Why clinical thermometers have a kink near the bulb

Clinical thermometers have a kink, also known as a constriction, in the capillary tube just above the bulb.

This kink serves a crucial purpose: it prevents the mercury from flowing back into the bulb immediately after the thermometer is removed from the body. This allows a person to have enough time to take an accurate temperature reading. The mercury level remains at the highest temperature recorded until the thermometer is deliberately reset by shaking it with a few sharp jerks.