The clinical thermometer
This is a thermometer specially designed for measuring the temperature of the human body, and so it is only necessary for it to have a range of a few degrees on either side of the normal body temperature (Fig. 14.4). The thermometer is generally placed beneath the patient’s tongue and left there for at least two minutes to ensure that it fully acquires the body temperature. When the thermometer is taken from the mouth the mercury thread does not contract back into the bulb, but remains in the stem. The reason for this is that the stem has a narrow constriction in its bore just
above the bulb. When the thermometer is removed from the mouth the sudden
cooling and contraction of the mercury in the bulb causes the thread to break at the constriction, and so it stays in the stem at its original reading. This nables the temperature tobe read at leisure. Before being used again, the mercury in the stem must be returned to the bulb by shaking. The average body temperature of a healthy person is taken as 37°C.