The 1928 - 29 RCA theremins used glass tube type oscilators to produce electronic sounds. These were in turn, controlled by the radio magnetic field which surrounds the antennae, and is generated by the large coils located on the left, and right sides of the instrument.
The Larger coil on the left of this image produce the radio magnetic field which surrounds the vertical pitch antenna, which allows notes to be changed in pitch.
The smaller coil located on the right side of the instrument produced a radio magnetic field which surrounds the ring like volume antenna mounted on the side of the instrument. This provides control over dynamic, and expressive volume.
The amplification circuit is housed on a lower rack inside the instrument.
The audio signal was routed from the amplifier to an external speaker.
Some models of the RCA theremin actully had the speaker built right in!
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