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If the business side of Cahill's project was bold, the engineering was bordering on madness. The Telharmonium was based on additive synthesis, implemented half a century before computers with pure mechanics: 145 gigantic rotors generated sound that was mixed like colors on a palette.

The main challenge for the performer was the keyboard: to the twelve equal-tempered keys Cahill added two more rows — tuned slightly sharp and slightly flat, for pure intervals — leaving musicians to play on 36 keys per octave. When the 200-ton machine was moved to Broadway in 1906, the move took over thirty shipments — hence the legend of "30 railroad cars"; in reality the whole load would have fit into one or two boxcars.

One of its biggest fans was Mark Twain, who said he had "postponed death to hear this." Physics killed the project: the massive signal bled into telephone lines, and Wall Street bankers picking up the phone for serious deals heard Rossini overtures instead of their partners — enraging the telephone companies enough to bury the invention.