The Radio Amateur's Hand Book / A Complete, Authentic and Informative Work on Wireless Telegraphy and Telephony

by A. Frederick Collins

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.

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.

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.