How did John Ambrose Fleming’s thermionic valve detect radio signals?

John Ambrose Fleming’s thermionic valve—also known as the vacuum diode—detected radio signals by acting as an electronic rectifier.12

Radio signals consist of high-frequency alternating current (AC), which cannot be directly detected by a telephone receiver. Fleming’s device solved this by converting the AC signal into a direct current (DC) that the receiver could process.21

The valve functioned based on the Edison effect, where a heated cathode emits electrons. Because the device contained two electrodes, it allowed current to flow only when the anode was positively charged relative to the cathode. When the alternating radio signal made the anode positive, electrons flowed from the cathode to the anode; when the signal polarity reversed, the flow was blocked. This one-way conduction effectively rectified the signal, allowing the weak direct current to be detected.231