Wiktionary
n. (context scientific usually dated or in textbooks English) A tube with two ends and an artificial semipermeable membrane in the middle.
Usage examples of "u-tube".
So the U-tubes produce a set of binary digits that is like a picture of the standing wave--a graph of the harmonics that make up the musical note that is being played on the speaker.
The crescents of mercury in all those U-tubes are shifting up and down, opening and closing the contacts, but systematically: it is not turbulent sloshing around, but a coherent progression of discrete controlled shiftings, informed by some program.
So the U-tubes produce a set of binary digits that is like a picture of the standing wave—a graph of the harmonics that make up the musical note that is being played on the speaker.