Crystals

A crystal is a material that is composed of particles arranged in a structured pattern.  When energy is applied to a crystal, it causes the crystal to resonate at a specific frequency called the resonant frequency.  The resonant frequency is determined by the type of crystal used, the way it is cut, and the energy applied.  An electrical oscillator can be made from a crystal through the piezoelectric effect, which is the relationship between mechanical stress and electrical potential.  Applying an electrical pulse to a crystal causes electrical stress that induces a brief vibration.  Sustained resonance can be obtained by amplifying the resulting electrical signal and feeding it back into the crystal.  This process results in an electrical oscillator that is extremely stable and temperature independent.