It is used to measure the minimum
temperature within a period (usually daily) and is an alcohol-in-glass thermometer.
There is a very light, about 2cm dumb-bell shaped dark glass index within the alcohol
column. The index may be moved freely in the alcohol, but does not readily emerge
from the liquid due to the surface tension. When temperature falls and the alcohol
contracts, the index will be dragged towards the bulb by the surface of the alcohol
meniscus. When temperature rises again, the alcohol will flow over the index with
it remains stationary. Consequently, the end of the index nearest to the meniscus
indicates the lowest temperature reached since the last reset. In practice,
the minimum thermometer rests horizontally inside a screen with bulb on left side.
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