Jeffrey traces the history of the e-meter from the earliest versions in the 1890s to Volney Mathison invention of the Hubbard electropsychometer.

Jeffrey Augustine’s written history of the e-meter: https://scientologymoneyproject.com/2020/06/03/a-brief-history-of-how-scientologys-e-meter-came-into-existence-parts-1-10/

This a downloadable 92 page PDF which discusses how 18th century electrical measuring devices migrated into 19th electrophysiology and psychology; early 20th century polygraphs; and Volney Mathison electropsychometer which L. Ron Hubbard took as his own device and renamed it the Scientology e-meter.