Kink is used to stop the mercury expanding or condensing. For example in laboratory thermometer there is no kink, so when we take out a laboratory thermometer from a jar filled with hot water the temperature again comes to the room temperature.. and when we put a clinical thermometer in hot water, even after removing it from the jar the temperature remain the same.. it is mostly used by the doctors for checking the body temperature.

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Kink prevent the backward flow of mercury when we are taking the reading.