Trautonium

Each lamp's capacitor voltage over a few of the master's periods, from extinction (60 V) up toward striking (90 V). The master's every discharge kicks the dividers up by the sync pulse (the shaded band under the strike line): a divider inside the band fires with it and starts again in step, so it fires on every n-th flash. One that the kick can't reach in time fires on its own and slides past.

The circuits stay where their capacitors tune them, whatever note is played. Each flash of a lamp rings them, so the harmonics that fall under a peak come out loud: move up the keyboard and different harmonics light up, as a voice keeps its vowel on every pitch.

A slide along the wire bends every harmonic together while the circuits' bands stay put. Dividers in step add lines below the note at f/2, f/3 …; a slipping divider sits a little flat of them.