Six's thermometer

Six's Maximum and Minimum thermometer consists of a U-shaped glass tube with two separate temperature scales set along each arm of the U.

[2] The maximum and minimum readings are recorded by two small steel markers which are sprung into the capillary tube so that they can slide, but only if a force is applied to them, either by being pushed by the mercury or under the influence of an external magnet.

Before a new maximum or minimum reading can be taken, the thermometer must be reset by moving the markers to the top of the mercury, usually by hand using a small magnet to slide them along the tube.

The markers thus record the furthest point reached by the mercury in each arm of the tube, and thereby the highest and lowest temperatures since the last reset.

Typically the thermometer is reset every 24 hours to measure the diurnal temperature variation, the positions of the ends of the markers nearest to the mercury are examined.

When manual control is operated, the plate is pushed away from the U-shaped tube, freeing the markers which then drop under gravity to the surface of the mercury.

A maximum–minimum thermometer. The scales are Fahrenheit on the inside of the U and Celsius on the outside. The current temperature is 23 degrees Celsius, the maximum recorded is 25, and the minimum is 15; both read from the base of the small markers in each arm of the U tube. The bulbs are hidden by a plastic housing.
Detail of the thermometer bulbs of the maximum-minimum thermometer shown above. The left-hand (minimum arm) bulb is full of alcohol. This bulb measures the temperature by the expansion and contraction of the liquid. The right-hand (maximum arm) bulb contains alcohol and a bubble of low-pressure gas or alcohol vapor. This bulb accommodates the expansion in the other bulb and allows the train of alcohol and mercury to move in the U-shaped tube as the temperature changes. [ 3 ]
Marker showing the maximum temperature. Mercury has retreated to about 26.5°C as the temperature has dropped below its maximum of about 28.3°C.