Detector receiver in a nutshell

This self-made model likely dates back to the 1920s or 1930s. CT images first showed that the receiver was a detector receiver. Only absolutely essential components were included: a self-wound coil and detector glass. A voltage source was not necessary. The spring contact was moved with the coral piece over the glass to find a hot spot for receiving signals. The frequency could not be changed, which meant that only the local transmitter could be received.