How long should a mercury thermometer be left in the armpit?

A mercury thermometer typically requires 5 to 8 minutes for an axillary (underarm) measurement—there is no need to measure for longer. Mercury thermometers operate based on the principle of thermal expansion and contraction: when exposed to heat, the mercury expands and rises within the thermometer tube; the extent of this rise corresponds to the temperature indicated by the calibrated scale marked on the outside of the thermometer, allowing the temperature reading to be observed. Prior to measurement, always shake the thermometer vigorously to lower the mercury column below the 35°C mark. Avoid drinking fluids, eating, or engaging in excessive physical activity both before and immediately after measurement. For optimal accuracy, it is best to lie supine or sit quietly during measurement.