What is the purpose of a kink in a thermometer?

A 'kink' in the glass tube which breaks the mercury as it contracts, storing the highest temperature reading. The glass tube is shaped like a lens to magnify the thin mercury thread. Shaking the thermometer resets the mercury back into the bulb.

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. That's the reason a person before taking a new reading must give a sudden shake to the clinical thermometer so that the mercury level comes to normal .

it doesn't. it was imagined up in the middle ages.