..............................................................................RESONANCE................................................................................
Resonance occurs everwhere in nature, a typical example example being a fluttering flag or perhaps a violin string when it is plucked and was perhaps more dramatically uillustrated when a suspension bridge in America was destroyed by its effects
A simple example is the pendulum on a clock, ............It swings back and forth with considerable momentum ( especially if the clock is large ) and it is sustained with only a very feeble push from the mechanism,......... the tiny push is magnified by the effect of resonance.
The number of times the pendulum swings back and fore each minute is determined by the length of the bar that connects the clock mechanism to the weight not by the size of the push given by the mechanism.
So how does a pendulum work ?....... well it is a matter of energy conversion,........ at the bottom of its stroke the pendulum is moving at maximum speed but has lost its potenetial energy ( it can go no further down ) and at the top of its stroke it has only potential energy and no movement ( it stops for an instant before reversing direction.
So it is a cycle of continous energy conversions (oscillations )......potential to kinetic, ...kinetic to potential, potential to kinetic, and so on,........ in fact if there were no loses ( air resistance being the main one here ) it would swing back and fore for ever once started without any push from the clock............the push from the clock only serves to make up for the losses and does not contribute to the oscillation once it has built up
So what has this to do with the crystal set ? ...............well the effect of resonance is used to magnify the wanted signal and reject others.
Here the energy conversions ( oscilations ) are not kinetic and potential as in the clock but are capacitive energy and inductive energy ( properly termed electrostatic and electromagnetic ),............A capacitor and an inductor can both store energy when they are connected together but due to the different propreties of their reactances not at the same time
If a charged capacitor ( electrostatic energy )is connected to an inductor it will immediately start to discharge into the inductor ( just like the pendul;um at the top of its swing starting on the way down ) slowly at first as the inductor will oppose any change in voltage across it but a point will be reached where the current flowing will build up to a maximum ( like the pendulum at the bottom of its stroke ) and then the current will slowly decrease as the capacitor becomes discharged and will finally stop ( like the pendulum at its other peak ) the inductor now holding all the energy ( electromagnetic energy )
Just like the pendulum these energy conversions would continue forever once started if there were no losses, here the main loss being the resistance of the inductors wire,.........so it can be seen that the feeble pushes from the antenna are magnjfied by the resonant circuit of the capacitor and inductor.
Just like the pendulum the size of the push is not important its the timing of it, ........or frequency,.......and the frequency that the tuned circuit will respond to is determined by the size of the capacitor and inductor,...........in the crystal set the frequency is selected by the capacitor being made variable although it could be done by varying the inductor as old fashioned car radios used to do