Three-Electrode Cold-Cathode Tubes

Figure 7. Phenomena occurring in a two-electrode cold-cathode gas-filled tube.

Figure 8. A three-electrode cold-cathode gas-filled tube of the general type sometimes used in selective-ringing circuits as explained on page 358. (Courtesy Western Electric Co.)

There are two gaps to be broken down and to be made conducting. The design sometimes is such that the starter gap between the cathode and starter anode breaks down at a lower potential than the main gap from cathode to main anode.

Figure 9. A half-wave rectifier, and the shapes of the impressed and the rectified waves, a and b. Waves c, d, e, and f are the major components of the rectified wave b.

The tube may be used as a relay in this way: Both anodes are made positive but are maintained below the breakdown value and hence conduct negligible current. The device to be operated is placed in series with the main anode. An incoming control or signal impulse is injected in series with the starter anode. This impulse "fires" the starter anode, causing ionization. This supplies sufficient ions to permit the main anode to conduct and operate the device in series with it.