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VACUUM TUBES
503
heated by the passage of an electrical current through it supplied by the battery marked A and called the "A" battery. Under this condition, after the electrons leave the conductor, which is called the filament, they appear in the space surrounding it and some of them will actually bombard the glass envelope and eventually return to the filament.
From Chapter XXVI it will be clear that since the electrons are negatively charged particles, the moment that any of them leave the filament some of its negative charge has been removed and it therefore becomes positively charged. Since there is then a positively charged body surrounded by negatively charged particles (electrons) these latter will be attracted by the positive charge, thus all the electrons emitted will eventually be recaptured by the filament.
Figure 3 5 1 — When a metal is heated in a vacuum, electrons are evaporated from it and appear in the surrounding space as negatively charged particles.
1. TWO-ELEMENT VACUUM TUBE
The first vacuum tube to be developed was of the two-element type consisting of a filament and a plate as shown in Figure 352.
In this type a plate is inserted in the glass envelope and is given a positive charge by the battery marked "B" in the figure, the positive pole of which is connected to the plate and the negative to the filament. When the current from the "A" battery starts toflow through the filament it raises its temperature and electrons are emitted in the manner described in the previous section. Since now the plate is given a positive charge by the "B" battery, many of the electrons emitted will be attracted by this positive charge and will consequently stream across the space between these two elements, continuing through the connecting wires and battery, eventually returning to the filament in the direction shown by the arrows. This constitutes a flow of direct current, not only in the wires comprising the circuit, but also in the space between the filament and plate. There is thus produced a direct-current flow in a vacuum which is just as truly an electrical current as when this takes place in a metal conductor.
Figure 352 — Electrons emitted by the filament are attracted by the positive charge on the plate and continue through the connecting wires back to the filament.
Vacuum tubes of this type are commonly used as rectifiers.