an instrument for measuring temperature, often a sealed glass tube that contains a column of liquid, as mercury, that expands and contracts, or rises and falls, with temperature changes, the temperature being read where the top of the column coincides with a calibrated scale marked on the tube or its frame.
1630s, from French thermomètre (1620s), coined by Jesuit Father Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (see -meter). An earlier, Latinate form was thermoscopium (1610s). The earliest such device was Galileo's air-thermometer, invented c.1597.
An instrument used to measure temperature. There are many types of thermometers; the most common consist of a closed, graduated glass tube in which a liquid expands or contracts as the temperature increases or decreases. Other types of thermometers work by detecting changes in the volume or pressure of an enclosed gas or by registering thermoelectric changes in a conductor (such as a thermistor or thermocouple).
