Thermometer is a device used to measure temperature, by
using materials that change in some way when either heated or cooled. In a
mercury or alcohol thermometer, the liquid expands as it is heated and
contracts when it is cooled, so the length of the liquid column is longer or
shorter depending on the temperature. Modern thermometers are calibrated in
standard temperature units such as Fahrenheit or Celsius.
In 1596, Galileo Galilei is often claimed to be the inventor
of the thermometer. However the instrument he invented was a thermoscope, the
predecessor to the thermometer. The
thermoscope is a thermometer without a scale. It indicates differences in
temperature if the temperature is higher or lower.
Savants had found out that it might be possible to use air
and water to invent a "ruler" or scale that would mark the grades
from cold to hot and back again. They experimented with thermoscope involving a
column of air in a tube with one end in a container of coloured water. In 1610,
Galileo tried it with wine instead, and so is credited with the first alcohol
thermometer.
The Italian, Santorio Santorio (1561-1636) is generally appreciated
for applying a scale to an air thermoscope around 1612 and thus is thought to
be the inventor of the thermometer as a temperature measuring device. Santorio's
instrument was an air thermometer. Its accuracy was poor due to the effects of
varying air pressure on the thermometer.
The first sealed thermometer was designed in 1641 for the
grand duke of Tuscany. It used alcohol, and it had degree marks. But the man who
used the freezing point of water as the "zero" or starting point was
a Londoner, Robert Hooke, in 1664. An astronomer called Roemer in Copenhagen
chose ice and the boiling point of water as his two reference points, and
started keeping weather records.
Daniel Gabriel Fahrenheit (1686-1736) was the German
physicist who invented the alcohol thermometer in 1709, and the mercury
thermometer in 1714. In 1724, he introduced the temperature scale that bears
his name - Fahrenheit Scale.
In 1724, a German instrument maker called Gabriel Fahrenheit
settled on mercury as the most suitable liquid for measuring temperature.
He calibrated his
first thermometer using a mixture of ice and water with sea salt as his zero.
But salt water has a much lower freezing point than ordinary water, so for his
purposes he chose his freezing point as 30, and the temperature inside the
mouth of a healthy human as 96.
With those three points, he established the
boiling point of water at 212 and later adjusted his freezing point of water to
32. This way, he could count 180 degrees between boiling and freezing, at sea
level.
Two decades later, Linnaeus - the Swede who invented the
taxonomic system naturalists now use for naming species - and a Swedish
astronomer called Anders Celsius separately worked out a scale of just one
hundred degrees between freezing and boiling points. Because there were 100
steps between the two states, it was called a "centigrade" scale.
Lord Kelvin took the whole process one step further and led
to the invention of the Kelvin Scale in 1848. The Kelvin Scale measures the
ultimate extremes of hot and cold. Kelvin developed the idea of absolute
temperature, often called the "Second Law of Thermodynamics", and
developed the dynamical theory of heat.
He used the centigrade scale, but started from absolute
zero, the point at which all molecular motion stops, the lowest conceivable
temperature in the universe. This turned out to be -273.16C.