Electric Pianos began being developed at around 1930, the same time as the Electric Guitar. A true electric Piano has moving parts that hit small tuned pieces of metal. The sound from these pieces of metal is amplified electrically producing a Piano like sound. In the Mid 50s two company’s were making the electric piano popular. Wurlizer and Fender.
Wurlitzer
1973 Fender Rhodes Electric Piano (Click to Listen)
The Fender company produced the Electric Piano design of Harold Rhodes and became known as the Fender Rhodes. The Fender Rhodes sound is synonymous with the 1970’s.
Hits like Elton Johns “Daniel” from 1973 and Billy Joel’s “Just the way you are” from 1977 are signature Fender Rhodes songs.
Another inventor working in the new field of electric powered instruments was Laurens Hammond. Using Tone wheel technology taken from Morse Code equipment Hammond developed an Organ like sound. He started selling his Hammond Organs in 1935. Though originally intended to be a lower cost alternative to church organs, the Hammond B3 became popular with Bands of the 1960’s and 1970’s. The Leslie speaker system spun two Speaker cones around at high speed to create a Tremelo effect.
https://www.youtube.com/watch?v=deB_u-to-IE
The Hammond B-3 Organ with a Leslie Speaker Cabinet
(Click Image to Listen)