mechanical stimulus is
instructive.
An earlier and most important telegraphic application, based upon
information given in the preceding lecture, was made in 1895 by Prof.
Popoff, of Russia, and will be mentioned shortly (_see_ page 62). I
now proceed to developments of syntonic or attuned telegraphy on the
true Hertz-wave principle, the preliminary experiments on which are
mentioned above in connection with the figures on page 27.