copper wires rest across one another the contact-resistance may fall
from 8,000 to 7 ohms by the impact of an electric wave. He has
recently devised a tripod arrangement, in which a light metal stool
with three slightly-oxidised legs stands on a polished plate of steel.
The contact points must be oxidised, but not too heavily, and the
stool makes a bad electrical contact until a wave falls upon it.[35]
The decoherence is effected by giving the stool a tilt by means of an
electromagnet.
These single or multiple-point kumascopes labour under the
disadvantage that only a very small current can be passed through the
variable contact when used as a relay arrangement, without welding
them together so much that a considerable mechanical shock is required
to break the contact and reset the trap.
The logical development of the single contact is, therefore, the
infinite number of contacts existing in the tube of metallic filings,
which has been the form of kumascope most used for many years. In its
typical form it consists of a tube of insulating material with
metallic plugs at each end, and between them a mass of metallic
powder, filings, borings, granules or small spheres, lightly touching
one another. Imperfect contact must be arranged by light pressure, and
in the majority of cases the resistance is very large until an
electric wave falls upon the tube, when it drops suddenly to a small
value and remains there until the tube is given a slight shake or the
granules disturbed in any way, when the resistance suddenly rises
again. This type of responder is a non-restoring kumascope, and
requires the continual operation of some external agency to keep it in
a condition in which it is receptive or sensitive to electric waves.
Much discussion and considerable research have taken place in
connection with the action and improvement of these metallic powder
kumascopes. As regards materials, the magnetic metals, nickel, iron
and cobalt, in the order named, appear to give the best results. The
noble metals, gold, silver and platinum, are too sensitive, and the
very oxidisable metals too insensitive, for telegraphic work, but an
admixture may be advantageously made.
Omitting the intermediate developments of invention, it may be said
that Mr. Marconi was the first to recognise that to secure great
sensibility in an electric wave detector of this type the following
conditions must be fulfilled: An exceedingly small mass of metallic
filings must be placed in a very narrow gap between two plugs, the
whole being contained in a vessel which is wholly or partly exhausted
of its air. Mr. Marconi devoted himself with great success to the
development of this instrument, and in a very short time succeeded in
transforming it from an uncertain laboratory appliance, capable of
yielding results only in very skilled hands, into an instrument
certain and simple in its operations as an ordinary telegraphic relay.
He did this, partly by reducing its size, and partly by a most
judicious selection of materials for its construction. As made at
present, the Marconi metallic filings tube consists of a small glass
tube, the interior diameter of which is not much more than one-eighth
of an inch, which has in it two silver plugs which are bevelled off
obliquely. These are placed opposite to each other, so as to form a
wedge-shaped gap, about a millimetre in width at the bottom and two,
or at most three, millimetres in width at the top (see Fig. 16). The
silver plugs exactly fill the aperture of the tube, and are connected
to platinum wires sealed through the glass. The tube has a lateral
glass tube fused into it, by which the exhaustion is made, which is
afterwards sealed off, and this tube projects on the side of the wider
portion of the gap between the silver plugs. The sensitive material
consists of a mixture of metallic filings, five per cent. silver and
ninety-five per cent. nickel, being carefully mixed and sifted to a
certain standard fineness. In the manufacture of these tubes, great
care is taken to make them as far as possible absolutely identical.
Each tube when finished is exhausted, but not to a very high vacuum.
The tube so finished is attached to a bone holder, by which it can be
held in a horizontal position. The object of bevelling off the plugs
in the Marconi tube is to enable the sensitiveness of the tube to be
varied by turning it round, so that the small quantity of filings lie
in between a wider or narrower part of the gap.[36]
[Illustration: FIG. 16.--MARCONI SENSITIVE TUBE OR METALLIC FILINGS
KUMASCOPE. PP, silver plugs; TT, platinum wires; F, nickel and silver
filings.]
Other ways of adjusting the quantity of the filings to the width of
the gap have been devised. Sometimes one of the plugs is made movable.
In other cases, such as the tubes devised by M. Blondel and Sir Oliver
Lodge, there is a pocket in the glass receptacle to hold square
filings, from which more or less can be shaken into the gap.
An interesting question, which we have not time to discuss in full, is
the cause of the initial coherence of the metallic filings in a Branly
tube. It does not seem to be a simple welding action due to heat, and
it certainly takes place with a difference of potential, which is very
far indeed below that which we know is required to produce a spark. On
the other hand, it seems to be proved that in a Banly tube, when acted
upon by electric waves, chains of metallic particles are produced. The
effect is not peculiar to electric waves. It can be accomplished by
the application of any high electromotive force. Thus Branly found
that coherence may be produced by the application of an electromotive
force of twenty or thirty volts, operating through a very high water
resistance, and thus precluding the passage of any but an excessively
small current. Again, the coherence seems to take place in some cases
when metallic particles are immersed in a liquid, or even in a solid,
insulator. Processor Branly has, therefore, preferred to speak of
masses of metallic granules as _radio-conductors_, and Professor Bose
has divided substances into positive and negative, according as the
operation of electromotive force is to increase the coherence of the
particles or to decrease it.
It has been asserted that for every particular Branly tube, there is a
critical electromotive force, in the neighbourhood of two or three
volts which causes the tube to break down and pass instantly from a
non-conductive to a conductive condition, and that this critical
electromotive force may become a measure of the utility of the tube
for telegraphic purposes. Thus, C. Kinsley (_Physical Review_, Vol.
XII., p. 177, 1901) has made measurements of this supposed critical
potential for different "coherers," and subsequently tested the same
as receivers at a wireless telegraph station of the U.S.A. Signal
Corps. The average of twenty-four experiments gave in one case 2.2
volts as the breaking down potential of one of these coherers or
Branly tubes, 3.8 volts for a second and 5.5 volts for the third.
These same instruments, tested as telegraphic kumascopes, showed that
the first of the three was most sensitive.
On the other hand, W. H. Eccles (_Electrician_, Vol. XLVII., pp. 682
and 715, 1901) has made experiments with Marconi nickel-silver
sensitive tubes, using a liquid potentiometer made with copper
sulphate, to apply the potential so that infinitesimal spark contacts
might be avoided and the changes in potential made without any
abruptness. He states that if the coherer tube is continuously tapped,
say at the rate of fifty vibrations per second, whilst at the same
time an increasing potential is applied to its terminals and the
current passing through it measured on a galvanometer, there is no
abrupt change in current at any point. He found that when the current
and voltage were plotted against each other, a regular curve was
obtained, which after a time becomes linear. A decided change occurs
in the conductivity of the mass of metallic filings when treated in
this manner at voltages lower than the critical voltage obtained by
previous methods. He ascertained that there was a complete
correspondence between the sensitiveness of the tubes used as
telegraphic instruments and the form of the characteristic curve of
current and voltage drawn by the above-described method.
In the same manner, K. E. Guthe and A. Trowbridge (_Physical Review_,
Vol. II., p. 22, 1900) investigated the action of a simple ball
coherer formed of half a dozen steel, lead or phosphor-bronze balls in
slight contact. They measured the current _i_ passing through the
series under the action of a difference of potential _v_ between the
ends, and found a relation which could be expressed in the form
v = V(1 - e^{ki}),
where V and _k_ are constants.
The current through this ball coherer is, therefore, a logarithmic
function of the potential difference between its ends, of the form
i = log(v - V)
and exhibits no discontinuity.
The inference was drawn that the "resistance" is due to films of water
adhering to the metallic particles through which electrolytic action
occurs.
A good metallic filings tube for use as a receiver in Hertzian wave
telegraphy should exhibit a constancy of action and should cohere and
decohere, to use the common terms, sharply, at the smallest possible
tap. It should not have a current passed through it by the external
cell of more than a fraction of a milliampere, or else it becomes
wounded and unsensitive.
The investigations which have already taken place seem to show pretty
clearly that the agency causing the masses of filings to pass from a
non-conductive to a conductive condition is electromotive force, and
that, therefore, it is the electromotive force set up in the aerial by
the incident waves which is the effective agent in causing the change in
the metallic filings tube, when this is used as a telegraphic kumascope.
This transformation of the tube from a non-conductor to a conductor is
made to act as a circuit-closer, completing the circuit, by means of
which a single cell of a local battery is made to send current through
an ordinary telegraph relay, and so by the aid of a second battery
operate a telegraphic printer or recorder of any kind. Hence it is clear
that after one impact, the metallic filings tube has to be brought back
to its non-conductive condition, and this may be achieved in several
ways. (1) By the administration of carefully-regulated taps or shocks or
by rotating the tube on its axis; (2) by the aid of an alternating
current; (3) in those cases where filings of magnetic metals are
employed, by magnetism.
The decoherence by taps was discovered by Branly,[37] and Popoff,
following the example of Sir Oliver Lodge, employed an electric bell
arrangement for this purpose.[38]
Mr. Marconi, in his original receiving instruments, placed an
electromagnet under the coherer tube with a vibrating armature like an
electric bell.[39] This armature carries a small hammer or tapper,
which, when set in action, hits the tube on the under side, and
various adjusting screws are arranged for regulating exactly the force
and amplitude of the blows. This tapper is actuated by the same
current as the Morse printer, or other telegraphic recorder, so that
when the signal is received and the metallic filings tube passes into
the conductive condition and closes the relay circuit, this latter in
turn closes the circuit of the Morse printer or other recorder, and at
the same time, a current passes through the electromagnet of the
tapper and the tube is tapped back. This sequence of operations
requires a certain time which limits the speed of receiving. The
tapper has to be so arranged that it is possible to receive and to
record not only the _dot_ but a _dash_ on the Morse system. The _dash_
is really a series of closely adjacent dots, which run together in
virtue of the inertia and inductance of the different parts of the
whole receiving apparatus. The adjustment has so to be made that,
whilst the _dash_ is being recorded and a continuous tapping is kept
up, yet, nevertheless the continued electromotive force in the aerial,
due to the continually arriving trains of waves, is able to act
against the tapping and to keep the filings in the tube in the
conductive condition. Hence, the successful operation of the
arrangement requires attention to a number of adjustments, but these
are not more difficult, or even as difficult, as those involved in the
use of many telegraphic receivers employed in ordinary telegraphy with
wires.
Mr. Marconi also introduced devices for preventing the sparks at the
contacts of the electromagnetic hammer from directly affecting the
tube, and also to prevent the electric oscillations which are set up
in the aerial from being partly shunted through other circuits than
that of the sensitive tube. We pass on to notice the remaining devices
for restoring the metallic filings tube to a condition of
sensitiveness or receptiveness.
A method for doing this by alternating currents is due to Mr. S. G.
Brown.[40] The pole pieces of the coherer tube are made of iron, and
they are enveloped in magnetising coils traversed by an alternating
electric current. Between these pole pieces is placed a small quantity
of nickel or iron filings, and under the action of the electromotive
force, due to an electric wave acting on them, may be made to cohere
in the usual fashion; but the moment that the wave ceases, the
alternating magnetism of the electrodes causes the filings to drop
apart or decohere. In place of the alternating current, Mr. Brown
finds that a revolving permanent magnet can be used to produce the
alternating magnetisation of the pole pieces of the sensitive tube or
coherer.
The third method of causing the decoherence of the filings is that due
to T. Tommasina. He found that when a Branly tube is made with filings
of a magnetic metal, such as iron, nickel and cobalt, the decoherence
of the filings can be produced by means of an electromagnet placed in
a suitable position under the tube.[41] The explanation of this fact
seems to be that, when an electric wave falls upon the tube or when
any other source of electromotive force acts upon it, chains of
metallic particles are formed, stretching from one electrode to the
other. Tommasina contends that he has proved the existence of these
chains of particles by experiments made with iron filings; and R.
Malagoli,[42] in referring to Tommasina's assertion, states that it
can be witnessed in the case of brass filings placed between two
plates of metal and immersed in vaseline oil, when a difference of
potential is made between the plates.
T. Sundorph[43] says he has confirmed Tommasina's discovery of the
formation of these chains of metallic particles in the coherer. The
filings do not all cling together, but certain chains are formed which
afford a conducting path for the current subsequently passed through
the coherer from an external source. Accordingly, Tommasina's method
of causing decoherence in the case of filings of magnetic metals is to
pull them apart by an external magnetic field; and he stated that
decoherence can be effected more easily and regularly in this way than
by tapping. Whilst on this point, it may be mentioned that C.
Tissot[44] says that he has found that the sensitiveness of a coherer
formed of nickel and iron filings can be increased by placing it in
the magnetic field, the lines of which are parallel to the axis of
the tube. According to MM. A. Blondel and G. Dobkevitch, this is
merely the result of an increased coherence of the particles.
* * * * *
Quite recently, Sir Oliver Lodge and Dr. Muirhead have employed as a
self-restoring coherer or kumascope a steel disc revolved by
clockwork, the edge of which just touches a globule of mercury covered
with a thin film of paraffin oil. The contact is made between the
mercury and the steel by the electric wave generating an electromotive
force in the aerial, sufficient to break through the thin film of oil.
When the wave stops, the circuit is again interrupted automatically.
This device is used without a relay to actuate directly a syphon
recorder as used in submarine telegraphy. The working battery employed
with it must only have an electromotive force of about a tenth of a
volt. It may be used also with a telephone in circuit and can,
therefore, be employed either for telegraphic or telephonic
reception.[45]
[Illustration: FIG. 17.--ITALIAN NAVY SELF-RESTORING KUMASCOPE. C,
carbon plug; I, iron plug; M, mercury globule; A, aerial; B, battery;
T, telephone; S, adjusting screw.]
One of the most sensitive of these self-restoring kumascopes is the
carbon-steel-mercury coherer, the invention of which has been
attributed to Castelli, a signalman in the Italian Navy,[46] but also
stated on good authority to have been the invention of officers in the
Royal Italian Navy, and has, therefore, been called the Italian Navy
coherer.[47] This instrument has been arranged in several forms, but
in the simplest of these it consists of a glass tube, having in it a
plug of iron and a plug of arc-lamp carbon, or two plugs of iron with
a plug of carbon between them. The plugs of iron, or of iron and
carbon, are separated by an exceedingly small globule of mercury, the
size of which should be between one and a-half and three millimetres.
The plugs closing the tube must be capable of movement, one of them by
means of a screw, as shown in the diagram (Fig. 17), taken from a
patent specification communicated to Mr. Marconi by the Marchese Luigi
Solari, of the Royal Italian Navy. One of the plugs of this tube is
connected to the aerial and the other to the earth, and they are also
connected through another circuit composed of a single dry cell and a
telephone. The arrangement then forms an extremely sensitive detector
of electric waves or of small electromotive forces, or, if a wave
falls on the aerial, the electromotive force at once improves the
contact between the mercury and the plugs, and therefore causes a
sudden increase in the current through the telephone, giving rise to a
sound; but when the wave ceases, or the electromotive force is
withdrawn, the resistance falls back again to its origin value, and
the arrangement is, therefore, self-acting, requiring no tapping or
other device for restoring it to receptivity.
A very ingenious form of combined telephone and coherer has been
devised by T. Tommasina.[48] In this instrument the diaphragm of an
ordinary Bell telephone carries upon it a very small carbon or
metallic coherer. This coherer is connected in between the aerial and
the earth, and is also in circuit with a battery and the electromagnet
of a telegraphic relay. When this relay operates it closes the circuit
of another battery which is placed in series with the telephone coil.
The moment the current passes through the telephone coil it attracts,
and therefore vibrates, the diaphragm and shakes up the metallic
filings. If an observer, therefore, places the telephone to his ear,
he hears a sound corresponding to every train of waves incident upon
the aerial. With this arrangement, one can obtain two different kinds
of results, according to the nature of the cohering powder placed in
the cavity in the diaphragm. First, if the powder consists of a
non-magnetic metal, gold, silver, platinum or the like, the receiver
will be very sensitive; and at the same time the current passing
through it when it is cohered will be sufficient to work a sensive
recording apparatus in series with the telephone coil. Secondly, if
the metallic powder placed in the cavity is a magnetic metal, the
receiver will be somewhat less sensitive, but will work with more
precision, because of the magnetic action of the magnet of the
telephone upon the cohering powder. If no recording apparatus is used,
the observer must write down the signals as heard in the telephone,
since corresponding to a short spark at the transmitting station, a
single tick or short sound is heard at the telephone, and
corresponding to a series of rapidly successive sparks, a more
prolonged sound is heard in the telephone. These two sounds, as
already explained, constitute the dot and the dash of the Morse
signals.
We may, in the next place, refer to that form of kumascope in which
the action of the wave or of electromotive force is not to decrease
the resistance of a contact, but to increase that of an imperfect
contact. As already mentioned, Professor Branly discovered long ago
that peroxide of lead acts in an opposite manner to metallic filings,
in that when placed in a Branly tube it increases its resistance under
the action of an electric spark, instead of decreasing it. Again,
Professor Bose has found that fragments of metallic potassium in
kerosene oil behave in a similar manner, and that certain varieties of
silver, antimony and of arsenic, and a few other metals, have a
similar property. Branly tubes, therefore, made with these materials,
or any arrangements which act in a similar manner, have been called
"anti-coherers." The most interesting arragement which has been called
by this name is that of Schaefer.[49] Schaefer's kumascope is made in
the following manner: A very thin film of silver is deposited upon
glass, and a strip of this silver is scratched across with a diamond,
making a fine transverse cut or gap. If the resistance of this divided
strip of silver is measured, it will be found not to be infinite, but
may have a resistance as low as forty or fifty ohms if the strip is
thirty millimetres wide. On examining the cut in the strip with a
microscope, it will be found that the edges are ragged and that there
are little particles of silver lying about in the gap. If, then, an
electromotive force of three volts or more is put on the two separated
parts of the strip, these little particles of silver fly to and fro
like the pith balls in a familiar electrical experiment, and they
convey electricity across from side to side. Hence a current passes
having a magnitude of a few milliamperes. If, however, the strip is
employed as a kumascope and connected at one end to the earth and at
the other end to an aerial, when electric waves fall upon the aerial,
the electrical oscillations thereby excited seem to have the property
of stopping this dance of silver particles and the resistance of the
gap is increased several times, but falls again when the wave ceases.
If, therefore, a telephone and battery are connected between two
portions of the strip, the variation of this battery current will
affect the telephone in accordance with the waves which fall upon the
aerial, and the arrangement becomes therefore a wave-detecting device.
It is said to have been used in wireless telegraph experiments in
Germany up to a distance of ninety-five kilometres.
We must next direct attention to those wave-detecting devices which
depend upon magnetisation of iron, and here we are able to record
recent and most interesting developments. More than seventy years ago
Joseph Henry, in the United States, noticed the effect of an electric
spark at a distance upon magnetised needles.[50] Of recent times, the
subject came back into notice through the researches of Professor E.
Rutherford,[51] who carried out at Cambridge, England, in 1896, a
valuable series of experiments on this subject. He found that if a
magnetised steel needle or a very small bundle of extremely thin iron
wires is magnetised and placed in the interior of a small coil, the
ends of which are connected to two long collecting wires, then an
electric wave started from a Hertz oscillator at a distance causes an
immediate demagnetisation of the iron. This demagnetisation he
detected by means of the movement of the needle of a magnetometer
placed near one end of the iron wire. Although Rutherford's wave
detector has been much used in scientific research, it was not, in the
form in which he used it, a telegraphic instrument, and could not
record alphabetic signals.
Not long ago Mr. Marconi invented, however, a telegraphic instrument
based upon his discovery that the magnetic hysteresis of iron can be
annulled by electric oscillations. In one form, Mr. Marconi's magnetic
receiver is constructed as follows[52] (see Fig. 18): An endless band
of thin iron wire composed of several iron wires about No. 36 gauge,
arranged in parallel, is made to move slowly round on two pulleys,
like the driving belt of a machine. In one part of its path the wire
passes through a glass tube, on which are found two coils of wire, one
a rather short, thick coil, and the other a very fine, long one. The
fine, long coil is connected with a telephone, and the shorter coil is
connected at one end to the earth and the other to the aerial. Two
permanent horse-shoe magnets are placed as shown n Fig. 18, with their
similar poles together, and, as the iron band passes through their
field, a certain length of it is magnetised, and owing to the
hysteresis of the material, it retains this magnetism for a short time
after it has passed out of the centre of the field. If then an
electric oscillation, coming down from the aerial, is passed through
the shorter coil, it changes the position of the magnetised portion of
the iron and, so to speak, brings the magnetised portion of iron back
into the position it would have occupied if the iron had had no
hysteresis. This action, by varying the magnetic flux through the
secondary coil, creates in it an electromotive force which causes a
sound to be heard in the telephone connected to it. If at a distant
place a single wave or train of waves is started and received by the
aerial, this will express itself by making an audible tick in the
telephone, and if several groups of closely adjacent wave trains are
sent, these will indicate themselves by producing a rapid series of
ticks in the telephone, heard as a short continuous noise and taken as
equivalent to the Morse _dash_.
[Illustration: FIG. 18.--MARCONI MAGNETIC RECEIVER. W_{1}W_{2}, wheels;
I, iron wire band; P, primary coil; S, secondary coil; T, telephone;
A, aerial; E, earthplate.]
It was by means of this remarkably ingenious instrument that Mr.
Marconi was able, in the summer of 1902, to detect the waves sent out
from Poldhu on the coast of Cornwall, and receive messages as far as
Cronstadt in the Baltic, in one direction, and as far as Spezzia in
the Mediterranean in another direction, and also to receive messages
across the Atlantic from the power stations situated in Glace Bay,
Nova Scotia, and from one at Cape Cod in Massachusetts, U.S.A., in
December, 1902.
There can be no question that this magnetic detector of Mr. Marconi's,
used in connection with a good telephone and an acute human ear, is
the most sensitive device yet invented for the detection of electric
waves and their utilisation in telegraphy without continuous wires. It
is marvellously simple, ingenious and yet effective, as a Hertzian
wave telegraphic receiver.
Whilst on the subject of magnetic wave detectors, the author may
describe experiments that he has been recently making to construct a
Hertzian wave detector on the Rutherford principle, which shall be
strictly quantitative. All the receivers of the coherer type and
electrolytic type give no indications that are at all proportional to
the energy of the incident wave. Their indications are more or less
accidental and depend upon the manner in which the receiver was last
left. There is a great need for a quantitative wave detector, the
indications of which shall give us a measure of the energy of the
arriving wave. It is only by the possession of such an instrument that
we can hope to study properly the sending powers of various
transmitters or the efficiency of different forms of aerial or devices
by which the wave is produced. This magnetic receiver is constructed
as follows:
A coil of fine wire is constructed in sections like the secondary coil
of an induction coil, and in the instrument already made, this coil
contains thirty or forty thousand turns of wire. In the interior of
this coil are placed a number of little bundles of fine iron wire
wound round with two coils, a fine wire coil which is a magnetising
coil, and a thicker wire coil which is a demagnetising coil. These
sets of coils are joined up, respectively, in series or in parallel.
Then, associated with this form of induction coil is a commutator of a
peculiar kind, which performs the following functions when a battery
is connected to it and when it is made to revolve by a motor or by
clockwork. First, during part of the revolution, the commutator closes
the battery circuit and magnetises the iron cores, and whilst this is
taking place the secondary circuit of the induction coil is
short-circuited and the galvanometer is disconnected from it.
Secondly, the magnetising current is stopped, and soon after that the
secondary coil is unshort-circuited and connected to the galvanometer,
and remains in this condition during the remainder of the revolution.
This cycle of operations is repeated at every revolution. If then an
electrical oscillation is sent into the demagnetising coils, and if it
continues longer than one revolution of the commutator, it will
demagnetise the iron core during that period of time in which the
battery is disconnected and the galvanometer connected. The
demagnetisation of the iron which ensues produces an electromotive
force in the secondary coil and causes a deflection of the
galvanometer, and this deflection will continue and remain steady if
the oscillation persists. Moreover, since this deflection is due to
the passage through the galvanometer of a rapid series of discharges,
it is large when the oscillations continue for a long time and are
powerful, and small when they continue for a short time or are weak.
We can, therefore, with this arrangement, receive on the galvanometer,
just as on the mirror galvanometer used in submarine cable work, a dot
or dash, and, moreover, the magnitude of these deflections is a
measure of the energy of the wave.
It is probable that when this arrangement is perfected it will become
exceedingly useful for making all kinds of tests and measurements in
connection with Hertzian telegraphy, even if it is not sensitive
enough to use as a long distance receiver.
Of late years a variety of wave-detecting devices have been brought
forward which depend upon electrolysis. One of the best known of these
is that by De Forest and Smythe.[53] In this arrangement, a tube
contains two small electrodes like plugs, which may be made of tin,
silver or nickel, or other metal. The ends of these plugs are flat and
separated from each other by about one two-hundredth of an inch.
Sometimes the end of one of these plugs is made cup shaped and the cup
or recess is filled with a mass of peroxide of lead and glycerine. In
the interval between the electrodes is placed an electrolyzable
mixture, which consists of glycerine or vaseline mixed with water or
alcohol, and a small quantity of litharge and metallic filings. These
metallic filings act as secondary electrodes. When a small
electromotive force is applied between the terminals of the electrodes
of this tube through a very high resistance of twenty or thirty
thousand ohms, an exceedingly small current passes through this
mixture, and it causes an electrolytic action which results in the
production of chains of metallic particles connecting the two
electrodes together. If, in addition to this, one terminal or
electrode of the arrangement is connected to an aerial wire and the
other terminal to the earth, then on the arrival of an electric wave
creating oscillations in the wire, these oscillations pass down into
the electrolytic cell, where they break up the chains of metallic
particles and thus interrupt the current passing through the telephone
quite suddenly, which is heard as a slight tick by an ear applied to
it. As soon as the wave ceases, the chain of metallic particles is
re-established, so that the appliance is always in a condition to be
affected by a wave. It is said that this breaking up and reformation
of the chains of metallic particles is so rapid that a short spark
made at the transmitting station is heard as a tick in the telephone,
but a rapid succession of oscillatory sparks is heard as a short
continuous sound; hence the two signals necessary for alphabetical
conversation can be transmitted.
Another receiver which has some resemblance to the above, although
different in principle, is that of Neugschwender.[54] In this
arrangement, which to a certain extent resembles the Schaefer detector,
a glass plate has upon it a deposit of silver in the form of a strip,
which is cut across at one place, thus interrupting it. If the cut is
breathed upon or placed in a moist atmosphere, a little dew is
deposited upon the glass, which bridges over the cut in the metal and
creates an electric continuity. Hence a small current can be passed
across the gap and through a telephone by one or two cells of a
battery. If, however, an electric oscillation passes across the gap on
its way from an aerial to the earth, then the continuity of the liquid
film is destroyed, and the current is interrupted and a sound created
in the telephone.
The opinion has been expressed by Sir Oliver Lodge that in this case
the interruption of the circuit which occurs is really due to the
coalescence of minute water particles into larger drops, as when
vapour is condensed into rain, and hence the continuity of the
material is interrupted.
We must then make a brief reference to other kumascopes which depend
upon the heating power of an electrical oscillation, which it
possesses in common with every other form of electric current.
Professor R. A. Fessenden[55] has constructed a very ingenious thermal
receiver in the following manner: An extremely fine platinum wire,
about 0.003 of an inch in diameter, is embedded in the middle of a
silver wire about one tenth of an inch in diameter, like the wick of
a candle. This compound wire is then drawn down until the diameter of
the silver wire is only 0.002 of an inch, and hence the platinum wire
in its interior, being reduced in the same ratio, will have been drawn
to a diameter of 0.00006 of an inch. A short piece of this drawn wire
is then bent into a loop and the ends fixed to wires. The tip of the
loop is then immersed in nitric acid and dissolved in the silver,
leaving an exquisitely fine platinum wire a few hundredths of an inch
in length and having a resistance of about thirty ohms. This little
loop is sealed into a glass bulb like a very small incandescent lamp,
or it may be enclosed in a small silver bulb and the air may be
exhausted. If an electrical oscillation is sent through this
exceedingly fine platinum wire it heats it and rapidly increases its
resistance. The electrical oscillations produced in an aerial are sent
through a number of these loops arranged in parallel, and the loops
are short-circuited by a telephone, joined in series with a source of
very small electromotive force produced by shunting a single cell or
opposing to one another two cells of nearly equal electromotive force.
Any variation of resistance of the little platinum loops due to the
heat produced by the oscillations, by suddenly altering the current
flowing through the telephone, will cause a sound to be heard in it.
The electrical oscillations when passing through the loops are
therefore detected by the heat which they generate in these
exquisitely fine platinum wires.
Finally, one word must be said on the subject of electrodynamic
receivers, due to the same inventor. An exceedingly small silver ring
is suspended by a quartz fibre and has a mirror attached to it in the
manner of a galvanometer. This ring is suspended between two coils
joined in series, which are placed either in the circuit of the aerial
or in the secondary circuit of the small air core transformer inserted
between the aerial and the earth. When electrical oscillations travel
down the aerial they induce other electrical oscillations in the
silver ring, and if the ring is so placed that its normal position is
with its plane inclined at an angle of forty-five degrees to the plane
of the fixed coils, then the ring will be slightly deflected every
time an oscillation occurs in the aerial.
Omitting further mention of the details of the kumascopes in use and
the receiving aerial, we must next proceed to consider the receiving
arrangements taken as a whole.
In the original Marconi system, the sensitive tube or coherer was
inserted between the bottom of the receiving aerial and the earth.[56]
Accordingly, when the incident electric wave strikes the receiving
aerial and creates in it an oscillatory electromotive force, this last
will, if of sufficient amplitude, cause the particles of the coherer
to cohere and become conductive. This sudden change from a nearly
perfect non-conductivity to a conductive condition is made to act as a
switch or relay, closing or completing the circuit of a single cell,
and so sending a current through an ordinary telegraphic relay,
closing or completing the circuit of a single cell, which may in turn
actuate another recording telegraphic instrument, such as a Morse
printer. To prevent the oscillations from passing into the relay
circuit, small choking or inductance coils are inserted between the
ends of the sensitive tube and the relay and cell and serve to confine
the oscillations to the tube.
It has already been pointed out that in the transmitting aerial the
amplitude at the potential vibrations increases from the bottom to the
top, and when vibrating in its fundamental manner there is a potential
node at the earth connection and a potential loop or antinode at the
top. The same is true of the receiving aerial. Hence, if the kumascope
employed is a Branly metallic filings tube and is inserted near the
base of the aerial, the difference of potential between its two ends
will be small.
It has also been mentioned that a receiver of this type acts in virtue
of electromotive force or potential difference, and hence the proper
place to insert the coherer is not at the base of the aerial, but
between the top of the aerial and the earth. This, however, could not
be done by running up another wire from the earth, as that would
amount to putting the coherer between the tops of two identical
aerials, and between its ends there would be no difference of
potential. Professor Slaby, in conjunction with Count von Arco, has
given an ingenious solution of this problem. If we take two equal
lengths of wire, bent at right angles, and connect the point of
intersection with the earth, placing one of these wires vertically and
the other horizontally, we then have an arrangement which responds to
the impact of electric waves, and has electrical oscillations set up
in it in such fashion that the common point of the two wires has a
very small amplitude of potential, but the two extremities have equal
and large variations. If, then, we insert a coherer tube between the
earth and the outer extremity of the horizontal wire, it is influenced
in the same manner as it would be by the potential variations at the
top of the vertical wire. In other words, it is acted upon by a large
difference of potential instead of a small one. It is not found
necessary to stretch the horizontal wire out straight; it may be
coiled into a spiral with open turns, and the slight decrease in
capacity and increase in inductance resulting from this can be
compensated by cutting off a short piece of it.
[Illustration: FIG. 19.--SLABY RECEIVER. A, aerial; E, earth plate; F,
coherer; M, multiplier; C, condenser; R, relay; B, battery; E, earth
plate.]
In this way we have an arrangement (see Fig. 19) in which the outer
extremity of this open spiral experiences variations or potential which
exactly correspond with those at the summit of the vertical aerial. The
receiving arrangements are then completed as in Fig. 19, one end of the
coherer being attached to the outer end of the spiral and the other end
through a condenser to the earth, a relay and a voltaic cell being
arranged as shown in the diagram. The mode of operation of this receiver
is as follows: When the wave strikes the aerial it sets up in it
electrical oscillations with a potential antinode at the summit, and at
the same time a potential antinode is created at the outer end of the
spiral attached near the base of the aerial, this spiral being called by
Professor Slaby a _multiplicator_. As long as the coherer tube remains
non-conductive, the local cell cannot send a current through the relay,
but, as soon as the resistance is broken down by the impact of a wave,
the local cell sends a current through the coherer tube which, passing
down to the earth through the base of the aerial and up through the
earth connection to the condenser, completes its circuit through the
relay. Many variations of this arrangement have been made by Slaby and
Von Arco and by the Allgemeine Elektricitaets Gesellschaft of Berlin.
In 1898, Mr. Marconi made a great advance in the construction of his
receiving apparatus by the insertion of his "jigger" or oscillation
transformer in the aerial receiving circuit.[57] In this arrangement,
the primary coil of an air core transformer wound in a particular way
is inserted between the receiving aerial and the earth, and the
secondary circuit is cut in the middle and connected to the two
surfaces of a condenser, these surfaces being also connected through
the circuit of an ordinary telegraphic relay and a single cell (see
Fig. 20). The ends of the secondary circuit of this oscillation
transformer are also connected to the terminals of the coherer tube,
and these again are short-circuited by a small condenser.
[Illustration: FIG. 20.--MARCONI RECEIVER. A, aerial; J, jigger; CC,
condensers; F, filings tube; T, tapper; R, relay; B, battery; M, Morse
printer.]
The operation of this receiver is as follows: The oscillations set up
in the aerial pass through the primary circuit of the jigger, and
these induce other oscillations in the secondary circuit; the
electromotive force or difference of potential between the primary
terminals being transformed up in any desired ratio. It is this
exalted electromotive force which is made to act on the coherer tube,
and, inasmuch as the jigger operates in virtue of a current passing
through its primary circuit and this current is at a maximum at the
lower end of the aerial, the arrangement is exceedingly effective,
because it, so to speak, converts current into voltage. At the lower
end of the aerial, although the amplitude of the potential
oscillations is a minimum, the amplitude of the current oscillations
is a maximum, and the jigger transforms these large current
oscillations into large potential oscillations, _provided it is
constructed in the right manner_. We can also transform up or increase
the amplitude of the small potential variations near the bottom of the
aerial by employing the principle of resonance. Many devices of this
kind, due to Professor Slaby and others, have been suggested and tried
but the details are rather too technical to be fully described here.
It will be noticed that the receiving aerial may be arranged in one of
two ways--it may be either earthed at the lower end or it may be
insulated. It has been claimed that there is a great advantage in
earthing the receiving aerial directly in that it eliminates
atmospheric disturbances.
We shall allude to this point more particularly later on. Meanwhile it
may be mentioned that the receiving arrangements, as a whole,
constitute a sensitive arrangement, as shown by Popoff, Tommasina and
by all the large experience of Mr. Marconi himself for detecting
changes in the electrical condition of the atmosphere, which are
doubtless of the nature of electrical oscillations. On the other hand,
the receiving arrangements may be perfectly insulated and some
experimentalists have asserted that by this method the greatest
freedom is secured from atmospheric disturbances. Amongst the
non-earthed arrangements the system invented by Professor F. Braun, of
Strassburg, and worked by Messrs. Siemens, of Berlin, may be
mentioned.[58]
[Illustration: FIG. 21.--BRAUN'S NON-EARTHED RECEIVER. I, induction
coil; CC, condensers; S, spark gap; J, transmitting jigger; K,
receiving jigger; F, filings tube; R, relay; B, battery.]
Professor Braun's arrangements are indicated in the diagram in Fig.
21. In this case an induction coil is used to create a discharge
between two spark balls, and to these two balls are connected the two
outer coatings of two condensers, the inner coatings of which are
connected together through the primary coil of an air core
transformer. The secondary coil of this transformer is connected to
two extension wires forming a Hertz resonator, and the length of these
wires is so adjusted with reference to the time period of the primary
circuit that they resonate to it, the whole length from end to end of
the secondary circuit being half a wave-length. The receiver, as shown
in the diagram, consists of a pair of quarter wave-length receiving
wires connected through two condensers, which are short-circuited by
the primary coil of an oscillation transformer. The secondary circuit
of this last oscillation transformer has two extension wires to it,
turned in the same manner, to respond to the primary oscillator; and
in the circuit of one of these extension wires is placed a coherer
tube, short-circuited by a relay and a local battery.
It will thus be seen that there is an entire abolition of ground
connection, which, Professor Braun claims, practically avoids all
atmospheric disturbances.[59] The details of the receiving arrangement
are as follows:--The coherer tube consists of an ebonite tube
containing hard steel particles of a uniform size, placed in the
adjustable space between two polished steel electrodes. It is found
that with this steel coherer, a small amount of magnetism in the
particles increases its sensitiveness, and to obtain this, a ring
magnet is employed in connection with a coherer tube. Receiving
apparatus arranged on this system is said to have been used for
telegraphing between Heligoland and Cuxhaven, a distance of thirty-six
miles.
All the immense experience, however, gained by Mr. Marconi and those
who have worked with his system, is in favour of using the earth
connection. There is no doubt that Hertzian wave telegraphy can be
conducted over short distances by means of totally insulated aerials,
but for long distances the earth connection is essential, for the
reasons that have been explained previously.
There are many of the details of the receiving arrangements which
remain to be considered. If the communication is received by a
telegraphic instrument like the Morse printer, which requires a
current of anything like ten milliamperes to work it, then an
important element in the receiving arrangement is the relay. The relay
that is generally used is a modified form of the Siemens polarised
relay, which is so adjusted as to make a single contact. For marine
work on board ship, it is essential that this relay shall be balanced
so that variations in position shall not affect it. Sometimes the
relay is hung in gimbals like a compass, and at other times suspended
from a support by elastic bands, so as to avoid jolting. In any case,
the relay must be so adjusted that no change of position will cause it
to close the circuit of the telegraphic printer or recorder. Its
sensibility ought to be such that it is actuated by a tenth of a
milliampere, and, if possible, even by less. The alteration of
sensibility in the ordinary contact form of relay is the pressure that
is necessary to bring the platinum points of the circuit closer
together, so as to pass the minimum current which will work the
telegraph printer.
The important matter, however, in connection with the use of the relay
in Hertzian wave telegraphy, is that it should be capable of
adjustment without extraordinary skill. It is no use to put into the
hands of an operator a relay which requires abnormal dexterity to make
it work at all.
* * * * *
It remains, then, to consider some of the questions connected with
practical Hertzian wave telegraphy and the problem of the limitation
of communication. These matters at the present moment very much occupy
the public attention, and many conflicting opinions are expressed
concerning them.
It may be observed at the outset that the difficulty of dealing with
the subject as freely as many desire is that Hertzian wave telegraphy
is no longer merely a subject of scientific investigation, but has
developed into a business and involves, therefore, other interests
than the simple advancement of scientific knowledge. We can, however,
discuss in a general manner some of the scientific problems which
present themselves for solution. The first of these is the
independence of communication between stations. It is desirable, at
the outset, to clear up a little misunderstanding. There is a great
difference between preventing the reception of communication when it
is not desired by the recipient, and preventing it when it is the
object of the latter to overhear if he can. It is, therefore,
necessary to distinguish between isolation and overhearing. We may say
that a station is _isolated_ when it is not affected by Hertzian waves
other than those it desires to receive; but that a station _overhears_
when it can, if it chooses, pick up communications not intended for
it, or cannot help receiving them against its will.
This distinction is a perfectly fair one. Any telegraph or telephone
wire can be tapped, if it is desired, but unless there is some fault
on the line, no station will receive a message against its desires.
Moreover, it may be noted that there are penalties attached to tapping
a telegraph wire, and at present there are none connected with the
misappropriation of an ether wave.
We shall, therefore, consider in the first place the methods so far
proposed for preventing any given receiver from being affected by
Hertzian waves sent out from other stations, except that of those from
which it is desired to receive them. The first method is that which
has been called the method of _electrical syntony_, and consists in
adjusting the electrical capacity and inductance of the various open
and closed circuits of the receiving and transmitting stations to be
put in communication so that they have the same electrical
time-period.[60]
In the Cantor Lectures before the Society of Arts in 1900, on
electrical oscillations and electric waves, the author has discussed
at length the conditions under which powerful electrical oscillations
can be set up in a circuit. It was there shown that every electric
circuit having capacity and inductance has a particular or natural
time-period of electrical oscillation depending on the product of
these qualities, and that, to accumulate powerful electrical
oscillations in it, the electromotive impulses on it must be delivered
at this rate. Illustrations were drawn from mechanics, such as the
examples furnished by vibrating pendulums and springs, and from
acoustics, as illustrated by the phenomena of resonance, to show that
small or feeble blows or impulses delivered at the proper time
intervals have a cumulative effect in setting up vibrations in a body
capable of oscillation. It is a familiar fact that if we time our
blows, we can achieve that which no single blow, however powerful, can
accomplish in throwing into vibration a body such as a pendulum, which
is capable of oscillation under the action of a restoring force.
Precisely the same is true of an electric circuit. We have already
seen that the receiving aerial has an alternating electromotive force
set up in it by the impact of the successive electric waves sent out
from the transmitter. It must, however, be remembered that the
transmitter sends out a series of trains of waves, not by any means a
continuous train, but one cut up into groups of probably ten to fifty
waves, each separated by intervals of silence, long, compared with the
duration of a single train of waves.
[Illustration: FIG. 22.--SEIBT'S APPARATUS FOR EXHIBITING ELECTRIC
RESONANCE. I, induction coil; S, spark gap; CC, condensers; L,
variable inductance; E, earth plate; WW, wire spirals; VV, vacuum
tubes.]
If, however, by a suitable adjustment of capacity and inductance, we
make the natural time-period of oscillation of the receiving aerial
circuits agree with those of the transmitting aerial, within certain
limits the former will only be receptive for waves of the frequency
sent out by the transmitter. It is quite easy to illustrate this
principle by numerous experiments. It can be done by means of an
apparatus devised by Dr. Georg Seibt for showing in an interesting
manner the syntonisation or tuning of two electric circuits. This
consists of two bobbins, each consisting of one layer of insulated
wire wound on a wooden rod (see Fig. 22). Each of these bobbins has a
certain electrical capacity with respect to the earth, when considered
as an insulated conductor, and it has also a certain inductance. If,
therefore, electromotive impulses are applied to one end of the bobbin
at regular intervals, electrical oscillations will be set up in it,
and, as already explained, if these are timed at a certain rate, the
bobbin will act like a closed organ-pipe to air impulses and
oscillations of potential will be accumulated at the opposite end,
which have much greater amplitude than the impressed oscillations at
the end at which they are applied. We can make the existence of the
amplitude oscillations of potential evident by attaching to one end of
the bobbin a vacuum tube, which will be illuminated thereby, or by
terminating it by a pointed piece of wire, so that an electrical brush
can be formed at the point, if the potential variations have
sufficient amplitude. We arrange also another closed oscillation
circuit, consisting of two Leyden jars and a variable inductance coil
and a pair of spark balls which are connected to an induction coil. In
this manner we can set up oscillations in the discharge circuit of
these Leyden jars, and we can vary the time period by altering the
inductance and the capacity. If we denote the capacity of the jars in
the microfarads by the letter C and the inductance in centimetres of
the discharge circuit of the jars by the letter L, it can then be
shown that the number of oscillations per second denoted by _n_ is
given by the expression--[61]
n = (5,000,000,000) / ([\sq]{CL}).
If now we adjust the Leyden jar circuit to a particular rate of
oscillation, we have between the terminals of the jar or condenser an
alternating difference of potential or electromotive force. If we
connect one side of the jars to the earth and the other side to the
foot of one of the spirals or bobbins above described, we shall find
perhaps that the vacuum tube at the other end is not rendered
luminous. When, however, we adjust the inductance in the discharge
circuit of the jar to a certain value to make the frequency of the
condenser oscillations agree with the natural time period of the
bobbin terminated by the vacuum tube, this latter at once lights up
brilliantly. Again, if we connect both these bobbins at the same time
to the discharge circuit of the Leyden jar, we shall find that we can
make an adjustment of the inductance of that circuit, such that either
of the bobbins at pleasure can be made to respond and be set in
electrical vibration, as shown by the illumination of the vacuum tube
at its upper end or by an electrical brush being formed at the
terminal. In making this adjustment of inductance, we are _tuning_, as
it is called, the Leyden jar discharge circuit to the resonating
bobbin. A very small variation of the inductance of the jar circuit
causes the vacuum tube to change in luminosity. If, however, the
natural time periods of these bobbins do not lie very far apart, then
a faint luminosity will make its appearance in both the vacuum tubes.
Supposing, therefore, that we connect to the oscillating circuit of
the jar a number of bobbins having different time periods of
oscillation, like organ-pipes, and supply them all with one common
alternating electromotive force. These bobbins, whose natural time
period is very different to that of the osciilating circuit or
impressed electromotive force, will not respond, but those bobbins of
which the natural time period lies near to, even if not quite exactly
the same as, that of the impressed electromotive force will give
evidence of being set in oscillation. A very violent electromotive
force will cause them all to respond to some slight extent, no matter
whether the period of that impluse is tuned to their common period
precisely or not.
At this point questions arise of great practical importance. A matter
which has been in dispute in connection with practical Hertzian wave
telegraphy is how far this electrical tuning is a sufficient solution
of the practical problem of isolation. It is not denied that
experiments such as those made with Seibt's apparatus can be shown on
a small scale; and, on a still larger scale, Mr. Marconi gave to the
author in September, 1900, a demonstration in practical telegraphic
work of sending two independent Hertzian wave messages and receiving
them on two independent receivers attached to the same aerial.
Since that date much experience has been gained and large power
stations erected, and a statement has been frequently made that
syntony is no protection against interference when one of the stations
is sending out very powerful waves. The contention has been raised
that large power stations producing electric waves will therefore play
havoc with Hertzian wave telegraphy on a smaller scale, such as the
ship to shore and intermarine communication. Under these
circumstances, it appeared to the author important to subject the
matter to a special test, and Mr. Marconi, therefore, offered to give
a demonstration, with this object, in support of the opinion that he
has expressed positively that waves from his power stations do not
interfere with the working of his ship installations. This matter is
vital to the whole question of practical Hertzian wave telegraphy, for
the ship to shore communication is of stupendous importance; and if
Mr. Marconi had done nothing else except to render this possible and
effective, he would have earned, as he has done, the gratitude of
humanity for all time. Accordingly, the author embraced the
opportunity of making some careful tests to settle the question
whether the powerful waves sent out from a station such as Poldhu did
or did not affect the exchange of messages between ship and shore
stations in proximity, equipped with Marconi apparatus of a suitable
type.
These experiments were carried out on the eighteenth of March last, at
Poldhu, in Cornwall, and a programme was arranged by the author of the
following kind. Close to the Poldhu station is an isolated mast, which
was equipped by Mr. Marconi with a Hertzian wave apparatus, similar to
that he places on ships. Six miles from Poldhu is the Lizard receiving
station, with which ships proceeding up or down the English Channel
communicate. It was arranged that a series of secret messages, some of
them in cipher, should be delivered simultaneously at certain known
times, both to the power station at Poldhu and to the small adjacent
ship station; and it was arranged that these messages should be sent
off simultaneously, the operators being kept in ignorance up to the
moment of sending as to the nature of the messages. At the Lizard, Mr.
Marconi connected two of his receiving instruments to the aerial, one
of them tuned to the waves proceeding from the power station at
Poldhu, and the other to those proceeding from the small ship station.
At the appointed time, these two sets of messages were received
simultaneously in the presence of the author, each message being
printed down independently on its own receiver; and Mr. Marconi read
off and interpreted all these messages perfectly correctly, not having
known before what was the message that was about to be sent. In
addition to this, precautions were taken to prove that the power
station at Poldhu was really emitting waves sufficiently powerful to
cross the Atlantic and not being made to sing small for the occasion.
To assist in proving this, the messages sent out from the power
station were also received at a station at Poole, two hundred miles
away, and the assistant there was instructed to telegraph back these
messages by wire as soon as he received them. These messages came back
perfectly correctly, thus demonstrating that the power station was
sending out power waves. The whole programme was carried out with the
greatest care to avoid any mistakes on the part of the assistants, and
provided an absolute demonstration of the truth of Mr. Marconi's
assertion that the waves from one of his power stations, such as
Poldhu, do not in the least degree interfere with the transmission and
reception of messages between ship and shore, effected by means of
certain forms of Marconi apparatus for producing and detecting waves
of a different wave length.[62] This complete independence of
transmission, however, is entirely due to the employment of a
receiving circuit of a certain type in Mr. Marconi's receivers. It
does not at all follow that a receiving circuit of any kind, even a
Marconi receiver not especially arranged, set up in proximity to a
power station would not be affected. This, however, is not an
important matter. Far more important is it to show, as has been shown,
that practically perfect isolation can be achieved if it is desired.
It must be noted, however, that, although the fact that electric
circuits have a natural time-period of oscillation of their own is a
scientific principle which carries us a considerable way towards a
solution of what is called syntonic Hertzian wave telegraphy, it is
not in itself alone in every respect an entire solution of the
practical problem. The degree to which it is a solution depends to a
considerable extent upon the nature of the detecting device, or
kumascope, which we are employing. The coherer, or Branly filings
tube, has the peculiarity that its passage from a non-conductive to a
conductive condition follows immediately when the difference of
potential between its ends is made sufficiently great. In other words,
if the tube is acted upon by a sufficient electromotive force, it is
not necessary that electromotive force should be repeated at intervals
to make this particular form of kumascope responsive. Again, if we
consider the nature of the oscillations which are sent out from any
transmitting aerial, we find that each group of oscillations
corresponding to a single spark consists of waves gradually decreasing
in amplitude. In other words, the first wave of the group is the
strongest, and the decay in amplitude is often very rapid. Supposing,
then, we construct a simple receiver consisting of an aerial having
inserted in its circuit a sensitive Branly filings tube. Such a
receiver is almost entirely non-syntonic; that is to say, it is
affected by any wave passing over it which is sufficiently powerful.
We may look upon it that if the first wave of the series is
sufficiently powerful to affect the kumascope, the conductive change
takes place whether or not the first wave is followed by others.
Accordingly, it is perfectly certain that if a transmitter is sending
out trains of waves of any period, a simple combination of coherer and
aerial will be influenced, if it is placed near enough to the
transmitter. On the other hand, it is possible to combine a kumascope
of a certain type with a receiving aerial and other circuits in such a
manner that when the waves that reach it are feeble it shall not
respond at all unless those waves have very nearly a time period of a
certain value.
At this stage, it may be perhaps well to explain a little in detail
what is meant by an easily responsive circuit, and, on the other hand,
by an irresponsive circuit, or, as we may call it, a _stiff_ circuit.
Supposing that we consider an aerial consisting of a simple straight
wire having small capacity and small inductance, such a circuit admits
of being sent into electrical oscillation, not only by waves of its
own natural time-period, but by the sudden application of any violent
electromotive impulse. If, on the other hand, we bestow upon the
circuit in any way considerable inductance, we then obtain what may be
called a stiff or irresponsive circuit, which is one in which
electrical oscillations can be accumulated only by the prolonged
action of impulses tuned to a particular period.
A mechanical analogue of this difference may be found in considering
the different behaviour of elastic bodies to mechanical blows. Take,
for instance, a piece of elastic steel and fix the bottom end in a
vice. The steel strip may be thrown into vibration by deflecting the
upper end. It has, however, a very small mass, and therefore any
violent blow or blows, even although not repeated, will set it in
oscillation. If, however, we add mass to it by fixing at the other end
a heavy weight, such as a ball of lead, and at the same time make the
spring stiffer, we have an arrangement which is capable of being sent
into considerable oscillation only by the action of a series of
impulses or blows which are timed at a particular rate.
Returning then to the electrical problem, we see that in order to
preserve a kumascope or wave detector from being operated on by any
vagrant wave or waves having a period very different to an assigned
period, it must be associated with an electrical circuit of the kind
above called a stiff circuit.
We will now consider the manner in which the problem has been
practically attacked by Mr. Marconi, Dr. Slaby, Sir Oliver Lodge and
others, who have invented forms of receiver and transmitter, which are
syntonic or sympathetic to one another.
Some of the methods which Mr. Marconi has devised for the achievement
of syntonic wireless telegraphy were fully described by him in a Paper
read before the Society of Arts on May, 17, 1901.[63]
[Illustration: FIG. 23.--MARCONI TRANSMITTER AND RECEIVER. I,
induction coil; A, aerial; E, earth plate; HH, choking coils; S, spark
gap; J, transmitting jigger; K, receiving jigger; R, relay; C,
condenser; F, filings tube; B, battery. Many practical details are
omitted.]
On referring to his Paper, it will be seen that in one form his
transmitter consists of an aerial, near the base of which is inserted
the secondary circuit of an oscillation transformer or transmitting
jigger. One end of this secondary circuit is attached to the aerial
and the other end is connected to the earth through a variable
inductance coil. The primary circuit of this oscillation transformer
is connected in series with a condenser, consisting of a battery of
Leyden jars, and the two together are connected across to the spark
balls which close the secondary circuit of an induction coil, having
the usual make and break key in the primary circuit. Mr. Marconi so
adjusts the induction of the aerial and the capacity of the condenser,
or battery of Leyden jars, that the two circuits, consisting
respectively of this battery of Leyden jars and the primary circuit of
the transformer, and on the other hand of the capacity of the aerial
and the inductance in series with it, and that of the secondary
circuit of the transformer have the same time period. In other words,
these two inductive circuits are tuned together. At the receiving end,
the aerial is connected in series with a variable inductance and with
the primary circuit of another oscillation transformer, the second
terminal of which is connected to the earth. The secondary circuit of
this last oscillation transformer is cut in the middle and is
connected to the terminals of a small condenser. The outer terminals
of this secondary circuit are connected to the metallic filings tube
or other sensitive receiver and to a small condenser in parallel with
it (see Fig. 23). The terminals of the condenser which is inserted in
the middle of the secondary circuit of the oscillation transformer are
connected through two small inductance coils with a relay and a single
cell. This relay in turn actuates a Morse printer by means of a local
battery. The two circuits of the oscillation transformer are tuned or
syntonised to one another, and also to the similar circuit of the
transmitting arrangement. When this is the case, the transmitter
affects the co-resonant receiving arrangement, but will not affect any
other similar arrangement, unless it is within a certain minimum range
of distance. Owing to the inductance of the oscillation transformer
forming part of the receiving arrangements, the receiving circuit is,
as before stated, very stiff or irresponsive; the sensitive tube is
therefore not acted upon in virtue merely of the impact of the single
wave against the aerial, but it needs repeated or accumulated effects
of a great many waves, coming in proper time, to break down the
coherer and cause the recording mechanism to act. An inspection of the
diagram will show that as soon as the secondary electromotive force in
the small oscillation transformer or jigger of the receiving
instrument is of sufficient amplitude to break down the resistance of
the coherer, the local cell in circuit with the relay can send a
current through it and cause the relay to act and in turn make the
associated telegraphic instrument record or sound.
Mr. Marconi described in the above-mentioned Paper some other
arrangements for achieving the same result, but those mentioned all
depend for their operation upon the construction of a receiving
circuit on which the time-period of electrical oscillations is
identical with that of a transmitting arrangement. By this means he
showed experiments during the reading of his Paper, illustrating the
fact that two pairs of transmitting and receiving arrangements could
be so syntonised that each receiver responded only to its particular
transmitter and not to the other.
With arrangements of substantially the same nature, he made
experiments in the autumn of 1900 between Niton, in the Isle of Wight,
and Bournemouth, a distance of about thirty miles, in which
independent messages were sent and received on the same aerial.
Dr. Slaby and Count von Arco, working in Germany, have followed very
much on the same lines as Mr. Marconi, though with appliances of a
somewhat different nature. As constructed by the General Electric
Company, of Berlin, the Slaby-Arco syntonic system of Hertzian
telegraphy is arranged in one form as follows:--The transmitter
consists of a vertical rod like a lightning conductor, say, 100 or 150
feet in height. At a point six or nine feet above the ground, a
connection is made to a spark ball (see Fig. 24), and the
corresponding ball is connected through a variable inductance with one
terminal of a condenser, the other terminal of which is connected to
the earth. The two spark balls are connected to an induction coil, or
alternating current transformer, and by variation of the inductance
and capacity the frequency is so arranged that the wave-length
corresponding to it is equal to four times the length of that portion
of the aerial which is above the spark ball connection. The method by
which this tuning is achieved is to insert in the portion of the
aerial below the spark balls, between it and the earth, a hot wire
ammeter of some form. It has already been shown that in the case of
such an earthed aerial, when electrical oscillations are set up in it,
there is a potential node at the earth and a potential anti-node or
loop at the summit, if it is vibrating in its fundamental manner;
also, there is a node of current at the summit of the aerial and an
anti-node at the base. This amounts to saying that the amplitude of
the potential vibrations is greatest at the top end of the aerial, and
the amplitude of the current vibrations is greatest at the bottom or
earthed end. Accordingly, the inductance and capacity of the lateral
branch of the transmitter is altered until the hot wire ammeter in the
base of the aerial shows the largest possible current.
[Illustration: FIG. 24.--SLABY-ARCO SYNTONIC TRANSMITTER AND RECEIVER.
I, induction coil; M, multiplier; B, battery; A, aerial; F, filings
tube; R, relay; E, earth plate; C, condenser.]
The corresponding receiver is constructed in a very similar manner. A
lightning conductor or long vertical rod of the same height as the
transmitting aerial is set up at the receiving station, and at a point
six or nine feet from the ground a circuit is taken off, consisting of
a wire loosely coiled in a spiral, the length of which is nearly equal
to, although a little shorter than, the height of the vertical wire
above the point of connection. The outer end of this loose spiral is
connected to one terminal of the coherer tube, and the other terminal
of the coherer is connected to the earth through a condenser of rather
large capacity. The terminals of this last condenser are
short-circuited by a relay and a single cell. When the adjustments are
properly made, it is claimed that the receiver responds only to waves
coming from its own syntonised or tuned transmitter. In this case the
length of the receiving aerial above the point of junction with the
coherer circuit is one quarter the length of the wave. A variation of
the above arangements consists in making this lateral circuit equal in
length to one-half of a wave, and connecting the coherer to its centre
through a condenser to the earth. The outer end of this lateral
circuit is also connected to the earth (see Fig. 24).[64]
Dr. Slaby claims that this arrangement is not affected by atmospheric
electricity, and that the complete and direct earthing of the aerial
and also in the second arrangement, of the receiver of the outer end
of the lateral conductor, conduces to preserve the receiver immune
from any electrical disturbances except those having a period to which
it is tuned.
[Illustration: FIG. 25.--LODGE-MUIRHEAD SYNTONIC RECEIVER. I,
induction coil; S, spark gap; A, aerial; CC, condensers; E, earth
plate; R, relay; L, variable inductance; F, filings tube; B, battery.]
A method has also been arranged by him for receiving on the same
aerial two messages from different transmitting stations
simultaneously. In this case, two lateral wires of different lengths
are connected to the receiving aerial, and to the outer end of each of
these is connected a coherer tube, the other end of which is earthed
through a condenser. One of these lateral wires is made equal, or
nearly equal, in length to the aerial, and the other is made longer to
fulfil the following condition.[65] If we call H the height of the
receiving aerial above point of junction of the lateral wires, then
the length of one lateral wire is made equal to H, and the height of
the aerial is adjusted to be equal to one-quarter of the wave length
of one incident wave. The other lateral wire may then be made of a
length equal to one-third of H, and it will then respond to the first
odd harmonic of that wave, of which the fundamental is in syntony with
the vertical wire. By suitably choosing the relation between the
wave-lengths of the two transmitting stations, it is possible to
receive in this manner two different messages at the same time on the
same aerial. Subsequently to the date of the above-mentioned
demonstration of multiplex wireless telegraphy by Mr. Marconi an
exhibition of a similar nature was given by Professor Slaby in a
lecture given in Berlin on December 22, 1900.[66]
Both the above-described syntonic systems of Mr. Marconi and Dr. Slaby
are "earthed" systems, but arrangements for syntonic telegraphy have
been devised by Sir Oliver Lodge and Professor Braun which are
"non-earthed."
Sir Oliver Lodge and Dr. Muirhead have devised also syntonic systems.
According to their last methods, the systonic transmitting and
receiving arrangements are as shown in Fig. 25.[67] On examining the
diagrams it will be seen that the secondary terminals of the induction
coil are, as usual, connected to a pair of spark balls, and that these
spark balls are connected by a condenser and by a variable inductance.
One terminal of the condenser is earthed through another condenser of
large capacity, and the remaining terminal of the first condenser is
connected to an aerial. It should, therefore, be borne in mind in
dealing with electrical oscillations that a condenser of sufficient
capacity is practically a conductor, and an inductance coil of
sufficient inductance is practically a non-conductor. Hence the
insertion of a large capacity in the path of the aerial wire is no
advantage whatever and makes no essential difference in the
arrangement. In order to obtain any powerful radiation, the length of
the aerial, or sky wire, as they call it, must be so adjusted that its
length is one-quarter the wave-length corresponding to the oscillation
circuit, consisting of the condenser and variable inductance.
The receiving arrangement consists of a similar sky wire or aerial
earthed through a condenser of large capacity and having in the
portion above this last condenser another condenser of similar
capacity. At the earthed side of this last condenser a connection is
made to a resonant circuit, consisting of a variable inductance, and
another condenser and a sensitive metallic filings tube of the Branly
type; also a portion of this resonant circuit is shunted by another
consisting of a battery and telegraphic relay, as shown in the
diagram. The circuit, including the coherer, is tuned to its own
aerial and also to that of the transmitting circuit, and under these
circumstances trains of waves thrown off at the transmitting aerial
will sympathetically affect the receiving aerial.
There is nothing in the arrangement which specially calls for notice.
It is simply a variation of other known forms of syntonic transmitter
and receiver, and possesses all the advantages and disadvantages
attaching to such electrical syntonic methods.
Professor Braun's syntonic system, the receiver and transmitter of
which have been described, is also in one form a non-earthed system.
Innumerable other patentees have taken out patents for devices which
are modifications in small degree of the above arrangements.
It may be well to note at this point the disadvantages that are
possessed by any form of coherer as a telegraphic kumascope in
connection with proposed arrangements for the isolation of Hertzian
wave stations. All the detectors of the coherer type really depend for
their actuation upon electromotive force; that is to say, upon the
application to the terminals of the detector of a certain
electromotive force. Although there may be no sharp and defined
critical electromotive force, yet, nevertheless, as a matter of fact,
if the electromotive force applied exceeds a certain value, then the
detector passes suddenly from one state of conductivity to another. It
may be of great conductivity, as in the case of the Branly coherer, or
of lesser conductivity, as in the case of the so-called anti-coherers,
of which the Schaefer kumascope may be taken as a type. Accordingly,
when these instruments are subjected to a train of waves, each
individual group of which is damped, their operation is largely
governed by the fact that if the first wave or oscillation set up in
the receiving circuit is powerful enough to break down the coherer,
then the receiving mechanism acts, no matter whether the first impulse
is followed by others or not.
In comparison with so-called coherers, those depending upon the
changes in the magnetisation of iron by electrical oscillations
certainly have an advantage, because this is a process which requires
the application of alternating electric currents decreasing in
strength for a certain time; and it is found, therefore, that the
magnetic receivers do not require to be associated with such a stiff
or irresponsive resonant circuit to confine their indications to
oscillations or waves of one definite period, and that they lend
themselves much more perfectly to the work of "tuning" or syntonising
stations than do those kumascopes depending upon the contact or
coherer principle.
We may then glance at the alternative solutions of the problem offered
by other investigators. M. Blondel has proposed to effect the
syntonisation of two stations, not by syntonising the receiver for the
exceedingly high-frequency oscillations of the individual electric
waves, but to syntonise it for the much lower frequency, corresponding
to that of the intervals between the groups of waves. Thus, for
instance, if an ordinary simple transmitting aerial is set up, the
production of sparks between the spark balls results in the emission
of short trains of waves, each of which may consist of half a dozen or
more individual waves, the time of production of the whole group being
very small compared with the interval between the groups. M. Blondel
proposes, however, to syntonise the receiver, not for the
high-frequency period of the waves themselves, which may be reckoned
in millions per second, but for the low-frequency period between the
groups of waves, which is reckoned in hundreds per second. Thus, for
instance, if sparks are made at the rate of fifty or a hundred per
second, they can be made to actuate the telephone receiver and so
produce in the telephone a sound corresponding to a frequency of 50 or
100; in other words, to make a low musical note or hum. This
continuous sound can be cut up, by means of a key placed in the
primary circuit of the transmitting arrangement, into long or short
periods, and hence the letters of the alphabet signal.
M. Blondel's arrangements comprise a Mercadier's monotone telephone
and either a coherer or a particular form of vacuum tube as a
kumascope. On August 16, 1898, M. Blondel deposited with the Academy
of Sciences in Paris a sealed envelope containing a description of his
improvements in syntonic wireless telegraphy, which was opened on May
19, 1900.[68] The arrangement of the receiving apparatus was as
follows:--A single-battery cell keeps a condenser charged until the
kumascope is rendered conductive by the oscillations coming down the
aerial; and under these circumstances the condenser discharges through
the telephone and causes a tick to be heard in it. If the trains of
waves are at the rate of 50 or 100 per second, these small sounds run
together into a musical note, and this continuous hum can be cut up
into long and short spaces, in accordance with the Morse alphabet
signals. The telephone must not be an ordinary telephone, capable of
being influenced by any frequency, but be one which responds only to a
particular note, and under these conditions the receiving arrangement
is receptive only when the trains of waves arrive at certain regular
predetermined intervals, corresponding with the tone to which the
telephone is sensitive.
* * * * *
A number of more or less imperfect arrangements, having the isolation
of communications for their object, have been devised or patented,
which are dependent upon the use of several aerials, each supposed to
be responsive only to a particular frequency; and attempts have been
made to solve the problem of isolation by MM. Tommasi, Tesla, Jegon,
Tissot, Ducretet and others.
We may then pass on to notice the attempts that have been made to
secure isolation by a plan which is not dependent on electrical
syntony. One of these, which has the appearance of developing into a
practical solution of the problem, is that due to Anders Bull.[69] In
the first arrangements proposed by this inventor, a receiver is
constructed which is not capable of being acted upon merely by a
single wave or train of waves or even a regularly-spaced train of
electric waves, but only by a group of wave trains which are separated
from one another by certain unequal, predetermined intervals of time.
Thus, for instance, to take a simple instance, the transmitting
arrangements are so devised as to send out groups of electric waves,
these wave trains following one another at time intervals which may be
represented by the numbers 1, 3 and 5; that is to say, the interval
which elapses between the second and third is three times that between
the first two, and the interval between the fourth and fifth is five
times that between the first two. This is achieved by making five
electric oscillatory sparks with a transmitter of the ordinary kind,
the intervals between which are settled by the intervals between holes
punched upon strips of paper, like that used in a Wheatstone automatic
telegraphic instrument. It will easily be understood that by a device
of this kind, groups of sparks can be made, say, five sparks rapidly
succeeding each other, but not at equal intervals of time. One such
group constitutes the Morse dot, and two or three such groups
succeeding one another very quickly constitute the Morse dash. These
waves, on arriving at the receiving station, are caused to actuate a
punching arrangement by the intermediation of a coherer or other
kumascope, and to punch upon a uniformly moving strip of paper holes,
which are at intervals of time corresponding to the intervals between
the sparks at the transmitting station. This strip of paper then
passes through another telegraphic instrument, which is so constructed
that it prints upon another strip a dot or a dash, according to the
disposition of the holes on the first strip. Accordingly, taken as a
whole, the receiving arrangement is not capable of being influenced so
as to print a telegraphic sign except by the operation of a series of
wave trains succeeding one another at certain assigned intervals of
time.
An improvement has been lately described by the same inventor,[70] in
which the apparatus used, although more complicated, performs the same
functions. At each station two instruments have to be employed; at the
transmitting station one to effect the conversion of Morse signals
into the properly arranged series of wave trains, and at the receiving
station an instrument to effect the re-conversion of the series of
wave trains into the Morse signals. These are called respectively the
dispenser and the collector. The details of the arrangements are
somewhat complicated, and can only be described by the aid of numerous
detailed drawings, but the inventor states that he has been able to
carry on Hertzian wave telegraphy by means of these arrangements for
short distances. Moreover, the method lends itself to an arrangement
of multiplex telegraphy, by sending out from different transmitters
signals which are based upon different arrangements of time intervals
between the electric wave trains. Although this method may succeed in
preventing a receiving arrangement from being influenced by vagrant
waves or waves not intended for it, yet an objection which arises is
that there is nothing to prevent any one from intercepting these wave
trains, and with a little skill interpreting their meaning. Thus, if
the record were received in the ordinary way on a simple receiver,
corresponding to a Morse dot would be printed five dots at unequal
intervals, and corresponding to a Morse dash would be printed two such
sets of five dots. A little skill would then enable an operator to
interpret these arbitrary signals. On the other hand, the inventor
asserts that he can overcome this difficulty by making intervals of
time between the impulses in the series so long that the latter become
longer than the intervals between each of the series of waves which
are despatched in continuous succession when the key is pressed for a
dash. In this case, when telegraphing, the series of dots would
overlap and intermingle with each other in a way which would make the
record unintelligible if received in the usual manner, but would be
perfectly legible if received and interpreted by a receiver adapted
for the purpose.
Another way of obliterating the record, as far as outsiders are
concerned, is to interpolate between the groups of signals an
irregular series of dots--_i.e._, of wave trains--which would affect
an ordinary coherer, and so make an unintelligible record on an
ordinary receiver, but these dots are not received or picked up by the
appropriate selecting instrument used in the Anders Bull system.
The matter most interesting to the public at the present time is the
long-distance telegraphy by Hertzian waves to the accomplishment of
which Mr. Marconi has devoted himself with so much energy of late
years. Everyone, except perhaps those whose interests may be
threatened by his achievements, must accord their hearty admiration of
the indomitable perseverance and courage which he has shown in
overcoming the immense difficulties which have presented themselves.
Five years ago he was engaged in sending signals from Alum Bay, in the
Isle of Wight, to Bournemouth, a distance of twelve or fourteen miles;
and to-day he has conquered twice that number of hundred miles and
succeeded in sending, not merely signals, but long messages of all
descriptions over three thousand miles across the Atlantic. Critics
there are in abundance, who declare that the process can never become
a commercial one, that it will destroy short-distance Hertzian
telegraphy, or that the multiplication of long-distance stations will
end in the annihilation of all Hertzian wave telegraphy. No one,
however, can contemplate the history of any development of applied
science without seriously taking to heart the lesson that the
obstacles which arise and which prove serious in any engineering
undertaking are never those which occur to armchair critics. Sometimes
the seemingly impossible proves the most easy to accomplish, whilst
difficulties of a formidable nature often spring up where least
expected.
The long-distance transmission is a matter of peculiar interest to the
author of these articles, because he was at an early stage in
connection with it invited to render Mr. Marconi assistance in the
matter.[71] The particular work entrusted to him was that of planning
the electrical engineering arrangements of the first power station
erected for the production of electric waves for long-distance
Hertzian wave telegraphy at Poldhu, in Cornwall. When Mr. Marconi
returned from the United States in the early part of 1900, he had
arrived at the conclusion that the time had come for a serious attempt
to accomplish wireless telegraphy across the Atlantic. Up to that date
the project had been an inventor's dream, much discussed, long
predicted, but never before practically taken in hand. The only
appliances, moreover, which had been used for creating Hertzian waves
were induction coils or small transformers, and the greatest distance
covered, even by Mr. Marconi himself, had been something like 150
miles over sea. Accordingly, to grapple with the difficulty of
creating an electric wave capable of making itself felt at a distance
of 3,000 miles, even with the delicate receiving appliances invented
by Mr. Marconi, seemed to require the means of producing at least four
hundred times the wave-energy that had been previously employed. The
author was, therefore, requested to prepare plans and specifications
for an electric generating plant for this purpose, which would enable
electrical oscillations to be set up in an aerial on a scale never
before accomplished.
This work involved, not merely the ordinary experience of an
electrical engineer, but also the careful consideration of many new
problems and the construction of devices not before used. Every step
had to be made secure by laboratory experiments before the
responsibility could be incurred of advising on the nature of the
machinery and appliances to be ordered. Many months in the year 1901
were thus occupied by the author in making small-scale experiments in
London and in superintendence of large-scale experiments at the site
of the first power station at Poldhu, near Mullion, in Cornwall,
before the plant was erected and any attempt was made by Mr. Marconi
to commence actual telegraphic experiments. As this work was of a
highly confidential nature, it is obviously impossible to enter into
the details of the arrangements, either as made by the writer in the
first instance, or as they have been subsequently modified by Mr.
Marconi. The design of the aerial and of the oscillation transformers
and many of the details in the working appliances are entirely due to
Mr. Marconi, but as a final result, a power plant was erected for the
production of Hertzian waves on a scale never before attempted. The
utilisation of 50 H.P. or 100 H.P. for electric wave production has
involved dealing with many difficult problems in electrical
engineering, not so much in novelty of general arrangement as in
details. It will easily be understood that Leyden jars, spark balls
and oscillators, which are quite suitable for use with an induction
coil, would be destroyed immediately if employed with a large
alternating-current plant and immensely powerful transformers.
[Illustration: FIG. 26.--WOODEN TOWERS SUPPORTING THE MARCONI AERIAL
AT POLDHU POWER STATION, CORNWALL, ENGLAND.]
In the initial experiments with this machinery and in its first
working there was very considerable risk, owing to its novel and
dangerous nature; but throughout the whole of the work from the very
beginning, no accident of any kind has taken place, so great have been
the precautions taken. The only thing in the nature of a mishap was
the collapse of a ring of tall masts, erected in the first place to
sustain the aerial wires, but which now have been replaced by four
substantial timber towers, 215 feet in height, placed at the corners
of a square, 200 feet in length. These four towers sustain a conical
arrangement of insulated wires (see Fig. 26) which can be used in
sections and which constitute the transmitting radiator or receiver,
as the case may be. Each of these wires is 200 feet in length and
formed of bare stranded wire.
At the outset, there was much uncertainty as to the effect of the
curvature of the earth on the propagation of a Hertzian wave over a
distance of many hundreds of miles. In the case of the Atlantic
transmission between the station at Poldhu in Cornwall and that at
Cape Cod in Massachusetts, U.S.A., we have two stations separated by
about 45 degrees of longitude on a great circle, or one-eighth part of
the circumference of the world. In this case, the versine of the arc
or height of the sea at the half-way point above the straight line or
chord joining the two places is 300 miles.
The question has recently attracted the attention of several eminent
mathematical physicists. The extent to which a free wave propagated in a
medium bends round any object or is diffracted depends on the relation
between the length of the wave and the size of the object. Thus, for
instance, an object the size of an orange held just in front of the
mouth does not perceptibly interfere with the propagation of the waves
produced by the speaking or singing voice, because these are from two to
six feet in length: but if arrangements are made by means of a Galton
whistle to produce air waves half an inch in length, then an obstacle
the size of an orange causes a very distinct acoustic shadow. The same
thing is true of waves in the ether. The amount of bending of light
waves round material objects is exceedingly small, because the average
length of light waves is about one-fifty-thousandth part of an inch. In
the case of Hertzian wave telegraphy, we are, however, dealing with
ether waves many hundreds of feet in length, and the waves sent out from
Poldhu have a wave-length of a thousand feet or more, say, one-fifth to
one-quarter of a mile. The distance, therefore, between Poldhu and Cape
Cod is only at most about twelve thousand wave-lengths, and stands in
the same relation to the length of the Hertzian wave used as does a body
the diameter of a pea to the wave-length of yellow light. There is
unquestionably a large amount of diffraction or bending of the electric
wave round the earth, and, proportionately speaking, it is larger than
in the case of light waves incident on objects of the same relative
size.
Quite recently Mr. H. M. Macdonald (see _Proc._ Roy. Soc., London,
Vol. LXXI., p. 251) has submitted the problem to calculation, and has
shown that the power required to send given electric waves 3,000 miles
along a meridian of the earth is greater than would be required to
send them over the same distance if the sea surface were flat in the
ratio of 10 to 3. Hence the rotundity of the earth does introduce a
very important reduction factor, although it does not inhibit the
transmission. Mr. Macdonald's mathematical argument has, however, been
criticised by Lord Rayleigh and by M. H. Poincare (see _Proc._ Roy.
Soc., Vol. LXXII., p. 40, 1903).
The accomplishment of very long distances by Hertzian wave telegraphy
is, however, not merely a question of power, it is also a question of
wave-length. Having regard, however, to the possibility that the
propagation which takes place in Hertzian wave telegraphy is not that
simply of a free wave in space, but the transmission of a semi-loop of
electric strain with its feet tethered to the earth, it is quite
possible that if it were worth while to make the attempt, an ether
disturbance could be made in England sufficiently powerful to be felt
in New Zealand.
Leaving, however, these hypothetical questions and matters of pure
conjecture, we may consider some of the facts which have resulted from
Mr. Marconi's long-distance experiments. One of the most interesting
of these is the effect of daylight upon the wave propagation. In one
of his voyages across the Atlantic, when receiving signals from Poldhu
on board the S.S. _Philadelphia_, he noticed that the signals were
received by night when they could not be detected by day.[72] In these
experiments Mr. Marconi instructed his assistants at Poldhu to send
signals at a certain rate from 12 to 1 a.m., from 6 to 7 a.m., from 12
to 1 p.m., and from 6 to 7 p.m., Greenwich mean time, every day for a
week. He has stated that on board the _Philadelphia_ he did not notice
any apparent difference between the signals received in the day and
those received at night until after the vessel had reached a distance
of 500 statute miles from Poldhu. At distances of over 700 miles, the
signals transmitted during the day failed entirely, while those sent
at night remained quite strong up to 1,551 miles, and were clearly
decipherable up to a distance of 2,099 miles from Poldhu. Mr. Marconi
also noted that at distances of over 700 miles, the signals at 6 a.m.,
in the week between February 23 and March 1, were quite clear and
distinct, whereas by 7 a.m. they had become weak almost to total
disappearance. This fact led him at first to conclude that the cause
of the weakening was due to the action of the daylight upon the
transmitting aerial, and that as the sun rose over Poldhu, so the wave
energy radiated, diminished, and he suggested as an explanation the
known fact of the dissipating action of light upon a negative charge.
Although the facts seem to support this view, another explanation may
be suggested. It has been shown by Professor J. J. Thomson that
gaseous ions or electrons can absorb the energy of an electric wave,
if present in a space through which waves are being transmitted.[73]
If it be a fact, as suggested by Professor J. J. Thomson, that the sun
is projecting into space streams of electrons, and if these are
continually falling in a shower upon the earth, in accordance with the
fascinating hypothesis of Professor Arrhenius, then that portion of
the earth's atmosphere which is facing the sun will have present in it
more electrons or gaseous ions than that portion which is turned
towards the dark space, and it will therefore be less transparent to
long Hertzian waves.[74] In other words, clear sunlit air, though
extremely transparent to light waves, acts as if it were a slightly
turbid medium for long Hertzian waves. The dividing line between that
portion of the earth's atmosphere which is impregnated with gaseous
ions or electrons is not sharply delimited from the part not so
illuminated, and there may be, therefore, a considerable penetration
of these ions into the regions which I may call the twilight areas.
Accordingly, as the earth rotates, a district in which Hertzian waves
are being propagated is brought, towards the time of sunrise, into a
position in which the atmosphere begins to be ionised, although far
from as freely as is the case during the hours of bright sunshine.
Mr. Marconi states that he has found a similar effect between inland
stations, signals having been received by him during the night between
Poldhu and Poole with an aerial the height of which was not sufficient
to receive them by day. It has been found, however, that the effect
simply amounts to this, that rather more power is required by day than
by night to send signals by Hertzian waves over long distances.
Some interesting observations have also been made by Captain H. B.
Jackson, R.N.,[75] on the influence of various states of the
atmosphere upon Hertzian wave telegraphy. These experiments were all
made between ships of the British Royal Navy, furnished with Hertzian
wave telegraphy apparatus on the Marconi system. Some of his
observations concerned the effect of the interpositon of land between
two ships. He found that the interposition of land containing iron
ores reduced the signalling distances, compared with the maximum
distance at open sea, to about 30 per cent. of the latter; whilst hard
limestone reduced it to nearly 60 per cent. and soft sandstone or
shale to 70 per cent. These results show that there is a considerable
absorption effect when waves of certain wave-length pass through or
over hard rocks containing iron ores. It would be interesting to know,
however, whether this reduction was in any degree proportional to the
dryness or moisture of the soil. Earth conductivity is far more
dependent upon the presence or absence of moisture than upon the
particular nature of the material which composes it other than water.
The observations of Captain Jackson, however, only confirm the already
well-known fact that Hertzian waves, as employed in the Marconi system
of wireless telegraphy, within a certain range of wave-length, are
considerably weakened by their passage through land, over land or
round land. In some cases he noticed that quite sharp electric shadows
were produced by rocky promontories projecting into the line of
transmission. His attention was also directed (_loc. cit._) to the
more important matter of the effect of atmospheric electrical
conditions upon the transmission. The effect of all lightning
discharges, whether visible or invisible, is to make a record on the
telegraphic receiver. On the approach of an atmospheric electrical
disturbance towards the receiving station on a ship, the first visible
indications generally are the recording of dots at intervals from a
few minutes to a few seconds on the telegraphic tape. Captain Jackson
states that the most frequent record is that of three dots, the first
being separated from the other two by a slight interval like the
letters E I on the Morse code, and this is the sign most frequently
recorded by distant lightning. But in addition to this, dashes are
recorded and irregular signs, which, however, sometimes spell out
words in the Morse code. He noted that these disturbances are more
frequent in summer and autumn than in winter and spring, and in the
neighbourhood of high mountains more than in the open sea. In settled
weather, if present, they reach their maximum between 8 p.m. and 10
p.m., and frequently last during the whole of the night, with a
minimum of disturbance between 9 a.m. and 1 p.m. Another important
matter noted by Captain Jackson is the shorter distance at which
signals can usually be received when any electrical disturbances are
present in the atmosphere, compared with the distance at which they
can be received when none are present. This reduction in signalling
distance may vary from 20 to 70 per cent, of that obtainable in fine
weather. It does not in any way decrease with the number of lightning
flashes, but rather the reverse, the loss in signalling distance
generally preceding the first indications on the instrument of the
approaching electrical disturbance. It is clear that these
observations fit in very well with the theory outlined above, viz.,
that the atmosphere when impregnated with free electrons or
negatively-charged gaseous ions is more opaque to Hertzian waves than
when they are absent. Captain Jackson gives an instance of ships whose
normal signalling distance was 65 miles, failing to communicate at 22
miles when in the neighbourhood of a region of electrical disturbance.
These effects in the case of wireless telegraphy have their parallel
in the disturbances caused to telegraphy with wires by earth currents
and magnetic storms.
Another effect which he states reduces the usual maximum signaling
distance is the presence of material particles held in suspension by
the water spherules in moist atmosphere. The effect has been noticed
in the Mediterranean Sea when the sirocco wind is blowing. This is a
moist wind conveying dust and salt particles from the African coast. A
considerable reduction in signalling distance is produced by its
advent.
Another interesting observation due to Captain Jackson is the
existence of certain zones of weak signals. Thus, for instance, two
ships at a certain distance may be communicating well; if their
distance increases, the signalling falls off, but is improved again at
a still greater distance. He advances an ingenious theory to show that
this fact may be due to the interference between two sets of waves
sent out by the transmitter having different wave-lengths.
Finally, in the Paper referred to, he emphasises the well-known fact
that long-distance signalling can only be accomplished by the aid of
an aerial wire and a "good earth." Summing up his results, he
concludes: (1) That intervening land of any kind reduces the practical
signalling distance between two ships or stations, compared with that
which would be obtainable over the open sea, and that this loss in
distance varies with the height, thickness, contour, and nature of the
land; (2) material particles, such as dust and salt, held in
suspension in a moist atmosphere also reduce the signalling distance,
probably by dissipating and absorbing the waves; (3) that electrical
disturbances in the atmosphere also act most adversely in addition to
affecting the receiving instrument and making false signals or
_strays_, as they are called; (4) that with certain forms of
transmitting arrangement, interference effects may take place which
have the result of creating certain areas of silence very similar to
those which are observed in connection with sound signals from a
siren.
It is clear, therefore, from all the above observations, that
Hertzian-wave telegraphy taking place through the terrestrial
atmosphere is not by any means equivalent to the propagation of a wave
in free or empty space; and that just as the atmosphere varies in its
opacity to rays of light, sometimes being clear and sometimes clouded,
so it varies from time to time in transparency to Hertzian waves, the
cause of this variation in transparency probably being the presence in
the atmosphere of negatively-charged corpuscles or electrons. If there
are present in the atmosphere at certain times "clouds of electrons"
or "electronic fogs," these may have the effect of producing a certain
opacity, or rather diminution in transparency to Hertzian waves, just
as water particles do in the case of sunlight.
We may, therefore, in conclusion, review a few of the outstanding
problems awaiting solution in connection with Hertzian wave wireless
telegraphy. In spite of the fact that this new telegraphy has not been
accorded a very hearty welcome by the representatives of official or
established telegraphy in Great Britain, it has reached a point,
unquestionably owing to Mr. Marconi's energy and inventive power, at
which it is bound to continue its progress. But that progress will not
be assisted by shutting our eyes to facts. Many problems of great
importance remain to be solved. We have not yet reached a complete
solution of all the difficulties connected with isolation of stations.
In the next place, the question of localising the source of the
signals and waves is most important. Our kumascopes and receiving
appliances at present are like the rudimentary eyes of the lower
organisms, which are probably sensitive to mere differences in light
and darkness, but which are not able to _see_ or _visualise_, in the
sense of locating the direction and distance of a radiating or
luminous body. Just as we have, as little children, to learn to see,
so a similar process has to be accomplished in connection with
Hertzian telegraphy, and the accomplishment of this does not seem by
any means impossible or even distant. We are dealing with
hemispherical waves of electric and magnetic force, which are sent out
from a certain radiating centre, and in order to localise that centre
we have to determine the position of the plane of the wave and also
the curvature of the surface at the receiving point. Something,
therefore, equivalent to a range finder in connection with light is
necessary to enable us to locate the distance and the direction of the
radiant point.
Lastly, there are important improvements possible in connection with
the generation of the waves themselves. At the present moment, our
mode of generating Hertzian waves involves a dissipation of energy in
the form of the light and heat of the spark. Just as in the case of
ordinary artificial illuminants, such as lamps of various kinds, we
have to manufacture a large amount of ether radiation of long wave
length, which is of no use to us for visual purposes--in fact,
creating ninety-five per cent, of dark and useless waves for every
five per cent. of luminous or useful waves--so in connection with
present methods of generating Hertzian waves, we are bound to
manufacture by the discharge spark a large amount of light and heat
rays which are not wanted, in order to create the Hertzian waves we
desire. It is impossible yet to state precisely what is the
efficiency, in the ordinary sense of the word, of a Hertzian wave
radiator; how much of the energy imparted to the aerial falls back
upon it and contributes to the production of the spark, and how much
is discharged into the ether in the form of a wave.
Nothing is more remarkable, however, than the small amount of energy
which, if properly utilised in electric wave making, will suffice to
influence a sensitive receiver at a distance of even one or two
hundred miles. Suppose, for instance, that we charge a condenser
consisting of a battery of Leyden jars, having a capacity of one
seventy-fifth of a microfarad, to a potential of 15,000 volts; the
energy stored up in this condenser is then equal to 1.5 joules, or a
little more than one foot-pound. If this energy is discharged in the
form of a spark five millimetres in length through the primary coil of
an oscillation transformer, associated with an aerial 150 feet in
height, the circuits being properly tuned by Mr. Marconi's method,
then such an aerial will affect, as he has shown, one of Mr. Marconi's
receivers, including a nickel silver filings coherer tube, at a
distance of over two hundred miles over sea. Consider what this means.
The energy stored up in the Leyden jars cannot all be radiated as wave
energy by the aerial, probably only half of it is thus radiated. Hence
the impartation to the ether at any one locality of about half a
foot-pound of energy in the form of a long Hertzian wave is sufficient
to affect sensitive receivers situated at any point on the
circumference of a circle of 200 miles radius described on the open
sea. Hertzian wave telegraphy is sometimes described as being
extravagant in power, but, as a matter of fact, the most remarkable
thing about it is the small amount of power really involved in
conducting it. On the other hand, Hertzian wave manufacture is not
altogether a matter of power. It is much more dependent upon the
manner in which the ether is struck. Just as half an ounce of dynamite
in exploding may make more noise than a ton of gunpowder, because it
hits the air more suddenly, so the formation of an effective wave in
the ether is better achieved by the right application of a small
energy than by the wrong mode of application of a much larger amount.
If we translate this fact into the language of electronic theory, it
amounts simply to this. It is the electron alone which has a grip of
the ether. To create an ether wave, we have to start or stop crowds of
electrons very suddenly. If in motion, their motion implies energy,
but it is not only their energy which is concerned in the wave making,
but the acceleration, positive or negative--_i.e._, the quickness with
which they are started or stopped. It is possible we may discover in
time a way of manufacturing long ether waves without the use of an
electric spark, but at present we know only one way of doing
this--viz., by the discharge of a condenser, and in the discharge of
large condensers of very high potentials it is difficult to secure
that extreme suddenness of starting the discharge which we can do in
the case of smaller capacities and voltages.
How strange it is that the discharge of a Leyden jar studied so
profoundly by Franklin, Henry, Faraday, Maxwell, Kelvin and Lodge
should have become an electrical engineering appliance of great
importance!
Whilst there are many matters connected with the commercial aspect of
Hertzian wave telegraphy with which we are not here concerned, there
is one on which a word may properly be said. The ability to
communicate over long distances by Hertzian waves is now demonstrated
beyond question, and even if all difficulties are not overcome at
once, it has a field of very practical utility, and may even become of
national importance. Under these circumstances, we may consider
whether it is absolutely necessary to place the signalling stations so
near the coast. The greater facility of transmission over sea has
already been discussed and explained, but in time of war, the masts
and towers which are essential at present in connection with
transmitting stations could be wrecked by shot or shell from an
enemy's battleship at a distance of five or six miles out at sea, and
would certainly be done within territorial waters. Should not this
question receive attention in choosing the location of important
signalling stations? For if they can, without prejudice to their use,
be placed inland by a distance sufficient to conceal them from sight,
their value as a national asset in time of war might be greatly
increased.
It has been often contended that whilst cables could be cut in time of
war no one can cut the ether; but wireless telegraph stations in
exposed situations on high promontories, where they are visible for
ten to fifteen miles out at sea and undefended by any forts, could
easily be destroyed. The great towers which are essential to carry
large aerials are a conspicuous object for ten miles out at sea; and a
single well-placed shell from a six-inch gun would wreck the place and
put the station completely out of use for many months. Hence if
oceanic telegraphy is ever to be conducted in a manner in which the
communication will be inviolable or, at any rate, not be capable of
interruption by acts of war, the careful selection of the sites for
stations is a matter of importance. A small station consisting of a
single 150-foot mast and a wooden hut can easily be removed or
replaced, but an expensive power station, the mere aerial of which may
cost several thousand pounds, is not to be put up in a short time.[76]
Meanwhile, whatever may be the future achievements of this new
_supermarine_ wireless telegraphy conducted over long distances, there
can be no question as to its enormous utility and present value for
intercommunication between ships on the ocean and ships and the shore.
At the present time, there are some forty or more of the transatlantic
ocean liners and many other ships equipped with this Hertzian wave
wireless telegraph apparatus on the Marconi system. Provided with this
latest weapon of applied science, they are able to chat with one
another, though a hundred miles apart on the ocean, with the ease of
guests round a dinner table, to exchange news or make demands for
assistance.
Ships that pass in the night, and speak each other in passing--
Only a signal shown, and a distant voice in the darkness;
So, on the ocean of life, we pass and speak one another,
Only a look and a voice, then darkness again, and a silence.
Abundant experience has been gathered to show the inexpressible value
of this means of communication in case of accident, and it can hardly
be doubted that before long the possession of this apparatus on board
every passenger vessel will be demanded by the public, even if not
made compulsory. Although the privacy of an ocean voyage may have been
somewhat diminished by this utilisation of ether waves, there is a
vast compensation in the security that is thereby gained to human life
and property by this latest application of the great energies of
nature for the use and benefit of mankind.