Found in a 1924 book by Gernsback, this circuit uses a double variable capacitor, a component not very common at the time. Listening was done on a loudspeaker at the time.
We found this circuit, which appears with variations on this site, in a European documentation from the...
This circuit that drives an electromechanical counter of the time was found in Radio Electronics magazine...
Found in a Hands On Electronics of the 80s, this circuit makes use of a gas sensor of the time....