High Vacuum Diodes

Author: J.B. Hoag

Electrons evaporate from a hot filament in a vacuum tube just the way water molecules evaporate from a liquid into the air.

Fig. 10 A. A high vacuum two-electrode tube or diode. (From E. & N. P.)

Figure 10 A shows a filament F and a metal plate P sealed in a glass bulb which has been highly evacuated so that there is essentially no gas in the tube. At the bottom left, there is a battery which is used to heat up the filament. The temperature of the filament can be changed by changing the amount of current flowing from the battery through the filament. This is accomplished by means of the rheostat in the lower left corner where the arrow serves to indicate the sliding contact or knob which increases or decreases the total amount of resistance wire in this so-called A-battery circuit. The plate is charged positively with respect to the filament by means of the battery whose voltage is marked V. When electrons, which are negatively charged, are emitted from the filament, they are attracted to the positively-charged plate and move through the empty space between these two electrodes in the vacuum. The circuit containing the battery V, the current-measuring instrument i, the filament, and the plate is called the B-battery circuit. Inasmuch as the filament serves as the source of electrons, it is obvious that the current can flow through this two-electrode tube, or diode, in but one direction. In the conventional sense the electricity leaves the positive terminal of the battery, flows along the wire to the plate P, then to the filament, then through the wires to the meter i, to the negative terminal of the battery. In the true electron sense, the current flows in the reverse direction.