Greenleaf W. Pickard was famous for discovering the silicon crystal detector in 1906. The simple "oscillating crystal" circuit below was designed to create a weak "local oscillator" current, which could be used for heterodyne reception of continuous-wave signals.
QST, March, 1920, page 44:

circuit layout    Did you ever hear of an oscillating crystal? Mr. G. W. Pickard, of the Wireless Specialty Apparatus Co., has done it, and here's the hook-up. The secret is the nine volts at the potentiometer. CW is received by heterodyne beats in the usual manner. The Editor would like to know what results are had with this hookup.