Alcohol thermometer to give a background for reading the scale. The range of usefulness of the thermometer is set by the boiling point of the liquid used. In the case of the ethanol-filled thermometer, the upper limit for measurement is 78 °C (172.4 °F), which makes it useful for measuring daytime, nighttime and body temperatures, although not for anything much hotter than these. Ethanol-filled thermometers are used in preference to mercury for meteorological measurements of minimum temperatures and can be used down to −70 °C (-94 °F). The physical limitation of the ability of a thermometer to measure low temperature is the
Six's thermometer of mercury, a metal which is liquid at normal temperatures. This is pushed around the tube by the thermal expansion and contraction of the alcohol in the first bulb as it responds to the external temperature. The vacuum in other bulb allows free movement of the alcohol and mercury. It is the alcohol which measures the temperature; the mercury indicates the temperature reading on both scales. This is unlike a normal mercury thermometer, in which the expansion and contraction of mercury itself indicates temperature. The thermometer shows a reading at the top of the mercury section on both the maximum