When certain crystals, e.g., tourmaline, are heated uniformly, one end becomes positively charged and the opposite end negatively charged. When cooled, the charges are reversed. This phenomenon is called ‘pyro-electricity’. If a crystal be broken up, each part of it exhibits the same properties. If the tourmaline be powdered and spread on a glass plate and warmed or cooled, the particles gather themselves in chains owing to polar charges.

Boracite, quartz and fluorspar are among the pyro-electric minerals
Riecke has shown that the electrification increases with increase of temperature. The emf is given by
        E = at + bt₂
Here, t is the change of temperature. a and b are constants.


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