what he needed to rectify alternating radio signals into DC signals required tooperate headphones. By hooking up Edison’s vacuum tube to a receiving antenna, the tube worked like a diode. When the signal voltage increased in one direction, it made the plate positive (+), and the signal got through. When the signal voltage increased in the other direction of the AC cycle, applying a negative (–) charge to the plate, the signal stopped.
The vacuum tube, also called the electron tube, required a source of electrons to function. In Edison’s original electron tube, the electron source, called the cathode, was the filament that, when heated red-hot, emitted electrons that flew off into the vacuum toward the positively charged plate, called the anode. The effect of heating the cathode to activate the electrons was called thermionic. Other electron tubes used ligh voltage to pull the electrons out of a cold cathode. Electronic emission also occurred by applying light energy to a photosensitive cathode. Tubes using this effect were called photoelectronic vacuum tubes. Although a variety of methods existed to remove electrons from the cathode, the thermionic vacuum tubes were the most widely used. The cathode was either heated by resistors within or used a separate source of power for heating. The vacuum tube consisted of a glass or metal enclosure with electrode leads brought out through the glass to metal pins molded into a plastic base (Fig. 1.5).
