This propelled a number of developing applications on piezoelectricity and pyroelectricity. In particular he had more experience than his brother in the study of pyroelectricity. Two effects which are closely related to pyroelectricity are ferroelectricity and piezoelectricity. A variety of effects can be harnessed to produce thermoelectricity, thermionic emission, pyroelectricity and piezoelectricity. Research into pyroelectricity became more sophisticated in the 19th century. The process of pyroelectricity has been known from ancient times. However, note that some piezoelectric materials have a crystal symmetry that does not allow pyroelectricity. Pyroelectricity is also a necessary consequence of ferroelectricity.