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.
It was invented by Englishman James Six in 1782; the same basic design remains in use.
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
last reset.
Description This describes the traditional construction.
Modern designs may substitute less toxic materials 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, the other contains
a vacuum.
In the bend of the U is a section of mercury, a metal which is liquid at normal temperatures.
This is pushed around the tube by the thermal expansion 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, 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 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.
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 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 or lower 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.
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.
Maintenance 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 extremum 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 magnetising 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 Thermometer The original Six's thermometer design contains
mercury, which has been banned for most uses in the European Union 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.
The thermometer operates in the same way as the mercury version.
Other types Digital version
Bimetal version Electronic version
See also Maximum minimum temperature system
References
Two hundred years of the Six's Self Registering Thermometer Austin and McConnell, Notes and
Records of the Royal Society of London * Vol. 35, No. 1, Jul., 1980 at JSTOR
A History of the Thermometer and Its Uses in Meteorology by Amit Batra, Johns Hopkins
University Press, 1966; ISBN 0-8018-7153-0 The Construction of a Thermometer by James
Six, Nimbus Publishing Ltd,1980; ISBN 0-9507036-0-5 External links
Article on Six's thermometer at the Museum of the History of Science at Florence, Italy
Explanation of the working of Six's thermometer