WIRELESS TELEGRAPHY
TELEGRAPHING WITHOUT WIRES.--Wireless telegraphy is an outgrowth of the
ordinary telegraph system. When Maxwell, and, later on, Hertz,
discovered that electricity, magnetism, and light were transmitted
through the ether, and that they differed only in their wave lengths,
they laid the foundations for wireless telegraphy. Ether is a substance
which is millions and millions of times lighter than air, and it
pervades all space. It is so unstable that it is constantly in motion,
and this phase led some one to suggest that if a proper electrical
apparatus could be made, the ether would thereby be disturbed
sufficiently so that its impulses would extend out a distance
proportioned to the intensity of the electrical agitation thereby
created.
SURGING CHARACTER OF HIGH-TENSION CURRENTS.--When a current of
electricity is sent through a wire, hundreds of miles in length, the
current surges back and forth on the wire many thousands of times a
second. Light comes to us from the sun, over 90,000,000 of miles,
through the ether. It is as reasonable to suppose, or infer, that the
ether can, therefore, convey an electrical impulse as readily as does a
wire.
It is on this principle that impulses are sent for thousands of miles,
and no doubt they extend even farther, if the proper mechanism could be
devised to detect movement of the waves so propagated.
THE COHERER.--The instrument for detecting these impulses, or
disturbances, in the ether is generally called a _coherer_, although
detector is the term which is most satisfactory. The name coherer comes
from the first practical instrument made for this purpose.
[Illustration: _Fig. 75._ WIRELESS TELEGRAPHY COHERER]
HOW MADE.--The coherer is simply a tube, say, of glass, within which is
placed iron filings. When the oscillations surge through the secondary
coil the pressure or potentiality of the current finally causes it to
leap across the small space separating the filings and, as it were, it
welds together their edges so that a current freely passes. The
bringing together of the particles, under these conditions, is called
cohering.
Fig. 75 shows the simplest form of coherer. The posts (A) are firmly
affixed to the base (B), each post having an adjusting screw (C) in its
upper end, and these screw downwardly against and serve to bind a pair
of horizontal rods (D), the inner ends of which closely approach each
other. These may be adjusted so as to be as near together or as far
apart as desired. E is a glass tube in which the ends of the rods (D)
rest, and between the separated ends of the rods (D) the iron filings
(F) are placed.
THE DECOHERERS.--For the purpose of causing the metal filings to fall
apart, or decohere, the tube is tapped lightly, and this is done by a
little object like the clapper of an electric bell.
In practice, the coils and the parts directly connected with it are put
together on one base.
THE SENDING APPARATUS.--Fig. 76 shows a section of a coil with its
connection in the sending station. The spark gap rods (A) may be swung
so as to bring them closer together or farther apart, but they must not
at any time contact with each other.
The induction coil has one terminal of the primary coil connected up by
a wire (B) with one post of a telegraph key, and the other post of the
key has a wire connection (C), with one side of a storage battery. The
other side of the battery has a wire (D) running to the other terminal
of the primary.
[Illustration: _Fig. 76._ WIRELESS SENDING APPARATUS]
The secondary coil has one of its terminals connected with a binding
post (E). This binding post has an adjustable rod with a knob (F) on its
end, and the other binding post (G), which is connected up with the
other terminal of the secondary coil, carries a similar adjusting rod
with a knob (H).
From the post (E) is a wire (I), which extends upwardly, and is called
the aerial wire, or wire for the antennæ, and this wire also connects
with one side of the condenser by a conductor (J). The ground wire (K)
connects with the other binding post (G), and a branch wire (L) also
connects the ground wire (K) with one end of the condenser.
[Illustration: _Fig. 77._ WIRELESS RECEIVING APPARATUS]
THE RECEIVING APPARATUS.--The receiving station, on the other hand, has
neither condenser, induction coil, nor key. When the apparatus is in
operation, the coherer switch is closed, and the instant a current
passes through the coherer and operates the telegraph sounder, the
galvanometer indicates the current.
Of course, when the coherer switch is closed, the battery operates the
decoherer.
HOW THE CIRCUITS ARE FORMED.--By referring again to Fig. 76, it will be
seen that when the key is depressed, a circuit is formed from the
battery through wire B to the primary coil, and back again to the
battery through wire D. The secondary coil is thereby energized, and,
when the full potential is reached, the current leaps across the gap
formed between the two knobs (F, H), thereby setting up a disturbance in
the ether which is transmitted through space in all directions.
It is this impulse, or disturbance, which is received by the coherer at
the receiving station, and which is indicated by the telegraph sounder.