<P> A maximum minimum thermometer, also known as Six's thermometer, is a thermometer which registers the maximum and minimum temperatures reached over a period of time, typically 24 hours . The original design contains mercury, but solely as a way to indicate the position of a column of alcohol whose expansion indicates the temperature; it is not a thermometer operated by the expansion of mercury; mercury - free versions are available . </P> <P> Mercury thermometers cover a wide temperature range from − 37 to 356 ° C (− 35 to 673 ° F); the instrument's upper temperature range may be extended though the introduction of an inert gas such as nitrogen . This introduction of an inert gas increases the pressure on the liquid mercury and therefore its boiling point is increased, this in combination with replacing the Pyrex glass with fused quartz allows the upper temperature range to be extended to 800 ° C (1,470 ° F). </P> <P> Mercury cannot be used below the temperature at which it becomes solid, − 38.83 ° C (− 37.89 ° F). If the thermometer contains nitrogen, the gas may flow down into the column when the mercury solidifies and be trapped there when the temperature rises, making the thermometer unusable until returned to the factory for reconditioning . To avoid this, some weather services require that all mercury - in - glass thermometers be brought indoors when the temperature falls to − 37 ° C (− 35 ° F). </P> <P> To measure lower meteorological temperatures, a thermometer containing a mercury - thallium alloy which does not solidify until the temperature drops to − 61.1 ° C (− 78.0 ° F) may be used . </P>

What prevents mercury from flowing out of the glass tube