HOW THE TRANSMITTING AND RECEIVING SETS WORK
The easiest way to get a clear conception of how a wireless
transmitter sends out electric waves and how a wireless receptor
receives them is to take each one separately and follow: (1) in the
case of the transmitter, the transformation of the low voltage direct,
or alternating current into high potential alternating currents; then
find out how these charge the condenser, how this is discharged by the
spark gap and sets up high-frequency currents in the oscillation
circuits; then (2) in the case of the receptor, to follow the high
frequency currents that are set up in the aerial wire and learn how
they are transformed into oscillations of lower potential when they
have a larger current strength, how these are converted into
intermittent direct currents by the detector and which then flow into
and operate the telephone receiver.
How Transmitting Set No. 1 Works. The Battery and Spark Coil
Circuit.--When you press down on the knob of the key the silver points
of it make contact and this closes the circuit; the low voltage direct
current from the battery now flows through the primary coil of the
spark coil and this magnetizes the soft iron core. The instant it
becomes magnetic it pulls the spring of the vibrator over to it and
this breaks the circuit; when this takes place the current stops
flowing through the primary coil; this causes the core to lose its
magnetism when the vibrator spring flies back and again makes contact
with the adjusting screw; then the cycle of operations is repeated.
A condenser is connected across the contact points of the vibrator
since this gives a much higher voltage at the ends of the secondary
coil than where the coil is used without it; this is because: (1) the
self-induction of the primary coil makes the pressure of the current
rise and when the contact points close the circuit again it discharges
through the primary coil, and (2) when the break takes place the
current flows into the condenser instead of arcing across the contact
points.
Changing the Primary Spark Coil Current Into Secondary Currents.--Now
every time the vibrator contact points close the primary circuit the
electric current in the primary coil is changed into closed magnetic
lines of force and as these cut through the secondary coil they set up
in it a _momentary current_ in one direction. Then the instant the
vibrator points break apart the primary circuit is opened and the
closed magnetic lines of force contract and as they do so they cut the
turns of wire in the secondary coil in the opposite direction and this
sets up another momentary current in the secondary coil in the other
direction. The result is that the low voltage direct current of the
battery is changed into alternating currents whose frequency is
precisely that of the spring vibrator, but while the frequency of the
currents is low their potential, or voltage, is enormously increased.
What Ratio of Transformation Means.--To make a spark coil step up the
low voltage direct current into high potential alternating current the
primary coil is wound with a couple of layers of thick insulated
copper wire and the secondary is wound with a thousand, more or less,
number of turns with very fine insulated copper wire. If the primary
and secondary coils were wound with the same number of turns of wire
then the pressure, or voltage, of the secondary coil at its terminals
would be the same as that of the current which flowed through the
primary coil. Under these conditions the _ratio of transformation_, as
it is called, would be unity.
The ratio of transformation is directly proportional to the number of
turns of wire on the primary and secondary coils and, since this is
the case, if you wind 10 turns of wire on the primary coil and 1,000
turns of wire on the secondary coil then you will get 100 times as
high a pressure, or voltage, at the terminals of the secondary as that
which you caused to flow through the primary coil, but, naturally, the
current strength, or amperage, will be proportionately decreased.
The Secondary Spark Coil Circuit.--This includes the secondary coil
and the spark gap which are connected together. When the alternating,
but high potential, currents which are developed by the secondary
coil, reach the balls, or _electrodes_, of the spark gap the latter
are alternately charged positively and negatively.
Now take a given instant when one electrode is charged positively and
the other one is charged negatively, then when they are charged to a
high enough potential the electric strain breaks down the air gap
between them and the two charges rush together as described in the
chapter before this one in connection with the discharge of a
condenser. When the charges rush together they form a current which
burns out the air in the gap and this gives rise to the spark, and as
the heated gap between the two electrodes is a very good conductor the
electric current surges forth and back with high frequency, perhaps a
dozen times, before the air replaces that which has burned out. It is
the inrushing air to fill the vacuum of the gap that makes the
crackling noise which accompanies the discharge of the electric spark.
In this way then electric oscillations of the order of a million, more
or less, are produced and if an aerial and a ground wire are connected
to the spark balls, or electrodes, the oscillations will surge up and
down it and the energy of these in turn, are changed into electric
waves which travel out into space. An open circuit transmitter of this
kind will send out waves that are four times as long as the aerial
itself, but as the waves it sends out are strongly damped the
Government will not permit it to be used.
The Closed Oscillation Circuit.--By using a closed oscillation circuit
the transmitter can be tuned to send out waves of a given length and
while the waves are not so strongly damped more current can be sent
into the aerial wire system. The closed oscillation circuit consists
of: (1) a _spark gap_, (2) a _condenser_ and (3) an _oscillation
transformer_. The high potential alternating current delivered by the
secondary coil not only charges the spark gap electrodes which
necessarily have a very small capacitance, but it charges the
condenser which has a large capacitance and the value of which can be
changed at will.
Now when the condenser is fully charged it discharges through the
spark gap and then the electric oscillations set up surge to and fro
through the closed circuit. As a closed circuit is a very poor
radiator of energy, that is, the electric oscillations are not freely
converted into electric waves by it, they surge up to, and through the
aerial wire; now as the aerial wire is a good radiator nearly all of
the energy of the electric oscillations which surge through it are
converted into electric waves.
How Transmitting Set No. 2 Works. With Alternating Current. The
operation of a transmitting set that uses an alternating current
transformer, or _power transformer,_ as it is sometimes called, is
even more simple than one using a spark coil. The transformer needs no
vibrator when used with alternating current. The current from a
generator flows through the primary coil of the transformer and the
alternations of the usual lighting current is 60 cycles per second.
This current sets up an alternating magnetic field in the core of the
transformer and as these magnetic lines of force expand and contract
they set up alternating currents of the same frequency but of much
higher voltage at the terminals of the secondary coil according to the
ratio of the primary and secondary turns of wire as explained under
the sub-caption of _Ratio of Transformation_.
With Direct Current.--When a 110 volt direct current is used to
energize the power transformer an _electrolytic_ interruptor is needed
to make and break the primary circuit, just as a vibrator is needed
for the same purpose with a spark coil. When the electrodes are
connected in series with the primary coil of a transformer and a
source of direct current having a potential of 40 to 110 volts,
bubbles of gas are formed on the end of the platinum, or alloy anode,
which prevent the current from flowing until the bubbles break and
then the current flows again, in this way the current is rapidly made
and broken and the break is very sharp.
Where this type of interrupter is employed the condenser that is
usually shunted around the break is not necessary as the interrupter
itself has a certain inherent capacitance, due to electrolytic action,
and which is called its _electrolytic capacitance_, and this is large
enough to balance the self-induction of the circuit since the greater
the number of breaks per minute the smaller the capacitance required.
The Rotary Spark Gap.--In this type of spark gap the two fixed
electrodes are connected with the terminals of the secondary coil of
the power transformer and also with the condenser and primary of the
oscillation transformer. Now whenever any pair of electrodes on the
rotating disk are in a line with the pair of fixed electrodes a spark
will take place, hence the pitch of the note depends on the speed of
the motor driving the disk. This kind of a rotary spark-gap is called
_non-synchronous_ and it is generally used where a 60 cycle
alternating current is available but it will work with other higher
frequencies.
The Quenched Spark Gap.--If you strike a piano string a single quick
blow it will continue to vibrate according to its natural period. This
is very much the way in which a quenched spark gap sets up
oscillations in a coupled closed and open circuit. The oscillations
set up in the primary circuit by a quenched spark make only three or
four sharp swings and in so doing transfer all of their energy over to
the secondary circuit, where it will oscillate some fifty times or
more before it is damped out, because the high frequency currents are
not forced, but simply oscillate to the natural frequency of the
circuit. For this reason the radiated waves approach somewhat the
condition of continuous waves, and so sharper tuning is possible.
The Oscillation Transformer.--In this set the condenser in the closed
circuit is charged and discharged and sets up oscillations that surge
through the closed circuit as in _Set No. 1_. In this set, however, an
oscillation transformer is used and as the primary coil of it is
included in the closed circuit the oscillations set up in it produce
strong oscillating magnetic lines of force. The magnetic field thus
produced sets up in turn electric oscillations in the secondary coil
of the oscillation transformer and these surge through the aerial wire
system where their energy is radiated in the form of electric waves.
The great advantage of using an oscillation transformer instead of a
simple inductance coil is that the capacitance of the closed circuit
can be very much larger than that of the aerial wire system. This
permits more energy to be stored up by the condenser and this is
impressed on the aerial when it is radiated as electric waves.
How Receiving Set No. I Works.--When the electric waves from a distant
sending station impinge on the wire of a receiving aerial their energy
is changed into electric oscillations that are of exactly the same
frequency (assuming the receptor is tuned to the transmitter) but
whose current strength (amperage) and potential (voltage) are very
small. These electric waves surge through the closed circuit but when
they reach the crystal detector the contact of the metal point on the
crystal permits more current to flow through it in one direction than
it will allow to pass in the other direction. For this reason a
crystal detector is sometimes called a _rectifier_, which it really
is.
Thus the high frequency currents which the steel magnet cores of the
telephone receiver would choke off are changed by the detector into
intermittent direct currents which can flow through the magnet coils
of the telephone receiver. Since the telephone receiver chokes off the
oscillations, a small condenser can be shunted around it so that a
complete closed oscillation circuit is formed and this gives better
results.
When the intermittent rectified current flows through the coils of the
telephone receiver it energizes the magnet as long as it lasts, when
it is de-energized; this causes the soft iron disk, or _diaphragm_ as
it is called, which sets close to the ends of the poles of the magnet,
to vibrate; and this in turn gives forth sounds such as dots and
dashes, speech or music, according to the nature of the electric waves
that sent them out at the distant station.
How Receiving Set No. 2 Works.--When the electric oscillations that
are set up by the incoming electric waves on the aerial wire surge
through the primary coil of the oscillation transformer they produce a
magnetic field and as the lines of force of the latter cut the
secondary coil, oscillations of the same frequency are set up in it.
The potential (voltage) of these oscillations are, however, _stepped
down_ in the secondary coil and, hence, their current strength
(amperes) is increased.
The oscillations then flow through the closed circuit where they are
rectified by the crystal detector and transformed into sound waves by
the telephone receiver as described in connection with _Set No. 1_.
The variable condenser shunted across the closed circuit permits finer
secondary tuning to be done than is possible without it. Where you
are receiving continuous waves from a wireless telephone transmitter
(speech or music) you have to tune sharper than is possible with the
tuning coil alone and to do this a variable condenser connected in
parallel with the secondary coil is necessary.