Six's thermometer

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A Maximum Minimum thermometer, also known as Six’s thermometer after its inventor. 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 thermometer is a thermometer which can measure the maximum and minimum temperature during a given time, devised by James Six in 1782. It is also known as a Maximum minimum thermometer. It is still in common use wherever a simple way is needed to measure the extremes of temperature at a location, e.g. in meteorology and horticulture.

[edit] Description

It consists of a U-shaped capillary tube with two separate temperature readings, one for the maximum temperature and one for the minimum temperature. There are bulbs at the top of each arm of the U-shaped tube. The one at the top of the minimum reading scale contains alcohol, the other contains a vacuum or low pressure alcohol vapour.

In the bend of the U is a section of mercury which is pushed around the tube by the expansion and contraction of the alcohol in the first bulb. It is the alcohol which measures the temperature, the mercury indicates the temperature reading on both scales.

At any given time the position of the mercury should be the same on both the maximum and minimum scales. If not then the instrument scales are not correctly positioned.

As the mercury moves it pushes 2 small steel markers which are sprung into the tube. They record the furthest point reached by the mercury in each arm of the tube. When the temperature reverses and the mercury is moved in the opposite direction by the expansion or contraction of the alcohol, the sprung markers remain in the tube at the furthest position they have been pushed by the mercury. They thus record the extremes of temperature experienced by the device since it was last reset.

The markers are reset by using a small magnet which can drag the markers along the tube so that they again rest on the surface of the mercury. In some designs the tube is horizontal and the markers un-sprung so the device is reset by turning it to the vertical so that gravity returns the markers to the mercury.

Its important to note that the alcohol is used as the thermometric liquid, while the mercury only serves as an indicator.