Liquid thermometer, an instrument for measuring temperature based on the thermal expansion of a liquid. It is used over the temperature range from -200 to 750°C. A liquid thermometer is a transparent glass (rarely quartz) bulb with a capillary of the same material sealed to it. The scale in °C is marked either on a thick-walled capillary (the so-called solid-stem liquid thermometer) or on a plate rigidly joined to it (a liquid thermometer with an external scale). A liquid thermometer with an enclosed scale (a clinical one, for example) has an outer glass (quartz) sheath. The scales have intervals from 10 to 0.01°C. The thermometric liquid fills the whole bulb and part of the capillary. Depending on the measuring range, a liquid thermometer is filled with pentane (for measuring temperatures from -200 to 35°C), ethyl alcohol (from -80 to 70°C), kerosene (from -20 to 300°C), mercury (from -35 to 750°C) and others. Mercury liquid thermometers are the commonest, since mercury remains liquid over the range from -38 to 356°C at normal pressure and up to 750°C at a slightly raised pressure (for which the capillary is filled with nitrogen). A gallium liquid thermometer measures temperatures over the range from 30 to 1200°C. Liquid thermometers are made from particular grades of glass and given a special heat treatment (ageing) that removes the shift of the zero point of the scale caused by the repeated heating and cooling of the thermometer (a correction for the shift of the scale zero has to be applied in precise measurements). The accuracy of a liquid thermometer is set by the interval of its scale. To secure the accuracy required and convenience in use, liquid thermometers with a shortened scale are used; the most accurate of them carry the 0°C point on the scale whatever temperature interval is marked on it. The accuracy of the measurement depends on how deeply the liquid thermometer is immersed in the medium being measured. The thermometer should be immersed up to the scale division being read, or up to a line specially marked on the scale (partial-immersion liquid thermometers). If this is impossible, a temperature correction for the emergent stem must be applied.