Take for example the induction coil. Early Induction coils existed before 1840, these were the work of Charles Page, James McGauley and Nicholas J. Callan to name a few productive tinkerers. But even in those early days it seemed to be used more often for sportingly shocking ones own body and unwitting test subjects. Both Page and Callahan shocked themselves often and deliberately in the study of these early induction coils. Neither of them believed this had any medical value. The one above appears to be the one patented by Henry Voelkner of Detroit as US456746 in 1891.
William Scheidel had his own induction coils which were advertised in quackery catalogs such as The Friedlander Company Catalog. Mr. Scheidel founded the Scheidel Coil Company company in Chicago in 1901, and while he did manufacture legitimate early X-ray equipment he never could let go of shocking peoples' testicles. The 1917 edition of the book Hawkin's Electricity writes simply of these devices "These are employed chiefly for the application of currents of varying voltage, strength and frequency and wave form to the patient..."
"In this type of interrupter an alkaline solution is used. By using an aluminum plate in place of the lead plate, it acts as the alternating current both as a rectifier and an interrupter. It consists of steel and aluminum electrodes in an alkaline solution, the combination or cell acts as a valve, permitting the current to pass from the steel to the aluminum plates, but not in the opposite direction (from the aluminum to the steel). In this way a uni-directional current is obtained, on which an electrolytic interrupter will operate."So it sounds like a real rectifier at first but really it's just a battery. Batteries produce DC power, which is unidirectional. It does not rectify any signal per se, but it can still be used for shocking people in the genitals. That may sound like hyperbole, but the very next page in Hawkin's Electricity details the metals for intra-uterine electrodes, urethral electrodes and the proper technique for applying current to hemorrhoids. These people were sadists. More here.