When temperature falls, the constriction prevents the mercury
from being drawn back into the reservoir and the upper part
of the column remains at the extreme position reached until
it cools down (or is shaken down in the case of the minimum
thermometer) giving the user a chance to get a reading.
The minimum thermometer uses clear alcohol instead of mercury.
A small index made of black glass is placed within the tube
and rests on the meniscus of the alcohol column. As the alcohol
expands and contracts, readings are obtained using the position
of the index.