INTRODUCTION TO ELECTRON TUBES

and extremely important. Practically everything that follows will be related in some way to the Edisoneffect. Be sure you have a good understanding of it before you go on.Q1. How can a sheet of copper be made to emit electrons thermionically?Q2. Why do electrons cross the gap in a vacuum tube?THE DIODE TUBEThe diode vacuum tube we are about to study is really Edison's old incandescent bulb with the platein it. Diode means two elements or two electrodes, and refers to the two parts within the glass containerthat make up the tube. We have called them filament and plate. More formally, they are calledCATHODE and PLATE, respectively. Sometimes the filament is called a HEATER, for obviousreasons-more on this later.Within a few years after the discovery of the Edison effect, scientists had learned a great deal morethan Edison knew at the time of his discovery. By the early 1900s, J.J. Thomson in England haddiscovered the electron. Marconi, in Italy and England, had demonstrated the wireless, which was tobecome the radio. The theoretical knowledge of the nature of electricity and things electrical wasincreasing at a rapid rate.J.A. Fleming, an English scientist, was trying to improve on Marconi's relatively crude wirelessreceiver when his mind went back to Edison's earlier work. His subsequent experiments resulted in whatbecame known as the FLEMING VALVE (the diode), the first major step on the way to electronics.OPERATION OF THE DIODE TUBEBefore learning about Fleming's valve, the forerunner of the modern diode, let's look at Edison'soriginal circuit. This time, however, we'll draw it as a schematic diagram, using the symbol for a diodeinstead of a cartoon-like picture. The schematic is shown in figure 1-3.Figure 1-3.—Schematic of Edison's experimental circuit.Note that this is really two series circuits. The filament battery and the filament itself form a seriescircuit. This circuit is known as the filament circuit.1-4