Modern electronics can trace its roots to the first electronic devices called vacuum tubes. Although, today, solid state devices have totally replaced the vacuum tube, the fundamental principle as to its usage remains relatively unchanged. For more than 40 years, until the late 1960s, the most important part in a consumer electronics product was the vacuum tube. It is with this historical perspective in mind that this section is presented so that readers will not lose sight of where it all started.
The vacuum tube got its start in 1883, when Edison was developing the incandescent lamp. To correct the premature burnout of the red-hot filament in light bulbs, Edison tried a number of experiments, one of which was to place a metal plate sealed inside a bulb and connect it to a battery and ammeter, as shown in Fig. 1.4. Edison observed that, when the filament was hot and the plate was positively (+) charged by the battery, the ammeter indicated a current flow through the vacuum, across the gap between the filament and the plate. When the charge on the plate was reversed to negative (–), the current flow stopped. As interesting as this phenomena was, it did not improve the life of Edison’s lamps and, as a result, he lost interest in this experiment and went on to other bulb modifications that proved more successful. For about 20 years, Edison’s vacuum tube experiment remained a scientific curiosity. In 1903, as radios were coming into use, J. A. Fleming, in England, found just.