Different authors have credited the invention of the thermometer to Cornelis Drebbel, Robert Fludd, Galileo Galilei or Santorio Santorio. The thermometer was not a single creation, however, but a development.
Philo of Byzantium and Hero of Alexandria knew of the principle that particular substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water. The enlargement and contraction of the air caused the position of the water/air interface to move along the tube.
Such a mechanism was later implemented to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the air. These devices were created by several European scientists in the 16th and 17th centuries, notably Galileo Galilei. As a result, devices were shown to produce this effect reliably, and the term thermoscope was adopted because it showed the changes in sensible heat (the concept of temperature was yet to arise). The varieties between a thermoscope and a thermometer are that the latter has a scale. Though Galileo is often mentioned to be the inventor of the thermometer, what he produced was thermoscopes.
Galileo Galilei also discovered that objects (glass spheres filled with aqueous alcohol) of slightly different densities would rise and fall, which is nowadays the foundation of the Galileo thermometer.
Today such thermometers are adjusted to a temperature scale.
The first clear diagram of a thermoscope was put out in 1617 by Giuseppe Biancani: the first showing a scale and thus constituting a thermometer was by Robert Fludd in 1638. This was a vertical tube, with a bulb at the highest point and the end immersed in water. The water level in the tube is controlled by the expansion and contraction of the air, so it is what we would now say to be an air thermometer.
The first individual to put a scale on a thermoscope is variously said to be Francesco Sagredo or Santorio Santorio in about 1611 to 1613.
The word thermometer (in its French form) first appeared in 1624 in La Récréation Mathématique by J. Leurechon, who mentions one with a scale of 8 degrees.
The above instruments suffered from the fault that they were also barometers, i.e. sensitive to air pressure. In about 1654 Ferdinando II de' Medici, Grand Duke of Tuscany, constructed sealed tubes part filled with alcohol, with a bulb and stem, the first modern-style thermometer, depending on the expansion of a liquid, and independent of air pressure. Many other scientists experimented with different liquids and designs of thermometer.
However, each creator and each thermometer was unique—there was no standard scale. In 1665 Christiaan Huygens suggested using the melting and boiling points of water as standards, and in 1694 Carlo Renaldini proposed implementing them as fixed points on a universal scale. In 1701 Isaac Newton put up the idea of a scale of 12 degrees between the melting point of ice and body temperature. Finally in 1724 Daniel Gabriel Fahrenheit made a temperature scale which now (slightly adjusted) has his name. He could do this because he made thermometers, using mercury (which has a high coefficient of expansion) for the first time and the quality of his production could provide a finer scale and greater reproducibility, leading to its general adoption. In 1742 Anders Celsius proposed a scale with zero at the boiling point and 100 degrees at the melting point of water, though the scale which now has his name has them the other way around.
In 1866 Sir Thomas Clifford Allbutt created a clinical thermometer that produced a body temperature reading in five minutes as opposed to twenty. In 1999 Dr. Francesco Pompei of the Exergen Corporation introduced the world's earliest temporal artery thermometer, a non-invasive temperature sensor which scans the forehead in about 2 seconds and provides a medically accurate body temperature.
Old thermometers were all non-cataloging thermometers. That is, the thermometer did not hold the temperature after it was implemented in a place with a different temperature. Determining the temperature of a pot of hot liquid required the user to leave the thermometer in the hot liquid until after registering it. If the non-registering (non-cataloging) thermometer was removed from the hot liquid, then the temperature indicated on the thermometer would immediately start changing to reflect the temperature of its new conditions (in this case, the air temperature). Registering thermometers are designed to hold the temperature indefinitely, so that the thermometer can be taken out and read at a later time or in a more convenient place. The first registering thermometer was designed and built by James Six in 1782, and the design, known as Six's thermometer is still in wide implementation today. Mechanical registering thermometers hold either the highest or lowest temperature recorded, until manually re-set, e.g., by shaking down a mercury-in-glass thermometer, or until an even more drastic temperature is experienced. Electronic registering thermometers may be made to remember the highest or lowest temperature, or to remember whatever temperature was present at a specified point in time.
Thermometers increasingly use electronic means to provide digital data or input to a computer.