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In a  thermometer, the mercury moves up and down the capillary tube as the temperature changes and  if it is removed  then it form contact with the patient and the reading would steadily fall as it cooled. As  a medical thermometer to get the  reading of the patient's temperature even after it has been removed from the patient, we  need to stop the mercury shrinking back into the reservoir. The kink in the clinical thermometer does this, it breaks the connection between the mercury in the capillary and the reservoir. So the reading given is accurate. Before it can be used again the mercury in the capillary has to be vigorously shaken back into the reservoir.