Max thermometer: what is it?

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A maximum thermometer is a glass mercury medical thermometer that has a carefully engineered restriction in the mercury column directly in front of the reservoir bulb. The purpose of the restriction is to keep a certain reading visible on the thermometer scale for as long as the user requests. In the absence of this feature, the mercury will rise to indicate its 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 prevented from returning by the restriction. The reading is then canceled and the mercury is returned to the environment by vigorously shaking the thermometer.

Traditional mercury thermometers have a glass rod with a thin hollow tube in the center that terminates 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 the tube up. When expansion has ceased, the upper 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 drops down the tube until it reaches a point where it indicates room temperature.

This arrangement works well if the reading can be observed immediately at the heat source. If the thermometer has to be moved or the reading has to be held for a period, a conventional mercury thermometer won’t work because the mercury cools and contracts immediately, thus leaving only a brief moment to accurately note the reading. To counteract this feature, the maximum thermometer has a short restriction directly above the bulb. When heated, the thermometer operates in the conventional way 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 restricted area.

This feature allows you to move or even keep the maximum thermometer for an extended period with the maximum temperature reading displayed. When the maximum reading is no longer required, the thermometer is vigorously shaken to return the mercury column to its ambient level for reuse. For this reason, maintaining an accurate reading requires that the maximum thermometer be handled very carefully after the reading has been taken.

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