CHAPTER 11
RESONANCE
YOU KNEW THE SPEED BY THE RATTLES

Ever sit at the wheel of your old Model A and say "Now we're doing 40-I can tell by the rattles"? If you changed your speed to 50 miles per hour, the rattles and vibrations also changed. You associated the various speeds of your automobile with the different rattles and vibrations that were heard.

What caused those rattles? Why did certain rattles show up at one speed and not at others?

Well, you know that all engines-even fine ones-vibrate. At different speeds, the frequencies of vibration are different. Every fender, bumper, and bolt in your automobile has a NATURAL FREQUENCY of vibration. If the engine is running at the correct speed to GENERATE the frequency of the right front fender, that fender will vibrate. If the engine is running at a speed that will produce the frequency of the rear bumper, that bumper will vibrate. And so on throughout the entire automobile.

SPECIAL RESONATORS

You can reinforce the sound of a musical note by using special resonators. Resonators on xylophones are round metal tubes; the violin has a wooden box; and the piano has a flat sounding board. These resonators are scientifically constructed to reinforce the vibrations coming from a weak source.

On a pipe organ or flute, the resonator is also the source of vibration. By changing the length of pipe in the organ or flute, the frequency (pitch) of the notes can be changed.

ELECTRICAL RESONATORS

You will run into special resonators in electricity, too. They will not be pipes or boxes, but just COILS and CONDENSERS in what are commonly called L-C circuits, or "TANK CIRCUITS."

Both types of connections are used in radio circuits. In some places they are used as a SOURCE of vibrations. In others, they are a part of a circuit used to REINFORCE other vibrations.

An L-C circuit that acts as a SOURCE of vibration is called an OSCILLATOR. The circuits used to reinforce the vibrations are called AMPLIFIERS.

WHAT HAPPENS IN AN L-C CIRCUIT

With musical resonators you may observe what is happening because you can actually FEEL and HEAR the vibrations. But in electrical resonators, you cannot observe the action as easily, since it is an ALTERNATING CURRENT that is GENERATED or REINFORCED.

Both mechanical and electrical resonators have several characteristics in common. Of these, here is the most important-neither MECHANICAL nor ELECTRICAL RESONATORS will RESPOND unless the CORRECT FREQUENCY Of VIBRATION is PRESENT. The frequency at which a RESONATOR will RESPOND is the RESONANT FREQUENCY of the object.

That is why your model A would rattle one way at 40.1 miles per hour, another at 42.3, and still another at 58.

Electrical resonators will not respond unless the source of power is delivering an a.c. of the RESONANT FREQUENCY. You learned in the chapter on coils that when a.c. is applied to a coil, the magnetic field is continually EXPANDING and COLLAPSING.

You also learned in the chapter on condensers, that when an a.c. is applied to a condenser, it will be continually CHARGING and DISCHARGING.

Now put the coil and condenser in an L-C circuit. When an a.c. of a RESONANT FREQUENCY is delivered to an L-C circuit, like the one in figure 97, the CHARGE and DISCHARGE of the condenser is IN HARMONY with the MAKE and COLLAPSE of the coil's magnetic field.

The circulation of electrons in the tank circuit is much like the swing of a pendulum. As long as the proper amount of energy is supplied to overcome the losses due to friction, the swinging back and forth will continue.

In a pendulum, you INCREASE the RATE of motion by DECREASING the length of the bar. Increasing the length of the bar DECREASES the rate of swing.

In a tank circuit, you INCREASE the RESONANT FREQUENCY by REDUCING the ELECTRICAL LENGTH of the L-C circuit. And you DECREASE the electrical length by REDUCING the INDUCTANCE of the coil and the CAPACITY of the condenser, or BOTH.

This is important

If you wish to DECREASE the resonant frequency you can do so by INCREASING the CAPACITY or INDUCTANCE.

MAKING USE OF RESONANCE

Each time you tune your receiver you are either increasing or decreasing the resonant frequency of the receiver's tank circuit.

Suppose your receiver is tuned to 4,740 kc., and you want to listen to a station on a frequency of 3,880 kc. The receiver's resonant frequency is too high-4,740 kc.-so you twist a knob until the receiver's resonant frequency is 3,880 kc. And in comes in your station.

If you wish to bring in a station of any new frequency, you simply adjust your receiver so that its resonant frequency is the SAME as that of the station you want to hear.

A VARIABLE CONDENSER DOES IT

Most receivers are tuned by adjusting a VARIABLE condenser, or several condensers ganged together on the same shaft. CLOSING the condensers-increasing the mesh-REDUCES the frequency. Opening the condenser tunes the receiver to a higher frequency.

SWITCHING COILS CHANGES BANDS

As you know, most Navy receivers are made to tune over several bands of frequencies. As an example of this the RAL receiver which tunes from 0.3 to 23 mc. in nine bands. To change from one band to another, you rotate a switch. Each time you turn this switch, a DIFFERENT set of COILS are connected into the circuit.

The coils used to tune the receiver to the LOWEST band have the GREATEST number of turns. For each successive higher frequency band, the coils have fewer and fewer turns of wire. Thus the coils used with the HIGHEST frequency band have the LEAST number of turns.

When you rotate the switch to change bands you don't always connect in other TUNING condensers. Usually the same variable condenser is used to tune the LOWEST and HIGHEST bands.

WHERE ELSE ARE RESONANT CIRCUITS USED?

Transmitters also contain many resonant circuits. Most Navy transmitters use a tank circuit to GENERATE an a.c. of radio frequencies. This part of a receiver is the OSCILLATOR. You change the frequency of the oscillator by changing either the capacity of the condensers or the inductance of the coils.

In addition to the transmitter's OSCILLATOR, other tank circuits are used to STRENGTHEN, or AMPLIFY, the oscillator's feeble a.c. You will hear more about this later in this manual.