Theremin

The whole circuit runs from 9-12 V. Connect nothing to the mains. Take the output through a small speaker — a harsh tone close to the ear tires you faster than you would think.

Theremin describes two oscillators above the audible range whose difference gives an audible tone. Pitch, volume and timbre control are each specified separately.

Detune two identical strings very slightly and pluck both. A slow "wah-wah" appears. That is the difference frequency, just still very low.

Build a simple LC or timer oscillator at around 200 kHz. Check it on a scope or a receiver — you should not be able to hear it.

Repeat the circuit as closely as you can. The nearer the two are in frequency, the lower the resulting tone.

Feed both into a mixer — a diode and a capacitor will do. The difference frequency appears at the output.

You hear a tone that is present in neither oscillator. The sound was born from the difference, not from a signal.

Slowly bring the frequencies together until the tone drops to nothing. That point is "zero beat", and everything else is measured from it.

Solder a vertical rod 30-40 cm long to the first oscillator's tuned circuit. Leave the second alone — it stays as the reference.

Move your palm toward the antenna. The tone rises. You have become part of the circuit — your body adds capacitance and pulls the frequency.

Measure the oscillator frequency with and without your hand. The shift is a fraction of a percent; the audible difference is whole octaves. That is the entire point of the beat method.

Note the distance at which the tone starts to change. Close to the antenna the notes bunch together — the instrument's scale is non-linear, which is why it is hard to play.

Build a separate circuit with a horizontal loop controlling the gain. Theremin splits pitch and volume between two hands.

The right hand leads the pitch, the left silences the sound between notes. Without the second hand the instrument wails continuously — that hand makes the rests.

The patent. US 1,661,058, "Method of and apparatus for the generation of sounds", filed 5 December 1925 and granted 28 February 1928. The name on the document is written "Leo Ssergejewitsch Theremin" — a Western transliteration; in Russian the inventor was Lev Sergeyevich Termen, which is why the instrument is called the termenvoks in Russian rather than the theremin.

The instrument came before the patent. Termen built the first versions around 1920 in Petrograd while working on measuring instruments: he noticed that a device sensitive to capacitance changed its tone when a hand moved nearby — and heard music in that rather than interference. In 1922 he demonstrated it to Lenin, who by the well-known account played a melody on it himself. Tours of Europe and the United States followed, and it was there, in 1925, that the American application was filed.

Why the beat method was chosen. The capacitance of a hand near an antenna is minute, on the order of a few picofarads. There is no way to control it directly: it is far too small to set an audio frequency on its own. But if it shifts the frequency of an oscillator running at hundreds of kilohertz very slightly, the absolute shift lands squarely in the audio range. In other words the circuit subtracts two large quantities to obtain a small one, and in doing so amplifies a tiny influence up to musical scale. Step 11 puts a number on it: fractions of a percent become octaves.

That is also the source of its main difficulty. The relationship between pitch and distance is non-linear: near the antenna notes are packed tightly, far away they are sparse. A theremin has no frets, no keys and nothing at all to hold on to, so it can only be played in tune by ear and by remembered hand position. Its finest player was Clara Rockmore, a violinist who developed a technique of precise, rapid finger movements for it.

What became of the inventor. Termen returned to the USSR in 1938, was arrested, and worked in a closed design bureau — a sharashka. There he created "The Thing", a passive listening device with no batteries and no wires, which operated for seven years in the US ambassador's office in Moscow. The same principle: a weak external influence alters the parameters of a resonant circuit. The invention that produced one of the first electronic musical instruments also worked as an intelligence tool — and that is part of the story, not a footnote to it.