is a thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...
which can measure the maximum and minimum temperature
A Maximum Minimum Temperature System or MMTS is a temperature recording system that keeps track of the maximum and minimum temperatures that have occurred over some given time period....
s reached over a period of time, usually during a day. It is commonly used wherever a simple way is needed to measure the extremes of temperature at a location, for instance in meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
Horticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...
It is also commonly known as a maximum minimum thermometer
, of which it is the earliest practical design and the most common type used.
It is an example of a registering thermometer
, that is a thermometer that keeps a record of where the temperature has been in the past.
It gives three readings: the current temperature, the highest temperature reached since it was last reset, and the lowest temperature reached since it was last reset.
It was invented by Englishman James Six
James Six was a British scientist born in Canterbury. He is noted for his invention, in 1780, of Six's thermometer, commonly known as the Maximum minimum thermometer...
in 1782, and named after him. The same basic design remains in use today.
This describes the traditional construction. Modern designs may substitute materials for less toxic ones but operate in the same way.
It 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
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, the other contains a vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
(or low pressure alcohol vapour).
In the bend of the U is a section of mercury
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, a metal which is liquid at normal temperatures. This is pushed around the tube by the thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
and contraction of the alcohol in the first bulb as it responds to the external temperature. The near vacuum in the other bulb allows free movement of the alcohol and mercury. It is the alcohol which measures the temperature; the mercury indicates the temperature reading on both scales. This is unlike a normal mercury thermometer
A mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German physicist Daniel Gabriel Fahrenheit in 1724 and is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the...
, in which the expansion and contraction of mercury itself indicates 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 reading 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 cannot easily slide unless a small force is applied to them.
Before a maximum or minimum reading can be taken, both markers must be returned to the top of the mercury, usually by hand using a small magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
to slide them along the tube. Any change in temperature after that time will push one of the markers along with it.
If the temperature is rising, then the maximum scale marker will be pushed. If it falls after the reset, then the moving mercury will push the minimum scale marker. Often both things happen one after another as the temperature changes, say through a twenty-four-hour period (see diurnal temperature variation
Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights.-Temperature lag:Temperature lag is an important factor in diurnal temperature variation...
). The markers thus record the furthest point reached by the mercury in each arm of the tube. They record the extremes of temperature experienced by the device since it was last reset. The thermometer is usually reset every day, but if left for longer the readings would show the highest and lowest temperatures encountered since the instrument was last reset.
To take a reading, 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 over the period of measurement.
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 immobilize the markers unless they are pushed by the thermal expansion of the device. When a central button is pressed, the plate is pushed away from the U-shaped tube, freeing the markers which then sink under gravity until they rest again on the surface of the mercury.
Another design has the U orientated horizontally and the markers completely free and unsprung. In this model, the reset is carried out by turning the U to the vertical so the markers sink to rest on the mercury and the U is then returned to the horizontal.
The Six's thermometer is notoriously 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 like a large old school fever thermometer. The centrifugal force will then force the mercury together again, hence the separation is corrected.
Should the markers be partly buried in the mercury, they can either be pulled up again with the magnet or put to a temperature change that leaves the markers uncovered. It is however important not to use a magnet to pull up the markers in the push-button type because there is a risk of damaging the weak magnet behind the scale or magnetising the steel in the markers with equally fatal consequences – either weakening or enhancing the pulling force against the capillary tube on all or part of the scale.
Mercury Free Maximum Minimum Thermometer
The original Six's thermometer design contains mercury which has been banned for most uses in the EU and some other parts of the world.
This does not mean the end of Six's thermometer: 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. The thermometer operates in exactly the same way as the old mercury model.