Six's thermometer

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.

Six's Maximum and Minimum thermometer is a thermometer that can show the maximum and minimum temperatures of a time period. British scientist James Six invented it in 1780. The same basic design is still in use.

Design

It is made of a U-shaped glass tube with two separate temperature scales. There is a scale on each arm of the U. One is for recording the maximum temperature and the other for the minimum temperature. The arms of the U-shaped tube end in sealed glass bulbs. The bulb at the top of the minimum reading scale arm is full of alcohol, the other tube contains a vacuum (or low pressure alcohol vapour).

How it works

In the bend of the U is a bit of mercury, a metal that is liquid at room temperature. This is pushed around by the thermal expansion and contraction of the alcohol in the first bulb as it responds to the outside temperature. The vacuum in the other bulb allows free movement of the alcohol and mercury. It is the metal indices that measures the temperature; the mercury shows the temperature reading on both scales. This is unlike a normal mercury thermometer, in which the expansion and contraction of mercury itself indicate temperature.

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. This is usually done by hand with 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 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. For this reason, the markers record the furthest point reached by the mercury in each arm of the tube. This corresponds to the highest and lowest temperatures since the last reset. Typically the thermometer is reset every day to measure temperature changes.