Singing Arc

Voltage across the arc against the current through it. A steady arc sits on its falling static curve, where the supply's load line crosses it. Singing, the operating point runs round a loop: above the curve while the current rises (the column is still cool), below it while it falls (still hot). The shaded strip on the left is where the column has collapsed and the hot carbons re-strike it; past the dashed line the arc hisses.

The column between the carbons (positive on the left, with its crater), its glow following the power u·i and its width the heat it holds, strobed so that one cycle takes about a second. The column's breathing is the sound: its pressure follows dP/dt.

Dots: the current (the musical branch's in slow motion). Lit: the capacitors switched in.

Each note grows out of the arc's own flicker and settles a little below Kelvin's frequency (blue tick). Hold keys together and the line steps down; past the hissing current, noise fills the top.