The Telharmonium was a machine for generating a kind of ``additive synthesis'' of musical sound for distribution over telephone lines. It is described in U.S. patent 580,035 by Thaddeus Cahill (1898), entitled ``Art of and Apparatus for Generating and Distributing Music Electrically''
Sound was generated electromechanically in the Telharmonium by so-called rheotomes, depicted in Figures G.1 and G.2 from the patent. A rheotome was a spinning metal shaft with cut-outs that caused a periodic electrical signal to be picked up by capacitive coupling. The rheotome is a clear forerunner of the Hammond organ tone wheels, shown in Fig.G.3.
Because rheotomes did not generate sinusoidal components, the Telharmonium is better considered an electromechanical descendant of the pipe organ than as an ``additive synthesis'' device in the modern sense (i.e., inspired by Bernoulli's modal decomposition insight and Fourier's famous theorem). However, the Hammond organ was conceptually an additive synthesis device (having drawbars on individual overtones), and it was clearly influenced by the Telharmonium.