Crystalline materials produce small amounts of electricity when a force is applied that changes their shape in some way. These are called piezoelectric materials. Quartz is an example of a piezoelectric substance. When small amounts of pressure are applied to a quartz crystal, a small voltage is produced from the changing charge created by the moving electrons. Phonographs using a crystal cartridge utilize the piezoelectric principle to convert the movement of the needle to an electrical signal which is later amplified and played through speakers. Microphones and barbecue lighters also use this principle.