MECHANICAL AND ELECTRICAL TUNING
There is a strikingly close resemblance between _sound waves_ and the
way they are set up in _the air_ by a mechanically vibrating body,
such as a steel spring or a tuning fork, and _electric waves_ and the
way they are set up in _the ether_ by a current oscillating in a
circuit. As it is easy to grasp the way that sound waves are produced
and behave something will be told about them in this chapter and also
an explanation of how electric waves are produced and behave and thus
you will be able to get a clear understanding of them and of tuning in
general.
Damped and Sustained Mechanical Vibrations.--If you will place one end
of a flat steel spring in a vice and screw it up tight as shown at A
in Fig. 34, and then pull the free end over and let it go it will
vibrate to and fro with decreasing amplitude until it comes to rest as
shown at B. When you pull the spring over you store up energy in it
and when you let it go the stored up energy is changed into energy of
motion and the spring moves forth and back, or _vibrates_ as we call
it, until all of its stored up energy is spent.
[Illustration: Fig. 34.--Damped and Sustained Mechanical Vibrations.]
If it were not for the air surrounding it and other frictional losses,
the spring would vibrate for a very long time as the stored up energy
and the energy of motion would practically offset each other and so
the energy would not be used up. But as the spring beats the air the
latter is sent out in impulses and the conversion of the vibrations of
the spring into waves in the air soon uses up the energy you have
imparted to it and it comes to rest.
In order to send out _continuous waves_ in the air instead of _damped
waves_ as with a flat steel spring you can use an _electric driven
tuning fork_, see C, in which an electromagnet is fixed on the inside
of the prongs and when this is energized by a battery current the
vibrations of the prongs of the fork are kept going, or are
_sustained_, as shown in the diagram at D.
Damped and Sustained Electric Oscillations.--The vibrating steel
spring described above is a very good analogue of the way that damped
electric oscillations which surge in a circuit set up and send out
periodic electric waves in the ether while the electric driven tuning
fork just described is likewise a good analogue of how sustained
oscillations surge in a circuit and set up and send out continuous
electric waves in the ether as the following shows.
Now the inductance and resistance of a circuit such as is shown at A
in Fig. 35, slows down, and finally damps out entirely, the electric
oscillations of the high frequency currents, see B, where these are
set up by the periodic discharge of a condenser, precisely as the
vibrations of the spring are damped out by the friction of the air and
other resistances that act upon it. As the electric oscillations surge
to and fro in the circuit it is opposed by the action of the ether
which surrounds it and electric waves are set up in and sent out
through it and this transformation soon uses up the energy of the
current that flows in the circuit.
[Illustration: Fig. 35.--Damped and Sustained Electric Oscillations.]
To send out _continuous waves_ in the ether such as are needed for
wireless telephony instead of _damped waves_ which are, at the present
writing, generally used for wireless telegraphy, an _electric
oscillation arc_ or a _vacuum tube oscillator_ must be used, see C,
instead of a spark gap. Where a spark gap is used the condenser in the
circuit is charged periodically and with considerable lapses of time
between each of the charging processes, when, of course, the condenser
discharges periodically and with the same time element between them.
Where an oscillation arc or a vacuum tube is used the condenser is
charged as rapidly as it is discharged and the result is the
oscillations are sustained as shown at D.
About Mechanical Tuning.--A tuning fork is better than a spring or a
straight steel bar for setting up mechanical vibrations. As a matter
of fact a tuning fork is simply a steel bar bent in the middle so that
the two ends are parallel. A handle is attached to middle point of the
fork so that it can be held easily and which also allows it to vibrate
freely, when the ends of the prongs alternately approach and recede
from one another. When the prongs vibrate the handle vibrates up and
down in unison with it, and imparts its motion to the _sounding box_,
or _resonance case_ as it is sometimes called, where one is used.
If, now, you will mount the fork on a sounding box which is tuned so
that it will be in resonance with the vibrations of the fork there
will be a direct reinforcement of the vibrations when the note emitted
by it will be augmented in strength and quality. This is called
_simple resonance_. Further, if you mount a pair of forks, each on a
separate sounding box, and have the forks of the same size, tone and
pitch, and the boxes synchronized, that is, tuned to the same
frequency of vibration, then set the two boxes a foot or so apart, as
shown at A in Fig. 36, when you strike one of the forks with a rubber
hammer it will vibrate with a definite frequency and, hence, send out
sound waves of a given length. When the latter strike the second fork
the impact of the molecules of air of which the sound waves are formed
will set its prongs to vibrating and it will, in turn, emit sound
waves of the same length and this is called _sympathetic resonance_,
or as we would say in wireless the forks are _in tune_.
[Illustration: Fig. 36.--Sound Wave and Electric Wave Tuned Senders
and Receptors. A - variable tuning forks for showing sound wave
tuning. B - variable oscillation circuits for showing electric wave
tuning.]
Tuning forks are made with adjustable weights on their prongs and by
fixing these to different parts of them the frequency with which the
forks vibrate can be changed since the frequency varies inversely with
the square of the length and directly with the thickness [Footnote:
This law is for forks having a rectangular cross-section. Those having
a round cross-section vary as the radius.] of the prongs. Now by
adjusting one of the forks so that it vibrates at a frequency of, say,
16 per second and adjusting the other fork so that it vibrates at a
frequency of, say, 18 or 20 per second, then the forks will not be in
tune with each other and, hence, if you strike one of them the other
will not respond. But if you make the forks vibrate at the same
frequency, say 16, 20 or 24 per second, when you strike one of them
the other will vibrate in unison with it.
About Electric Tuning.--Electric resonance and electric tuning are
very like those of acoustic resonance and acoustic tuning which I have
just described. Just as acoustic resonance may be simple or
sympathetic so electric resonance may be simple or sympathetic. Simple
acoustic resonance is the direct reinforcement of a simple vibration
and this condition is had when a tuning fork is mounted on a sounding
box. In simple electric resonance an oscillating current of a given
frequency flowing in a circuit having the proper inductance and
capacitance may increase the voltage until it is several times greater
than its normal value. Tuning the receptor circuits to the transmitter
circuits are examples of sympathetic electric resonance. As a
demonstration if you have two Leyden jars (capacitance) connected in
circuit with two loops of wire (inductance) whose inductance can be
varied as shown at B in Fig. 36, when you make a spark pass between
the knobs of one of them by means of a spark coil then a spark will
pass in the gap of the other one provided the inductance of the two
loops of wire is the same. But if you vary the inductance of the one
loop so that it is larger or smaller than that of the other loop no
spark will take place in the second circuit.
When a tuning fork is made to vibrate it sends out waves in the air,
or sound waves, in all directions and just so when high frequency
currents surge in an oscillation circuit they send out waves in the
ether, or electric waves, that travel in all directions. For this
reason electric waves from a transmitting station cannot be sent to
one particular station, though they do go further in one direction
than in another, according to the way your aerial wire points.
Since the electric waves travel out in all directions any receiving
set properly tuned to the wave length of the sending station will
receive the waves and the only limit on your ability to receive from
high-power stations throughout the world depends entirely on the wave
length and sensitivity of your receiving set. As for tuning, just as
changing the length and the thickness of the prongs of a tuning fork
varies the frequency with which it vibrates and, hence, the length of
the waves it sends out, so, too, by varying the capacitance of the
condenser and the inductance of the tuning coil of the transmitter the
frequency of the electric oscillations set up in the circuit may be
changed and, consequently, the length of the electric waves they send
out. Likewise, by varying the capacitance and the inductance of the
receptor the circuits can be tuned to receive incoming electric waves
of whatever length within the limitation of the apparatus.