Hearing aid technology took a great step forward with the introduction of the body-wearable Acousticon and similar carbon-type electrical aids. Substantial amplification was now available, although don’t think about Hi-Fi. These instruments used a carbon microphone or microphones connected in series with a battery, a magnetic earphone and a variable resistance. The resistance was used to control volume. See General information on carbon hearing aids for more info. The Acousticon I have was made by Dictograph Products Corporation of New York city. Documents 1 and 2 seem to be owner’s manuals, maybe printed at different times. I got them at an Estate sale along with my type SRD Acousticon.
Several Articles about hearing aids appeared in the October 1938 issue of Radio-Craft magazine. See Document 3. The first describes a new “Hi-Fi” hearing aid available for either battery or 115 volt AC-DC powering. WOW! See Fig. 1, schematic B, for the AC-DC model. It shows the earphone tip-jacks connected directly to one side of the power cord after the power switch! Polarized power plugs were not used in those days. When powered ON, one side of the power line was connected directly to the earphones on one’s head! The second Article describes the testing of hearing aids. The unit being tested looks as if it uses a carbon microphone.