The Theramin (or Theremin, opinion is divided on the correct spelling) was probably the first practical electronic instrument, credited with inspiring Bob Moog to invent his synthesiser. Invented by Lev Sergievitch Termen (normally Anglicised as Leon Theramin) in 1919, it was intended to be a classical instrument. Well, originally it was intended to be an improved radio, but after he discovered its capabilities he tried to develop it as an orchestral instrument - although it has made more of an impact in the popular music arena; few classical composers seemed interested in working with an instrument with no fixed pitch. Which is a shame, because it is hard to think of another instrument which has had such an influence, or which has been so completely different from anything which came before. Certainly when he toured Europe in 1927 crowds were such that the police had to be called.

Remember The Beach Boys' "Good Vibrations?" - well, the wobbly electronic thingummy in the background is a Theramin. And of course the 1950s would have been greatly the poorer without the Theramin sound tracks on those dreadful science-fiction B movies. Probably.

The amazing thing about the Theramin is that it is played without ever touching it. It generates an electronic tone whose frequency is modulated by a capacitor, one of whose plates is the player. By moving the hands the player can vary both pitch and volume at will - continuously variable.

It is tempting to think the instrument unique in its lack of fixed pitch, but of course there are a other instruments which have the same attribute - the slide trombone, for example, or the swannee whistle. Theramin is different in that it is an electronic instrument designed not to mimic the sound of an existing instrument but to be unique and distinctive; and it is also unique in being played without touching it. No other instrument of any significance has been designed to be played simply by moving the hands in the air nearby. Odd? Certainly. But curiously compelling in its ethereal sound and technical simplicity.

So what does it look like? The original one resembled a small coffin on legs, with an aerial and a handle; but the physics of the device are such that it could be almost any shape. One has been built out of an old desk drawer, for example, and the one used in Good Vibrations was like three or four shoeboxes joined end-to-end with a couple of knobs on one end.

Bands such as the Jon Spencer Blues Explosion use the theramin still, and it has a number of other devotees, but it remains a sadly neglected instrument despite the founding in 1997 of an International Theramin Festival in Portland, Maine, USA - the country from which Leon was reputedly kidnapped by the KGB in 1938, to reappear 30 years later despite wide assumptions of his death. A man whose life was as singular as his instrument.