Science Museum inventory number 1992-101.
The Sideman (1959) was the first commercially available drum machine*. It was supplied with a hand switch (see photos) which organists could use to start and stop the machine while sitting at the keyboard. ‘Sideman’ is jargon for a hired musician who is occasionally called on to play with a band (a session musician).
Although it was seen as a gigging instrument, the Sideman was a hefty object – it takes two people to lift it into the back of a car. Inside the Sideman, sounds are generated by electronic valves. These sounds are triggered when a rotating arm sweeps over contacts on a disk (see video). When the arm rotates faster, the rhythm plays faster. The rhythm pattern depends on the position of the contacts on the disk.
For photos of the inside of the Sideman, see Synthgear.
*Strictly speaking, the Sideman was the first commercially available drum machine to generate sounds electronically. The only earlier contenders were the Chamberlain Rhythmate (1949), which played back drum sounds that were pre-recorded on tape loops, and the Rhythmicon, an experimental instrument designed by Leon Theremin in 1931. Only ten Rhythmates were ever made. The Rhythmicon was never made commercially. It played complex, polyphonic rhythms with timings based on the pitch relationships of the harmonic series.
A Sideman without its sides! You can see the spinning disk of contacts here.
(via peahix, YouTube)