Min Max Thermometer


Six’s thermometer is a registering thermometer which can record the maximum and minimum temperatures reached over a period of time, for example, 24 hours. It is used to record the extremes of temperature at a location, for instance in meteorology and horticulture.


The thermometer consists of a U-shaped glass tube with two separate temperature scales set along each arm of the U. One of these is for recording the maximum temperature encountered and the other for the minimum temperature. The arms of the U-shaped tube terminate in sealed glass bulbs. The bulb at the top of the minimum reading scale arm is full of alcohol, the other contains a vacuum (or low-pressure alcohol vapour).[2]

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]

At the end of the U is a section of mercury, a metal that is liquid at normal temperatures. This is pushed around by the thermal expansion and contraction of the alcohol in the first bulb as it responds to the external temperature. The vacuum in the other bulb allows free movement of alcohol and mercury. It is the alcohol that measures the temperature; the mercury indicates the temperature reading on both scales. This is unlike a normal mercury thermometer, in which the expansion and contraction of mercury itself indicate temperature.[2]

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.

The thermometer shows a reading at the top of the mercury section on both the maximum and minimum scales; this shows the current temperature and should be the same on both scales. If the two readings are not the same, then the instrument scales are not correctly positioned or the instrument is damaged.[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. Any change in temperature after that will push one of the markers along with it. (If the markers are not reset, they will register maxima and minima only if they exceed the values already encountered.)

If the temperature rises, the maximum scale marker will be pushed. If it falls, the moving mercury will push the minimum scale marker. As the temperature varies, the markers will remain in their positions unless the temperature becomes higher (for maximum) or lower (for minimum) than already recorded, in which case the relevant marker is pushed further. 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. Their positions on the maximum and minimum scales show the highest and lowest temperatures encountered since the last reset.[4]

Design variations

In a variation of design, some models have unsprung markers held in place by a magnetic plate located behind the card showing the scales and close enough to the U-shaped tube to hold the markers in place unless they are pushed by the thermal expansion of the device. When a 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.

Another design has the U orientated horizontally and the markers completely free and unsprung. The reset is carried out by turning the U to the vertical so the markers sink to rest on the mercury, and returning it to the horizontal.


The Six’s thermometer is known for separations in the mercury column, in particular after shipment, though accidental knocks have been known causes as well. Separations can usually be corrected by swinging the thermometer as is done to reset a mercury clinical thermometer; the centrifugal force forces the mercury together again. Should a marker become partly buried in the mercury, it can either be pulled up again with the magnet or exposed to an extreme temperature that leaves the marker uncovered. If a strong external magnet is used to pull up the markers in the push-button type with the magnetic plate in place, there is a risk of damaging the weak magnet behind the scale, or magnetizing the steel in the markers either weakening or enhancing the pulling force against the capillary tube on all or part of the scale so that the thermometer no longer works properly.

Mercury-free maximum-minimum thermometers

The original Six’s thermometer design contains mercury, which has been banned for most uses in the European Union[5] and some other parts of the world. In 2006, S.Brannan & Sons Ltd, a UK company, was granted a patent for a mercury-free version of Six’s maximum-minimum thermometer: instead of mercury two immiscible liquids are used supporting an index[clarification needed]. The thermometer operates in the same way as the mercury version. Electronic thermometers often include a maximum-minimum registering feature


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