WIRELESS TELEGRAPHY.
The transmitting apparatus--The receiving apparatus--Syntonic
transmission--The advance of wireless telegraphy.
In our last chapter we reviewed briefly some systems of sending
telegraphic messages from one point of the earth's surface to another
through a circuit consisting partly of an insulated wire and partly of
the earth itself. The metallic portion of a long circuit, especially if
it be a submarine cable, is costly to install, so that in quite the
early days of telegraphy efforts were made to use the ether in the place
of wire as one conductor.
When a hammer strikes an anvil the air around is violently disturbed.
This disturbance spreads through the molecules of the air in much the
same way as ripples spread from the splash of a stone thrown into a
pond. When the sound waves reach the ear they agitate the tympanum, or
drum membrane, and we "hear a noise." The hammer is here the
transmitter, the air the conductor, the ear the receiver.
In wireless telegraphy we use the ether as the conductor of electrical
disturbances.[13] Marconi, Slaby, Branly, Lodge, De Forest, Popoff, and
others have invented apparatus for causing disturbances of the requisite
kind, and for detecting their presence.
The main features of a wireless telegraphy outfit are shown in Figs. 59
and 61.
THE TRANSMITTER APPARATUS.
We will first consider the transmitting outfit (Fig. 59). It includes a
battery, dispatching key, and an induction coil having its secondary
circuit terminals connected with two wires, the one leading to an
earth-plate, the other carried aloft on poles or suspended from a kite.
In the large station at Poldhu, Cornwall, for transatlantic signalling,
there are special wooden towers 215 feet high, between which the aërial
wires hang. At their upper and lower ends respectively the earth and
aërial wires terminate in brass balls separated by a gap. When the
operator depresses the key the induction coil charges these balls and
the wires attached thereto with high-tension electricity. As soon as the
quantity collected exceeds the resistance of the air-gap, a discharge
takes place between the balls, and the ether round the aërial wire is
violently disturbed, and waves of electrical energy are propagated
through it. The rapidity with which the discharges follow one another,
and their travelling power, depends on the strength of the induction
coil, the length of the air-gap, and the capacity of the wires.[14]
[Illustration: FIG. 59.--Sketch of the transmitter of a wireless
telegraphy outfit.]
[Illustration: FIG. 60.--A Marconi coherer.]
RECEIVING APPARATUS.
The human body is quite insensitive to these etheric waves. We cannot
feel, hear, or see them. But at the receiving station there is what may
be called an "electric eye." Technically it is named a _coherer_. A
Marconi coherer is seen in Fig. 60. Inside a small glass tube exhausted
of air are two silver plugs, P P, carrying terminals, T T, projecting
through the glass at both ends. A small gap separates the plugs at the
centre, and this gap is partly filled with nickel-silver powder. If the
terminals of the coherer are attached to those of a battery, practically
no current will pass under ordinary conditions, as the particles of
nickel-silver touch each other very lightly and make a "bad contact."
But if the coherer is also attached to wires leading into the earth and
air, and ether waves strike those wires, at every impact the particles
will cohere--that is, pack tightly together--and allow battery current
to pass. The property of cohesion of small conductive bodies when
influenced by Hertzian waves was first noticed in 1874 by Professor D.E.
Hughes while experimenting with a telephone.
[Illustration: FIG. 61.--Sketch of the receiving apparatus in a
wireless telegraphy outfit.]
We are now in a position to examine the apparatus of which a coherer
forms part (Fig. 61). First, we notice the aërial and earth wires, to
which are attached other wires from battery A. This battery circuit
passes round the relay magnet R and through two choking coils, whose
function is to prevent the Hertzian waves entering the battery. The
relay, when energized, brings contact D against E and closes the circuit
of battery B, which is much more powerful than battery A, and operates
the magnet M as well as the _tapper_, which is practically an electric
bell minus the gong. (The tapper circuit is indicated by the dotted
lines.)
We will suppose the transmitter of a distant station to be at work. The
electric waves strike the aërial wire of the receiving station, and
cause the coherer to cohere and pass current. The relay is closed, and
both tapper and Morse inker begin to work. The tapper keeps striking the
coherer and shakes the particles loose after every cohesion. If this
were not done the current of A would pass continuously after cohesion
had once taken place. When the key of the transmitter is pressed down,
the waves follow one another very quickly, and the acquired conductivity
of the coherer is only momentarily destroyed by the tap of the hammer.
During the impression of a dot by the Morse inker, contact is made and
broken repeatedly; but as the armature of the inker is heavy and slow to
move it does not vibrate in time with the relay and tapper. Therefore
the Morse instrument reproduces in dots and dashes the short and long
depressions of the key at the transmitting station, while the tapper
works rapidly in time with the relay. The Morse inker is shown
diagrammatically. While current passes through M the armature is pulled
towards it, the end P, carrying an inked wheel, rises, and a mark is
made on the tape W, which is moved continuously being drawn forward off
reel R by the clockwork--or electrically-driven rollers R^1 R^2.
SYNTONIC TRANSMISSION.
If a number of transmitting stations are sending out messages
simultaneously, a jumble of signals would affect all the receivers
round, unless some method were employed for rendering a receiver
sensitive only to the waves intended to influence it. Also, if
distinction were impossible, even with one transmitter in action its
message might go to undesired stations.
There are various ways of "tuning" receivers and transmitters, but the
principle underlying them all is analogous to that of mechanical
vibration. If a weight is suspended from the end of a spiral spring, and
given an upward blow, it bobs up and down a certain number of times per
minute, every movement from start to finish having exactly the same
duration as the rest. The resistance of the air and the internal
friction of the spring gradually lessen the amplitude of the movements,
and the weight finally comes to rest. Suppose that the weight scales 30
lbs., and that it naturally bobs twenty times a minute. If you now take
a feather and give it a push every three seconds you can coax it into
vigorous motion, assuming that every push catches it exactly on the
rebound. The same effect would be produced more slowly if 6 or 9 second
intervals were substituted. But if you strike it at 4, 5, or 7 second
intervals it will gradually cease to oscillate, as the effect of one
blow neutralizes that of another. The same phenomenon is witnessed when
two tuning-forks of equal pitch are mounted near one another, and one is
struck. The other soon picks up the note. But a fork of unequal pitch
would remain dumb.
Now, every electrical circuit has a "natural period of oscillation" in
which its electric charge vibrates. It is found possible to "tune," or
"syntonize," the aërial rod or wire of a receiving station with a
transmitter. A vertical wire about 200 feet in length, says Professor
J.A. Fleming,[15] has a natural time period of electrical oscillation of
about one-millionth of a second. Therefore if waves strike this wire a
million times a second they will reinforce one another and influence the
coherer; whereas a less or greater frequency will leave it practically
unaffected. By adjusting the receiving circuit to the transmitter, or
_vice versâ_, selective wireless telegraphy becomes possible.
ADVANCE OF WIRELESS TELEGRAPHY.
The history of wireless telegraphy may be summed up as follows:--
1842.--Professor Morse sent aërial messages across the Susquehanna
River. A line containing a battery and transmitter was carried on posts
along one bank and "earthed" in the river at each end. On the other bank
was a second wire attached to a receiver and similarly earthed. Whenever
contact was made and broken on the battery side, the receiver on the
other was affected. Distance about 1 mile.
1859.--James Bowman Lindsay transmitted messages across the Tay at
Glencarse in a somewhat similar way. Distance about 1/2 mile.
1885.--Sir William Preece signalled from Lavernock Point, near Cardiff,
to Steep Holm, an island in the Bristol Channel. Distance about 5-1/2
miles.
In all these electrical _induction_ of current was employed.
1886.--Hertzian waves discovered.
1895.--Professor A. Popoff sent Hertzian wave messages over a distance
of 3 miles.
1897.--Marconi signalled from the Needles Hotel, Isle of Wight, to
Swanage; 17-1/2 miles.
1901.--Messages sent at sea for 380 miles.
1901, Dec. 17.--Messages transmitted from Poldhu, Cornwall, to Hospital
Point, Newfoundland; 2,099 miles.
Mr. Marconi has so perfected tuning devices that his transatlantic
messages do not affect receivers placed on board ships crossing the
ocean, unless they are purposely tuned. Atlantic liners now publish
daily small newspapers containing the latest news, flashed through space
from land stations. In the United States the De Forest and Fessenden
systems are being rapidly extended to embrace the most out-of-the-way
districts. Every navy of importance has adopted wireless telegraphy,
which, as was proved during the Russo-Japanese War, can be of the
greatest help in directing operations.
[13] Named after their first discoverer, Dr. Hertz of Carlsruhe,
"Hertzian waves."
[14] For long-distance transmission powerful dynamos take the place of
the induction coil and battery.
[15] "Technics," vol. ii. p. 566.