WC>-> I've only ever seen that in car radios,  myself.

WC>I've seen them, along with "vibrators" to step up voltage
WC>for tubes.

I got my Car Radio Repairing experience on Vibrator Car Radios. They
were also popular in Farm Radios that used a 6 volt Car/Truck battery
for the power source. Adn in the earlier days they came in both Single
conversio nand Double conversion. The single conversion units just
chopped the DC adn then steped it up and rectified it where as the
Double conversions were synchrious in that that one set of contacts
Chopped the DC and another set of points unchopped(mechanicaly rectified
d) it

WC>OTOH my Cub Scout Crystal radio had an inductor wound on
WC>a Quaker Oats box with a slider for tuning.
WC>The capacitor was fixed and made from the genuine tin foil
WC>once found on gum wrappers you could peel off the paper backing.


While a little more precision in construction  my first Crystal Radio
also used a Fixed Capacitor and a Variable Inductance for tuning.
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 þ SLMR 2.1a þ This note from El Cajon California USA