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.
In the figure we have two possible configurations for multivibrators with relay. The idea comes...
This circuit serves to isolate analog signals, for example, audio signals in a secure link...
We found this circuit in an old English documentation. Its purpose is to synchronize circuits. The...