This circuit was found in a publication by Edelman from 1924. The power comes from the battery and the listening is through a high-impedance headphone.
This circuit was found in a 1967 US documentation. The FET is general purpose and the 6V zener....
This High Side Current Sensor was found in a 2005 Linear Technology manual. The current sensing...
Found in American documentation from the 90's, this circuit is very current and can be implemented with...