Question

why does a clenical termometer such a narrow zone

Answer 1

A clinical thermometer has a narrow zone or constriction in the capillary tube, just above the bulb. When the mercury expands, it pushes through the constriction. But when the thermometer is taken out of the patient's mouth, mercury does not flow back to the bulb. Basically the constriction or narrow zone of the thermometer prevents the backward flow of mercury into the bulb. So, the doctor or anyone who is using the thermometer can read the patient's body temperature without hurry, after removing the thermometer out of the patient's mouth.