PYROELECTRICITY AND PIEZOELECTRICITY 149 



Pyroelectricity : Historical notes. It was discovered 

 about 1700 that a tourmaline crystal placed in hot ashes attracted 

 the ashes. In 1756 Jpinus showed that the effect was electrical 

 and that the charges were opposite in sign at the two ends of the 

 crystal. Soon after, Canton* showed that the charges were equal 

 as well as opposite. He connected an insulated tin cup filled with 

 boiling water to a pith-ball electroscope. A tourmaline crystal 

 was dropped into the water and the pith ball showed no trace of 

 electrification, either then or during the subsequent cooling of the 

 water. Canton found also that the development of electrification 

 depended only on change of temperature from the neutral condition 

 and not on the absolute temperature. For if the crystal was in a 

 neutral condition at any temperature it showed polarity of one 

 kind if raised above that temperature, and polarity of the opposite 

 kind if cooled below that temperature. He broke a tourmaline 

 prism into three pieces and found that each piece exhibited the 

 same kind of action along the same axis as the whole crystal. He 

 found that Brazilian topaz acted like tourmaline, and his con- 

 temporary Watson discovered other pyroelectric gems. 



Haiiy about 1800 discovered that only crystals hemihedral with I 

 inclined faces developed pyroelectricity, and that the end forming 

 the most acute angles with the axis was positively electrified on.' 

 cooling. Other crystals may show electrification when not heatedj 

 uniformly, but this is due to local strains set up by the non-uniform 

 heating and a consequent local alteration, as it were, of thq 

 crystalline form. 



Analogous and antilogous poles. Riess termed the 

 pole on which + electricity appears on heating the analogous pole, 

 and the other the antilogous pole. 



A crystal may have as many as four pyroelectric axes. Quartz 

 has three axes, bisecting the three angles of 120 between the faces 

 of the prism. The axis of the prism is not a pyroelectric axis. 



Some methods of investigating pyroelectricity. The 

 earliest method consisted in merely heating a crystal and then 

 bringing one part of its surface near an electroscope. Hankel 

 heated the crystal in copper filings, passed it through a spirit 

 flame by which it was entirely reduced to the neutral condition, and 

 then allowed it to cool, when its electrification could be tested by 

 an electroscope. 



Kundt t heated the crystal in an air bath and after neutralisation 

 allowed it to cool. He then blew on to it, through muslin, a 

 powder consisting of a mixture of red lead and sulphur. The red 

 lead was positively electrified by the friction with the muslin and 

 the sulphur was negatively electrified. The positive region of the 

 crystal attracted the sulphur and the negative the red lead, and 

 the two regions became differently coloured. If a tourmaline prism 



'^Priestley on Electricity, 4th. ed. p. 298. 

 t Wied. Ann., 20 (1883), p. 692. 



