Ionovac Plasma Tweeter 14A435A

A small volume of air, enclosed in the walls of a quartz cell, is ionized by a radio frequency voltage applied to the internal and external electrodes of the cell, while an audio signal from an amplifier modulates this RF voltage. As a result, the ionized air generates a purple glow in the cell and varies in volume in accordance with the variation of the acoustic signal. Coupled to a suitable horn, its performance level is comparable to a 30 cm woofer, and its frequency response extends from around 3,500 Hz to over 20,000 Hz.

Literally translated from "Radiorama" January 1962.

Here you find 27 models, 26 with images and 10 with schematics for wireless sets etc. In French: TSF for Télégraphie sans fil.
All listed radios etc. from DuKane Corporation; St. Charles (IL)