From AMS Glossary
A modulating system used in some forms of chronometric radiosonde.
The meteorological transducing elements are so designed that each of them controls an electrical contact arm that sweeps a sector of a rotating wire helix or spiral. During a rotation of the helix or spiral each electrical contact momentarily touches the wire at a point in the cycle determined by the value of the meteorological parameter being measured. The chronometric pulses are translated into their meteorological equivalents at the receiving station by a chronograph operating in synchronism with the rotation of the helix or spiral.