y positive potential on anode Vacuum electrons Heated cathode gives off electrons Filament supply Figure 1.5 The idea behind Fleming's diode or oscillation valve thermionic emission. This makes the filament positively charged and the electrons are attracted back. If another element called an anode is introduced and is given a positive charge then it will attract the electrons away from the heated filament or cathode. Fleming used this effect to rectify the radio or wireless signals SO that the modulation of the alternating current signals was detected. The signals could then be displayed on a meter or heard in headphones. An American named Lee de Forest made the next major development. He saw the possibilities of Fleming's diode and started to work on copies of the device. In an attempt to be able to circumvent Fleming's patents he investigated a number of other ideas, placing a third electrode into the envelope between the cathode and anode. He called this electrode a grid because it consisted of a wire mesh or grid through which the electronics could pass. De Forest took out a number of patents for his idea, but it was not until around 1907 that he had developed the new valve, which he called an Audion, sufficiently for it to be used properly. Unknown to him at this 10 Newnes Guide to Radio and Communications Technology time, de Forest had struck on an idea of immense importance, but initially the triode valve was only used as a detector. It took a number of years before it was used as an amplifier. After their introduction, valves were not widely used. They were expensive to buy, and they were also very expensive to run. At this time valve technology was such that each valve required a separate battery to supply its filament, and as they consumed an amp or more each, the batteries did not last very long. This resulted in other developments coming to the fore. The main one of these was the crystal detector. These detectors consisted of a crystal that was usually made out of a substance called galena. A thin wir