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[Phys-L] Singing Kettle

We use an electric water kettle, made by Hamilton Beach; its capacity is 1.7 liters, electrified through its fixed base plate supplying 1.5kilowatts. The shape is a truncated parabola of revolution, focus up. When half filled, it shows a distinct acoustic pattern when the heating produces gas bubbles. The sound oscillates on a 10 second periodic time, from silence to a quiet pink noise and back, never louder than 60 dBm. I suppose there is a positive feedback loop of some kind.    Care to hazard a guess?

Brian W