Six's thermometer

It is also commonly known as a maximum–minimum, minimum–maximum, maxima–minima or minima–maxima thermometer, of which it is the earliest practical design.

The thermometer indicates the current temperature, and the highest and lowest temperatures since the last reset. It is an early type thermometer able to record both minimum and maximum daily temperatures.

The Six's thermometer cannot measure temperatures below -38.8 °C due to the solidification of mercury at this temperature. Although this is not an issue in most inhabited places, it can be one in particularly cold places, where thermometers using different liquids should be used to record these lower temperatures, such as a Rutherford-type minimum thermometer.

Description

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. One of these is for recording the maximum temperature encountered and the other for the minimum temperature.

In the bend of the U is a section of mercury, a metal that is liquid between −38.83 °C and 356.73 °C, which is pushed or pulled around the U 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 the alcohol and mercury. It is the contraction/expansion of alcohol that measures the minimum temperature, and the contraction/expansion of both alcohol and mercury that measures the maximum temperature. The top of the mercury indicates the temperature reading on both scales. This is unlike a normal mercury thermometer, in which the expansion and contraction of mercury alone indicate temperature.

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.

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.

Once a reading is taken, the thermometer can be reset to record future min/max temperatures by moving the markers to the top of the mercury, which is performed by hand using a small magnet to slide them along the tube in early versions of this thermometer, or by pushing a button that pushes a magnetic plate on the back of the thermometer away from the glass tube in later versions. In these later versions, the indices are held in place by their magnetic attraction to the magnetic plate behind, so when the magnetic plate is pushed away, the indices fall down to the top of the mercury column by gravity. 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.