Telharmonium

The twelve steel shafts, one a pitch class, all geared to one motor. Each drum is an alternator: its teeth sweep past the pole pieces, so it makes its shaft's turning rate times its teeth. A key lights its own alternator and the ones with 2, 3 … 8 times its teeth on the same shaft, its harmonics. Two keys an octave apart share drums. The motor's speed, and so every pitch, sags under a big chord.

Every key's n-th harmonic flows through bus n of its manual, one transformer coil behind its stop. The tick is what one note would put through it; the bar is the current that flows. Hold more keys and each one's share falls (robbing): the softer the stop, the more.

How each frequency reaches the listener: the telephone line (dashed; flat in the hall), then the receiver's diaphragm and its horn (solid), with the alternators' voicing set against the 1906 receiver. A city cable takes the highs away with distance; the diaphragm rings near its resonance; the horn's resonances ripple the bass.

Every harmonic is a steady line from its own alternator, switched in and out by the relays: each note starts and stops with a puff. Under a big chord all the lines bend down together as the motor slows, and come back when it lets go. With a weak receiver magnet, loud bass chords grow lines between them (sum and difference tones).