One of these was the Ondes Martenot, created in France by Maurice Martenot, and first presented in 1928. This instrument could make an ethereal sliding sound, as well as sounds for individual notes. It was played in orchestras, and is still in use today--in classical music as well as by pop musicians such as Radiohead. Here is a video that demonstrates the Ondes Martenot, played by Thomas Bloch:
Here is "Allegro," from Brandenburg Concerto number three, by J.S. Bach-- played with the Moog Synthesizer (and accompanied by wonderful coloful graphics!); this video was created and put on YouTube by "Puhzz;" the audio is not the original Switched-On Bach recording by Wendy, but one he made in 2013 as a tribute to her earlier work.
Soon, other companies became major competitors in producing sythesizers, but Robert Moog Music was where this type of electronic instrument began.