The receiving instruments form the most interesting and ingenious part
of a wireless station. They are the ears of the wireless station. They
are wondrously sensitive but yet simple and incapable of much
complication. The receiving station forms an exact counterpart of the
transmitter, and the train of actions taking place are the reverse of
those of the latter. The purpose of the transmitter is to change
ordinary electric currents into electrical oscillations and thus set up
electric waves, while the receptor converts the waves into oscillations
and thence into currents which are capable of manifesting themselves in
a telephone receiver. The instruments necessary for receiving comprise a
Detector
Telephone Receivers
Fixed Condenser
Tuning Device
[Illustration: FIG. 60.—Portable receiving set and case.]
Other instruments such as a potentiometer, test buzzers, variometers,
variable condensers, etc., complete the outfit and improve its
selectivity and sensitiveness.
[Illustration: FIG. 61.—Complete receiving outfit.]
The detector forms the most vital part of the receptor. In explaining
its action it may be well to recall and enlarge upon the description
already set forth on page 11, where it was explained that
electromagnetic or as they are more commonly called when identified with
wireless telegraphy, Hertzian waves have the power of exciting
oscillations in any conductor upon which they impinge. Electrical
oscillations, it will be remembered, are alternating currents of very
high frequency. They are generated in the aerial of the receiving
station by the action of the waves coming from the distant transmitting
station. These currents are exceedingly feeble, too feeble in fact to
operate any form of electrical apparatus except a telephone receiver,
which is one of the most sensitive instruments in existence.
[Illustration: FIG. 62.—Portable pack set. The receiving outfit is
contained in the left hand case; also the key and interrupter. The
tubular condenser, spark gap, and induction coil may be seen in the
right hand case.]
[Illustration: FIG. 63.—Complete receiving set, consisting of two
"Perikon" detectors, potentiometer, loose coupler, variable condenser,
etc.]
There are probably more different forms of detector than any other piece
of radiotelegraph apparatus. Those in most common use to-day are the
mineral detectors. A small crystal of certain minerals, iron pyrites,
silicon, galena, etc., is placed between two contact points which are
adjustable so that the pressure may be regulated and the most sensitive
portion of the mineral selected. A telephone receiver is shunted across
the terminals of the detector.
[Illustration: FIG. 64.—Showing construction of a "watch case"
telephone receiver.]
A telephone is shown in diagram in Fig. 64. It consists of a U shaped
permanent magnet of bar steel, so mounted as to exert a polarizing
influence upon a pair of little electromagnets, before the poles of
which an iron diaphragm is mounted. For convenience these elements are
assembled within a small cylindrical casing usually of hard rubber. The
permanent magnet exerts a continual pull upon the diaphragm tending to
distort it, concave inwards. When alternating currents are sent through
the receiver coils, that part of the alternation which is flowing in the
proper direction to form a magnetic field flowing in the same direction
as that of the permanent magnet will strengthen the latter and assist it
in attracting the diaphragm and causing it to further approach the
magnet. That portion of the current flowing in the opposite direction
detracts from the magnetic pull and allows the diaphragm to recede from
the magnet. The diaphragm thus takes up a vibrating motion corresponding
to the electrical waves supplied to the coil and it imparts motion to
the surrounding air, the result being sound.
[Illustration: FIG. 65.—Pickard adjustable telephone receivers for
wireless purposes.]
It might reasonably be asked why a telephone receiver could not be
directly connected to the aerial and ground so that it would respond
directly to the high frequency currents generated by the incoming waves
without the medium of a detector. There are two very good reasons why
such a method would not be possible, the first being that the little
magnet coils contained in the telephone receivers exert a choking action
upon alternating currents of high frequency which effectually blocks
their passage. Low frequency alternating currents, intermittent direct
currents and continuous direct currents will readily pass, producing a
sound-each time there is any change in their value. The purpose and
action of most types of detectors is to act as a valve allowing the
current to pass through in one direction but not permitting it to pass
in an opposite one. The high frequency oscillating currents may be
represented by a curved line crossing and recrossing a zero line and
gradually decreasing in amplitude as shown by A in Fig. 66.
[Illustration: FIG. 66.—Illustrating the valve action of a rectifying
detector.]
[Illustration: FIG. 67.—A new type of silicon detector in which a
crystal of arsenic may be brought to bear against the surface of one
of several silicon crystals.]
The detector, acting as a valve, eliminates one half of the alternating
current so that the result may be represented by B, in reality a
pulsating direct current which rises and falls but is able to flow
through the telephone receiver and produce a motion of the diaphragm
with consequent sound waves audible to the ear.
[Illustration: FIG. 68.—Diagram drawing analogy between rectifying
action of a detector and a pump.]
The accompanying sketches and the following analogy drawn between the
electric currents and the flow of a stream of water may serve to render
a better conception of how it is possible for the valve action of a
detector to rectify an alternating flow, continuously reversing its
direction to an intermittent current passing in one direction only. The
illustration shows two pumps A and B. Each pump is immersed in a pool of
water and consists of a cylindrical tube T and T' having a small
opening, O and O', at the lower end to admit the water and a piston, P
and P', operating up and down inside the tube. Every time that the
piston P is raised in the pump A it will draw in water through the small
hole O. As soon as it descends, however, the water will reverse its
direction and pass out. The action of the water represents that of an
alternating current because it passes in first one direction and then in
the other. The pump B is fitted with a valve whose action is to permit
the water to flow in one direction only. The valve is fitted to the
piston P'. It is a little flap which opens a hole in the piston when the
latter is descending and closes when it is rising. Suppose that the
piston is raised. Water will be drawn in through the little hole O'. As
soon as the piston reaches the limit of the stroke it commences to
descend. In falling it exerts a slight pressure on the valve which opens
and allows the water to pass through. The hole in the piston is larger
than the hole in the pump and so there is almost none of the water
forced back into the pool. The next up stroke of the piston draws more
water in, that which is on top flowing out through the overflow. The
nature of the stream passing through the hole O' is intermittent,
passing principally in one direction. It may be likened to the
intermittent direct current produced by the detector.
[Illustration: FIG. 69.—Pyron detector in which a fine wire is brought
to bear against a crystal of iron pyrites.]
Some of the many forms of detectors are interesting because of the
ingenious manner in which equivalent results are attained. The
illustration shows a type of detector known as the "Perikon." Two
minerals, zincite (oxide of zinc) and chalcopyrites (copper-iron
sulphide), are mounted in adjustable cups so arranged that the surfaces
of the minerals can be brought into variable contact with one another.
[Illustration: FIG. 70.—Perikon detector.]
Another very good rectifying detector is that consisting of a flat
surface of highly polished silicon mounted in a small cup. A flat brass
point mounted on the end of an adjustable thumbscrew is brought to bear
on the silicon.
[Illustration: FIG. 71.—Silicon detector.]
Other mineral detectors of value are the Pyron, molybenite and galena.
[Illustration: FIG. 72.—Electrolytic detector.]
The carborundum detector is a form of crystal rectifier consisting of a
fragment of carborundum held between two carbon blocks.
The electrolytic detector consists of a very fine platinum wire
(.001-.0003 of an inch in diameter) dipping into a small cup of dilute
nitric acid. A large platinum electrode is sealed in the bottom of the
cup so as to make an electrical connection with the liquid. This form of
detector is exceedingly sensitive, probably more so than any other. The
electrolytic detector requires a battery. When a slight current passes
through the circuit, very minute bubbles are formed at the wire,
insulating it from the liquid and thus shutting off the battery current
from the telephone receivers. However, upon the arrival of any electric
waves and consequent high frequency oscillations the latter destroy the
bubbles clustering around the little wire and permit the current to
flow. Upon the cessation of the high frequency currents the bubbles
immediately form again, only to become broken down by each train of
oscillations produced in the aerial. The intermittent currents can be
detected by a buzz in the telephone receivers.
[Illustration: FIG. 73.—Electrolytic detector in circuit.]
The carborundum detector also requires a battery although its action is
somewhat different from that just described.
[Illustration: FIG. 74.—Potentiometer.]
When a battery is used in connection with a detector, an instrument
known as a potentiometer becomes necessary. A potentiometer is simply a
device for accurately adjusting the voltage of a battery to a value
where it will render the detector the most responsive to the incoming
signals.
[Illustration: FIG. 75.—Diagram showing how potentiometer is connected
in circuit.]
The Tuning Coil is a device for accurately adjusting the oscillation
circuits to receive the waves.
Its action may be illustrated to a certain extent by pressing down the
loud pedal of a piano and at the same time whistling a note loudly and
clearly. Listen carefully and some of the wires in the piano will be
heard sounding the note whistled. At each vibration of the note of the
whistle a wave of pressure went forth from the lips and reaching the
wires gave them all a tiny impulse. The impulses followed each other
rapidly at definite intervals giving each of the wires the same push
each time. The wires which are tuned to produce the note on the piano
corresponding to that of the whistle will vibrate energetically enough
to produce a sound themselves. They are the wires to which the impulses
are rightly tuned so that each one adds to the motion it has already
acquired. We all know how a child sitting in a swing may be made to
swing back and forth by giving a succession of little impulses properly
timed. The small pushes are superimposed on one another, the result
being a single large motion.
[Illustration: FIG. 76.—Analogy between swinging and tuning.]
The "impulses" generated in the receiving aerial are exceedingly weak
and in order to produce an effect must be timed so as to follow one
another in proper succession. Tuning devices are for this purpose and by
their means the receiving circuits and instruments may be carefully
adjusted to the same wave length or "note" as the transmitter so that
the high frequency currents in the aerial will arrive at the proper time
to oscillate or surge back and forth to produce the maximum results.
In this way it is possible to convey intelligence over long distances by
the repetition of small impulses without it being necessary to send any
very energetic ones. By arranging the stations so that each one emits
its own definite wave different in period or length from that of the
others it is possible to operate several stations at the same time in
the same neighborhood without interfering with one another. The
apparatus is then said to be selective because the instruments can be
adjusted in a few seconds to receive from any desired station and to
exclude others.
[Illustration: FIG. 77.—Receiving a message in a Marconi transatlantic
station.]
The tuning coil consists of a cylinder wound with bare copper wire
spaced so that the turns do not touch one another. Variable contacts
called "sliders" are so arranged that connection can be made almost
instantly to any desirable turn of wire. The tuning coil is connected to
the aerial and receiving apparatus in the manner illustrated in Fig. 79.
By moving the sliders back and forth the wave length of the system may
be added to or detracted from and any desired "tune" quickly reached so
that it is possible to listen to any station desirable and exclude the
others. The cylinder over which the wire is wound usually consists of a
thick cardboard tube treated so as to be moisture proof. Bare wire is
preferable to all forms of insulated wire. The coil is usually three to
four inches in diameter and eight to twelve inches long.
[Illustration: FIG. 78.—Tuning coil of the double slide type.]
[Illustration: FIG. 79.—Diagram showing fixed condenser in circuit.]
Tuning coils are known as "single slide," "double slide" and "three
slide" according to the number of contacts they are fitted with.
[Illustration: FIG. 80.—Fixed condenser.]
The loading coil is a supplementary tuning coil used to furnish extra
inductance in case it is desirable to obtain a greater range of
resonance or tuning.
It is merely a single slide tuning coil connected in series with the
regular tuning device. It is not always a necessity but is often part of
the equipment when it may be necessary to adjust the apparatus to
receive long wave lengths.
Condensers are devices for collecting and storing electricity. They play
a very important part in both the transmitting and receiving operations.
Condensers and Leyden jars have already been described in connection
with the transmitting apparatus.
The condensers used in receiving are very much smaller in size and
capacity but are the same in principle. There are two general types of
receiving condensers called "fixed" and "variable" accordingly as the
capacity is alterable or not.
Fixed condensers consist of a few sheets of tinfoil interposed between
sheets of paraffined paper or in some cases mica. The condenser is
inclosed in a suitable case, usually a hollow molded block of insulating
composition, and is provided with suitable terminals to facilitate
connection.
[Illustration: FIG. 81.—Rotary variable condenser.]
When a conductor is charged with electricity it has the power of
exerting an opposite charge in any adjacent conductors. The two halves
of a condenser constitute adjacent conductors, the separating medium in
between being called the dielectric. An alternating current will pass
through a condenser because the charge on the plates keeps changing from
negative to positive and back from positive to negative again. A direct
current will not pass through a condenser.
These facts are utilized to considerable advantage in the receptor of a
wireless station. As has already been explained, the high frequency
oscillatory currents will not readily pass through the coils of the
telephone receivers, but a path is provided through the condenser. The
detector rectifies the alternating current into a direct current which
the condenser opposes and forces to pass through the telephone receiver
and produce sounds.
When a battery is used in connection with a detector a condenser is also
necessary to oppose the direct current of the battery and prevent it
from flowing around through the tuning coil instead of through the
detector. The capacity of the condenser may be smaller if the resistance
of the telephone receiver is very great for the reason that as the wire
grows smaller it offers greater impedance to the current. The opposite
also holds true and condensers of large capacity are better fitted for
use with telephone receivers of low resistance.
[Illustration: FIG. 82.—Interior of rotary variable condenser showing
construction.]
Variable condensers are divided into two general types, the "rotary" and
the "sliding" plate, accordingly as the plates forming the condenser are
adjusted with a rotary or a sliding motion. The rotary type consists of
a number of movable semi-circular aluminum plates which swing between a
series of fixed semi-circular plates of a slightly larger diameter. The
plates must not touch one another and move back and forth with perfect
freedom. The dielectric is formed by the air spacing between the plates.
[Illustration: FIG. 83.—Dr. Seibt's rotary variable condenser. The plates
are turned from a solid casting and the separation between is only .01
inch.]
The advantage of an air dielectric is that no losses of energy take
place through hysterisis. Hysterisis is the lagging which takes place in
the process of charging and discharging. A thumb knob is fitted to the
movable plates and provided with a pointer moving over a graduated scale
so that the degree of capacity in use is indicated.
[Illustration: FIG. 84.—Sliding plate variable condenser.]
In the sliding plate type of variable condenser the plates are either
square or rectangular in shape and move back and forth in grooves cut in
a hardwood frame as shown in the illustration.
Variable condensers are used for tuning and adjusting the receiving
circuit in the same way that a tuning coil is employed, namely to
increase or decrease the electrical length of the circuit so that it
will respond to different wave lengths. The condensers are capable of
finer adjustment than tuning coils because the change is gradual and
even and is not in jumps from one step to another as from one turn to
the next turn of the coil. If the desired point of resonance should
happen to come between two wires of the coil and not in a position to be
reached by the slider, the variable condenser can be adjusted to reach
the exact degree of resonance and thus bring the circuit into finer
adjustment than would otherwise be possible. The exact way in which this
is accomplished and the effect upon the circuit will be left to the next
chapter.
[Illustration: FIG. 85.—Diagram showing arrangement of rotary variable
condenser in receiving circuit.]