This is because it allows the mercury to rise up in a capillary tube when the bulb is in contact with the body but does not allow to fall back into the bulb after removing thermometer from body part. Thus , due to constriction , the temperature of human body can be taken easily and accurately.
kink prevents the mercury present in the capillary to fall back in the bulb. So it separates the mercury at high temperature and cool mercury and allows the user to note the reading when the thermometer is taken out.Kink in the Mercury thermometer helps to prevents the falling of mercury present in capillary tube into bulb. It ensures that the user takes the correct reading of temperature.
A slight bend or kink in the capillary tube of the clinical thermometer near the bulb ensures that the mercury does not move back into the bulb when the thermometer is taken out of a person's mouth for reading.The clinical thermometer has a kink (or constriction) in its tube to prevent the back flow of mercury into the bulb whereas a laboratory thermometer has no kink.The kink breaks the connection between the mercury in the capillary and the bulb containing it at the lower end of the thermometer. ... However, a laboratory thermometer has to measure temperature change continuously. The mercury level has to go up and down continuously. So, there is no kink in a laboratory thermometer.
If kink is absent in the thermometer : The instrument will continue to function exactly as it did when the scale/kink was present. The only difference is that the operator will no longer be able to extract any useful information from the thermometer.
The Italian, Santorio Santorio (1561-1636) is generally credited with having applied a scale to an air thermoscope at least as early as 1612 and thus is thought to be the inventor of the thermometer as a temperature measuring device.