Generally thought to be the first electronic music instrument, the first Telharmonium was built by Thaddeus Cahill in 1897. The instrument was conceptually similar to, although much more complex than, a Hammond organ. It used a bank of tone generators, which probably output a waveform close to but not quite a sine wave, and a set of mixing controls to synthesize different waveforms; as such, it can be considered an additive synthesis instrument. Note that at the time, de Forest had yet to invent the triode tube, and so there was no such thing as an audio amplifer. That meant that the Telharmonium's tone generators had to generate enough power for the waveform to drive a telephone receiver directly, at a level high enough to be audible. As a result, the tone generators were in fact high-voltage electrical alternators, converting DC to alternating currents, and geared to run at different speeds.
Cahill built three versions of the instrument. The first, of which construction started in Cahill's home and then moved to a mill building in Holyoke, MA, was a prototype and proof-of-concept unit intended to attract the interest of financial backers, an effort which was successful. Based on this, Cahill began construction of the "Mark II", the first fully functional version, in 1902. This was completed in the mill building in 1906, and then disassembled and moved to a performance venue in New York City, which was christened Teleharmonic Hall. The main floor was fashioned into a performance theater, while the Teleharmonium's machinery was installed in the basement, on a sturdy concrete foundation. The Teleharmonium in transit filled several railroad cars.
In the Mark II version, a complex keyboard switching mechanism selected the right generators corresponding to the harmonics of each played note. Three manuals were arranged so as to be able to play microtones in order to accommodate the differences between the perfect ratios of the harmonics and the equal tempered scale (it wasn't pre-wired for the performer, the way the later Hammond organ would be); this also had the advantage of allowing the performer to play in a variety of scales and intonations, but it also meant that in order to play a scale, the performer had to jump back and forth between the manuals. Because of this, performances were often executed with two or more performers at the console. Blending of harmonics was controlled by a set of organ-like stops mixed with switches of various sorts, which evolved as time went on and performers requested additional features from Cahill.
Cahill's concept was that the Telharmonium's music would be performed live and distributed into homes via the telephone system, in addition to the live audiences in Teleharmonic Hall. The telephone subscribers would be the main source of revenue. However, in 1906 there was no such thing as a proper loudspeaker. In Teleharmonic Hall, this was overcome by accoustically amplifying telephone receivers using horns, but installing large horns in the homes of telephone subscribers was not an option. So, in order to be audible through a telephone from across the room, the Teleharmonium had to drive substantially more current into the phone lines than they were designed to carry. This worked, in terms of making the music audible to subscribers, but it caused crosstalk with other calls in the telephone network, and it sometimes damaged network equipment. The music enjoyed a few years of popularity, but a financial recession killed the telephone subscriber market, and by 1910, Teleharminc Hall was out of business..
Undaunted, Cahill tried again, building a Mark III version of the machine that was installed in a basement of another building in New York. The plan this time was to work with the phone companies to provide special network facilities to carry the transmissions to special receivers in subscribers' homes. But all of this required money that Cahill didn't have, and after the Mark III version failed to attract enough subscribers, Cahilll declared bankruptcy in 1914. The Mark II machine (still in the basement at Telharmonic Hall) and the Mark III machine were both scrapped.
No artifacts of the Teleharmonium exist today except for newspaper articles, reviews, and a few photos. After Cahill died in 1934, his family tried for years to interest a museum in the remaining Mark I Telharmonium, but found no takers; they finally scrapped it in 1962. No recordings of the Telharmonium are known to exist.
More information, and photos, at 120 Years.net.