Drops of Knowledge
Small lessons from the counter. Turn the knobs.
Result —
Notes
A black light gives off long-wave ultraviolet, UV-A, and very little visible light. Things that fluoresce soak up that UV and give it back as light you can see.
Uranium glass fluoresces bright green. Optical brighteners, added to many papers, absorb around 340–370 nm and re-emit blue at about 420–470 nm. Banknote paper has no brighteners, which is why checking a banknote for fluorescence is a common counterfeit test.
Choosing a black light here also throws the switch for the whole site.
Sources Blacklight (tube peaks near 350 and 370 nm) · Uranium glass · Optical brightener, Wikipedia, read 5 Oct 2026
- Glass
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- Logo
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Why the glass matters: heat it and it grows
Notes
Pyrex reached kitchens in 1915 as borosilicate glass. Corning started making soda–lime Pyrex in the 1940s, about 30 years before the all-lowercase logo first appeared in 1975. So a U.S. piece from the 1940s to 1998 could be either glass, whatever its logo says. Since 1998, U.S. Pyrex is soda–lime.
French Pyrex, made since 1922, is borosilicate with the uppercase logo.
Expansion is only part of the story: borosilicate can still crack under rapid or uneven temperature changes, and U.S. soda–lime Pyrex is tempered.
Sources Pyrex (citing the Corning Museum of Glass) · Borosilicate glass · Soda–lime glass, Wikipedia, read 5 Oct 2026
- Capacitor
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- Tuned to
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- Math
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Notes
Inside the radio, a coil (L) and a tuning capacitor (C) ring at one frequency: f = 1 / (2π√LC). Turning the knob changes the capacitor: less capacitance, higher frequency.
Because f follows one over the square root of C, a knob that changes C evenly bunches the high frequencies together, as the curve shows. The Sony on our shelf, No. 04, reads AM 530 to 1600 on its dial.
The coil and capacitor values are illustrative, chosen to span the band. They are not measured from a particular radio.
Sources LC circuit · AM broadcasting, Wikipedia, read 5 Oct 2026 · dial: No. 04’s listing photo
Groove speed —
Notes
A record spins at a fixed speed, so the groove passes under the needle fastest at the outside edge and slowest near the label: same spin, less groove per second.
78s ruled until 1948. Columbia’s 33⅓ LP and RCA Victor’s 7-inch 45 then fought the “War of the Speeds” from 1948 to 1950, and the 12-inch 33⅓ LP eventually prevailed.
No. 03, the 7 Up crate, is just over 12 inches inside; its listing suggests it as a record box.
Sources Phonograph record, Wikipedia, read 5 Oct 2026 · radii are illustrative