The first British commercially  available high-vacuum variable capacitors have been announced by the English Electric Valve Co. So 64 years ago, started a ...

Chunky resistors and capacitors, vacuum tubes with cathodes aglow ... of any old radio was connected to the tuning knob: the big variable capacitor with its interdigitating metal plates.

We’re talking of course about the air-dielectric variable capacitor, the tuning element for millions of radio receivers back in the day but now long ago replaced by much flimsier polymer ...