Diode Tube

The diode tube we are about to study is really Edison’s old incandescent bulb with the plate in it. Diode means two elements or two electrodes, and refers to the two parts within the glass container that make up the tube.

We have called them filament and plate. More formally, they are called CATHODE and PLATE, respectively. Sometimes the filament is called a HEATER.

Within a few years after the discovery of the Edison effect, scientists had learned a great deal more than Edison knew at the time of his discovery. By the early 1900s, J.J. Thomson in England had discovered the electron. Marconi, in Italy and England, had demonstrated the wireless, which was to become the radio.

The theoretical knowledge of the nature of electricity and things electrical was increasing at a rapid rate. J.A. Fleming, an English scientist, was trying to improve on Marconi’s relatively crude wireless receiver when his mind went back to Edison’s earlier work.

His subsequent experiments resulted in what became known as the FLEMING VALVE (the diode), the first major step on the way to electronics.


Before learning about Fleming’s valve, the forerunner of the modern diode, let’s look at Edison’s original circuit. This time, however, we’ll draw it as a schematic diagram, using the symbol for a diode instead of a cartoon-like picture. (The schematic shown below).

A schematic of Edison's experimental circuit for the incandescent lightbulb.

Note that this is really two series circuits. The filament battery and the filament itself form a series circuit. This circuit is known as the filament circuit.

The path of the second series circuit is from one side of the filament, across the space to the plate, through the ammeter and battery, then back to the filament. This circuit is known as the plate circuit.

You will note that a part of the filament circuit is also common to the plate circuit. This part enables the electrons boiled from the filament to return to the filament.

No electron could flow anywhere if this return path were not completed. The electron flow measured by the ammeter is known as plate current. The voltage applied between the filament and plate is known as plate voltage.

You will become familiar with these terms and with others that are commonly used with diodes and diode circuits as we progress.

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