Before moving forward in time past the age of the Theremin, I must move backward slightly to talk about one of the first ever synths, more like an Organ, but even an Organ is a synthesizer by definition.
In order to create the Telharmonium, Thaddeus Cahill benefitted from a combination of the telephone and alternating current, both being recently invented. The telephone allowed audio to be converted to electrical signals for transmission and then reconverted back to audio for the receiver. Alternating Current is essentially a sine wave. The overtone theory provided that combining sine waves could produce complex tonalities. Through a combination of very large transformers, alternators and extremely long lines of cable, Cahill was able to create the entire harmonic series and control the volumes of those harmonics through switches and a piano-like keyboard, that would control “Wheels” or rotating disks within disks for each harmonic. This creation was the genesis of the Hammond Organ 25-30 years later, but there were a few more advancements needed to reduce the size of the instrument. Cahill's Telharmonium was built inside two stories of a factory, weighed several tons and required many railroad carsto transport it. The electrical power required to move each tone wheel was immense as well, to create the overtone series. The keyboard was far ahead of it’s time, in that, like a piano, it was able to produce variable dynamics through touch.
The Telharmonium was invented in Holyoke Massachusetts, then carted via around 200 Rail cars to New York City.
Make Noise company makes a Eurorack Modular Synthesizer that is a throw-back to the Telharmoium. You can listen to a couple of songs that feature this on my youtube page at Ralphykeys Artist. Hope you will check it out.