Becquerel, Antoine Cesar, beh KREHL or behk uh REHL, on TWAHN say ZAHR (1788-1878), was a French physicist who performed pioneering experiments in electricity and electrochemistry. His many inventions include the first constant-current electric pile, the electromagnetic balance, and the thermoelectric needle, which measures internal body temperature. He discovered that the energy developed by contracting muscle tissue is transformed into heat.