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A maximum thermometer is a medical mercury-in-glass thermometer that features a carefully designed restriction in the mercury column directly ahead of the reservoir bulb. The purpose of the restriction is to to keep any given reading visible on the thermometer scale for as long as the user requires. In the absence of this feature, the mercury will rise until it indicates the relevant temperature and then drop almost immediately to its ambient reading when the thermometer is removed from the heat source. In the case of a maximum thermometer, the mercury rises to its maximum reading and is stopped from returning by the restriction. The reading is then canceled and the mercury returned to ambient by shaking the thermometer vigorously.
Traditional mercury-in-glass thermometers feature a glass rod which is equipped at its center with a thin, hollow tube ending in a reservoir bulb. This hollow tube is partially filled with mercury, and the rod surrounding it is engraved with carefully calculated graduations indicating a set temperature gradient. When the bulb portion of the thermometer is exposed to heat, the mercury expands and pushes up the tube. When the expansion has ceased, the top surface of the mercury column can be compared to the scale to establish the temperature of the heat source. When the heat source is removed, the mercury contracts and descends in the tube until it reaches a point where it indicates the ambient temperature.
This arrangement works well if the reading can be observed immediately at the source of heat. If the thermometer has to be moved away or the reading stored for a period, a conventional mercury thermometer does not work because the mercury cools and contracts immediately, thereby leaving only a brief moment to accurately note the reading. To counteract this characteristic, the maximum thermometer includes a short restriction directly above the bulb. When heated, the thermometer operates in the conventional manner with the mercury column rising to indicate a maximum temperature. When the thermometer is removed from the heat source, however, the column cannot return to ambient due to the area of restriction.
This feature allows the maximum thermometer to be moved or even stored for a protracted period with the maximum temperature reading displayed. When the maximum reading is no longer required, the thermometer is shaken vigorously to return the mercury column to its ambient level for reuse. For this reason, maintaining an accurate reading requires the maximum thermometer be handled fairly carefully after the reading has been taken.