The principal instruments composing the apparatus used for sending the
wireless messages comprise an induction coil, or in its place a
transformer, a key, a spark gap, a condenser, and a helix.
The current supply available will determine the type of the instruments,
and whether an induction coil or a transformer is used. Unless current
mains for light and power are already installed, it must be generated by
an engine and dynamo, or recourse had to batteries. Induction coils may
be operated on either direct or alternating current. Dry cells are most
commonly employed to furnish the current for small induction coils, but
a storage or some form of renewable primary cell, such as the Fuller and
Edison, is necessary if the coil is a large one. When dry cells are
used, they should be connected in series multiple, as shown in the
accompanying diagram. This method of connecting distributes the load,
and considerably lengthens the life of the battery.
[Illustration: FIG. 28.—Diagram showing how batteries may be arranged
in "series" or "series multiple."]
[Illustration: FIG. 29.—The power plant of a Marconi transatlantic
station, showing engine and generator.]
When the source of current supply is alternating, an induction coil may
be operated as a transformer. Both induction coils and transformers are
instruments for raising the voltage of the ordinary available current
from a comparatively low value, 6-220 volts, to a quantity
(15,000–20,000 volts), where it can properly charge the aerial and
create a state of strain, or, as it is called in technical parlance, an
electro-static field.
[Illustration: FIG. 30.—If a magnet is suddenly plunged into a hollow
coil of wire a momentary electric current will be induced in the coil.]
Both the induction coil and transformer depend for their operation upon
the principles of magnetic induction. In 1831, Michael Faraday, a famous
English chemist and physicist, discovered that if a magnet be suddenly
plunged into a hollow coil of wire, that a momentary current of
electricity is generated in the coil. As long as the magnet remains
motionless, it induces no current in the coil, but when it is moved back
and forth, it sets up the currents. The source of electrical energy is
the mechanical work done in moving the magnet. The medium which changes
the mechanical energy into electricity is called the magnetic field. The
magnetic field is a peculiar state or condition of the space in the
immediate neighborhood of a magnet. Its real nature is very hard to
explain and not easily understood. Suffice it to say, however, that the
current is induced in the coil of wire only when the magnetic field is
changing, either decreasing or increasing. The change is produced by
moving the magnet because its influence on the coil will be great or
small accordingly as it is near or far.
[Illustration: FIG. 31.—Magnetic phantom formed by bar magnet.]
It is possible to show the existence of the magnetic field by placing a
sheet of glass over a bar magnet and then sprinkling iron filings on the
glass. They will settle down in curving lines as in Fig. 31, forming a
magnetic phantom. The curved lines formed by the filings represent the
direction of the lines of force which make up the magnetic field.
If we should examine the space in the immediate neighborhood of a coil
of wire carrying a current of electricity it would be found that a
similar state of affairs existed there and that the coil also possessed
a magnetic field composed of lines of force flowing around it.
This is readily shown by punching a small hole in a piece of cardboard
and passing a wire carrying a current of electricity through the hole.
If iron filings are sprinkled on the cardboard, they will arrange
themselves in circles around the wires, forming a magnetic phantom and
showing that a coil of wire carrying a current of electricity generates
a magnetic field in its vicinity. By forming the wire into a coil the
magnetic field generated is much stronger, for the then combined effect
of the wires is secured.
[Illustration: FIG. 32.—Magnetic phantom formed by wire carrying
current.]
[Illustration: FIG. 33.—Magnetic phantom formed by coil of wire carrying
current.]
The induction coil and transformer are simply instruments utilizing the
principle that a coil of wire carrying a current possesses a magnetic
field which will induce a current of electricity in another neighboring
coil.
[Illustration: FIG. 34.—Diagram of an induction coil.]
The induction coil consists essentially of a primary winding of heavy
wire wound around a soft iron core and surrounded by a secondary coil
consisting of many thousand turns of fine wire, carefully insulated. The
current from a battery is sent through the primary coil and sets up a
magnetic field. The magnetic field induces a current in the secondary
whose voltage is approximately proportional to the ratio of the turns of
the secondary to the primary. Thus, if the secondary contains one
hundred times as many turns of wire as the primary the induced voltage
will be one hundred times the voltage of the original primary current.
The purpose of the iron core is to concentrate the magnetic field and
make the coil more efficient. Since currents are only induced in the
secondary when the magnetic field is changing, an automatic device
called an interrupter or sometimes a vibrator, is employed to rapidly
turn the current flowing through the primary on and off. The interrupter
consists of a spring carrying a platinum point against which presses a
second piece of platinum on the end of an adjustable thumbscrew.
Platinum is necessary because the current of electricity would quickly
oxidize and burn up any other material. The interrupter spring is placed
near the end of the core so that the magnetism of the latter will draw
it forward away from the thumbscrew and interrupt the current. As soon
as the current ceases to flow the core loses its magnetism and the
spring returns to its former position repeating the cycle very rapidly a
large number of times per second. The interrupter is fitted with a
condenser shunted across its terminals to stop sparking at the platinum
points and also to make the currents in the secondary more intense.
[Illustration: FIG. 35.—Induction coil for wireless telegraph purposes.]
[Illustration: FIG. 36.—Induction coil, primary and secondary.]
The voltage of the currents in the secondary is high enough to leap
across an air gap in a torrent of sparks. The spark of an induction coil
intended for wireless work should be thick and heavy. It should be
sufficiently hot and flaming to ignite a piece of paper. A rapid
vibrator giving a high pitched spark is better than a slow one not only
because it causes a more intense and powerful spark but because the
human ear is the most sensitive to high pitched sounds and such a spark
is more easily read at the receiving station.
[Illustration: FIG. 37.—Interrupter for induction coil.]
[Illustration: FIG. 38.—Electrolytic interrupter.]
When the coil is a very large one and operated on the 110 volt current
an electrolytic interrupter is substituted for the mechanical type. One
pole of the current is connected to a lead plate placed in a jar
containing a mixture of sulphuric acid and water. The other side of the
current is connected to a platinum wire placed in a porcelain tube so
that only a small part of the lower end is in contact with the solution.
When the current passes a bubble forms at the end of the wire shielding
it from the liquid, and thus interrupting the current. The bubble is
almost immediately discharged however and the current allowed to flow an
instant before a new one forms. This operation is repeated continuously
at a frequency sometimes as high as a thousand per second. An
electrolytic interrupter is both an expensive and a troublesome device.
There are other types of interrupters of value in wireless service but
the limitations of space prohibit any account.
[Illustration: FIG. 39.—Open and closed core transformers.]
The transformer is acknowledged to be the best practice as a means of
stepping up the voltage of a circuit for wireless telegraph purposes.
Alternating current is necessary to operate a transformer. There are two
distinct types of transformers known as the "open" and "closed core"
accordingly as the shape of the latter is straight like that of an
induction coil or in the form of a hollow rectangle. The closed core
transformer consists of two coils of insulated wire, forming a primary
and a secondary, wound upon a rectangular core like that shown in Fig.
39B. The core is built up of sheets of iron called laminations, to
reduce the heating and increase the efficiency of the machine.
[Illustration: FIG. 40.—Lines representing direct and intermittent
direct currents.]
As noted above currents are only induced in a coil when the magnetic
field is changing. The interrupter is employed to rapidly "make" and
"break" the circuit. Every time that the circuit is made the primary
coil creates a field and every time it is broken it is destroyed. A
direct current is a current which passes in one direction only. It may
be represented by a straight line as A in Fig. 40. Its voltage is
usually very constant and does not vary greatly. In the case of electric
lighting circuits the normal voltage is usually 110. If an interrupter
is included in the circuit the current may be represented by a broken
line, the spaces corresponding to the periods when the current is
"broken" and the lines to the periods it is flowing. The interrupter
creates an intermittent direct current.
[Illustration: FIG. 41.—Diagram representing alternating current.]
An alternating current is one which reverses its direction and passes
first one way and then the other. It may be represented by the curved
line shown in Fig. 41. It starts at zero and rises to a maximum,
gradually dying away to zero, then passes in the opposite direction,
rising to a maximum and dying away again. This is repeated a definite
number of times per second; when the current rises from zero, reverses
and returns to zero it is said to have passed through a cycle. From a to
c represents a cycle—from a to b is an alternation. The usual frequency
of commercial alternating currents is 60 cycles or 7200 alternations per
minute.
[Illustration: FIG. 42.—High potential "Humming" transformer.]
[Illustration: FIG. 43.—High potential closed core transformer
for wireless work.]
[Illustration: FIG. 44.—Leyden jar set for oil immersion to prevent
losses from brush discharges.]
From these facts we may readily see why the troublesome interrupter may
be eliminated when alternating current is used. Every time that the
current rises and falls the magnetic field changes.
Considerable care must be used in proportioning the windings so that
they possess sufficient reactance. Reactance is the tendency of a coil
to resist the flow of an alternating current. A reactance coil is
sometimes placed in circuit with an open core transformer to prevent the
spark from arcing. Arcing is the tendency of the spark to pass across
the gap without charging the condenser and creating any high frequency
oscillations.
[Illustration: FIG. 45.—Oil immersed condenser]
The condenser, it will be remembered is the means of storing up the
energy, which suddenly rushing across the spark gap, produces the
oscillations necessary to generate the electric waves. A battery of
leyden jars may be used as a transmitting condenser in connection with
small induction coils. Their objection in large stations is that they
are very cumbersome and some energy is lost by the brush discharges
around the tops of the jars. The usual form of condenser consists of
alternate sheets of tinfoil and glass plates arranged in a pile. The
alternate sheets of tinfoil are connected together to form the terminals
of the instrument. The condenser is usually encased in a wooden box
poured full of wax or oil to increase the insulation and efficiency.
Condensers are arranged in units so that any desired capacity may be
readily secured by adding the proper number of units. The capacity of a
condenser is its relative ability to receive and retain an electrical
charge.
[Illustration: FIG. 46.—Diagram showing construction of condenser.]
The helix is an instrument consisting of copper or brass wire wound
around a frame of hard rubber or seasoned wood. A certain amount of
inductance is necessary in a wireless telegraph circuit in order to
develop high frequency oscillations. Inductance is the property of an
electric circuit by virtue of which lines of force are developed around
it. The helix furnishes the inductance in the circuit or at least the
greater part. Connections are established to the turns of the helix by
means of clips which snap on and off the wires.
[Illustration: FIG. 47.—Tubular condenser.]
[Illustration: FIG. 48.—Helix.]
[Illustration: FIG. 49.—Close coupled helix.]
[Illustration: FIG. 50.—Spark gap.]
[Illustration: FIG. 51.—Circuit showing tuned transmitting system
employing close coupled helix.]
The spark gap is the medium for discharging the aerial and condenser and
setting up the oscillations. It usually consists of a pair of electrodes
supported by suitable standards and so arranged that the distance
between the electrodes can be accurately adjusted. The electrodes
usually take the form of hollow faced cylindrical rods having flanges to
radiate the heat generated and prevent the spark from arcing. Various
metals are used for spark gaps. Silver is probably the best but its
expense is prohibitive. A special hard zinc alloy is most generally
used.
[Illustration: FIG. 52.—Photo of spark gap.]
[Illustration: FIG. 53.—Quenched spark gap.]
[Illustration: FIG. 54.—Diagram of aerial switch.]
[Illustration: FIG. 55.—Photo of aerial switch.]
Spark gaps take other forms, two of which are interesting and important
enough to describe here. The first is the rotary gap.
This consists of a number of small electrodes set around the periphery
of a wheel mounted upon the shaft of an electric motor. Two other
adjustable electrodes are so mounted that the small electrodes on the
revolving member pass between. When the motor is set in operation the
wheel revolves at a high rate of speed interrupting the spark and
causing a peculiar musical pitch to be emitted. A rotary spark gap
almost entirely eliminates the arcing of the spark.
The quenched gap consists of a number of disks of brass about five
inches in diameter having thin mica washers set between and arranged in
a pile as in the illustration. The quenched gap radiates considerably
more energy than any other form of gap and also has the advantage of
being practically noiseless. The crashing discharge of an ordinary gap
produces a very disagreeable penetrating noise hard to eliminate. In
most commercial stations the spark is muffled to a certain extent by
enclosing it in a cylinder of micanite or some other insulating
substance.
[Illustration: FIG. 56.—Anchor gap.]
The aerial switch is necessary for quickly connecting the aerial and
ground to either the transmitting or receiving apparatus. Amateurs very
often employ a small "double pole double throw" switch. The switch used
in commercial stations is built in the manner shown in Fig. 55.
[Illustration: FIG. 57.—Wireless key.]
An anchor gap is a simple little device consisting of a hard rubber ring
bearing two or three small electrodes or sparking points. It is a
necessary part of the transmitting apparatus wherever a loop aerial is
used. One electrode is connected to the transmitting apparatus and the
other two to the opposite sides of the aerial so that the currents
divide between the two halves and equalize.
[Illustration: FIG. 58.—Photo of wireless key.]
The key is a hand operated switch which controls the electric currents
passing through the transformer or coil shutting them on or off at will
and so controlling the electric oscillations in the antenna to send out
short or long trains of ether waves in accordance with the dot or dash
signals of the Morse alphabet.
[Illustration: FIG. 59.—Key and aerial switch.]
The key used in a wireless station is necessarily much larger and
heavier than those employed in ordinary Morse line work, in order to
carry the heavy currents used by the transmitter. In spite of their size
and weight, however, such keys when properly designed may be handled
with perfect ease.