A thermometer is a device that measures temperature or temperature gradient (the degree of warmth or coldness of an object). Some of the principles of the thermometer were known to Greek philosophers two thousand years ago. As Henry Carrington Bolton (1900) noted, the development of the thermometer “from a rough toy to a precision instrument occupied more than a century.
Italian physician Santorio Santorio (Sanctorius, 1561-1636) is generally credited as the invention of the first thermometer, but its standardization was completed in the 17th and 18th centuries. In the Dutch Republic, in the first decades of the 18th century, Daniel Gabriel Fahrenheit made two revolutionary breakthroughs in the history of thermometry. He invented the mercury-in-glass thermometer (the first widely used, accurate, practical thermometer) and the Fahrenheit scale (the first widely used standardized temperature scale).
A thermometer has two important elements:
- A temperature sensor in which some change occurs with a change in temperature (for example, the bulb of a mercury thermometer in a glass or the pyrometric sensor in an infrared thermometer);
- Some ways to convert this change into a numerical value (for example, the visible scale marked on a mercury-in-glass thermometer or a digital readout on an infrared model).
Thermometers are widely used in technology and industry to monitor processes in meteorology, medicine, and scientific research.
An infrared thermometer is a type of pyrometer (bolometer). While a single thermometer can measure degrees of temperature, the readings on two thermometers cannot be compared unless they follow an agreed-upon scale. Today there is an absolute thermodynamic temperature scale. Internationally accepted temperature scales are designed to approximate this based on fixed points and interpolated thermometers. The latest official temperature scale is the 1990 International Temperature Scale. It ranges from 0,65 K (−272,5 °C; -458,5 °F) to approximately 1.358 K (1,085 °C; 1.985 °F). Thermometer with Fahrenheit (symbol °F) and Celsius (symbol °C) units.
The Early Years of the Development of the Thermometer
Fifty-degree thermometers from the mid-10th century, with black dots representing odd degrees and white representing increments of 17 degrees, on display at the Galileo Museum; Used to measure atmospheric temperatures
Various authors attribute the invention of the thermometer to the Hero of Alexandria. But the thermometer was not a single invention, but an improvement. The Hero of Alexandria (10–70 AD) knew the principle of expansion and contraction of certain substances, especially air, and described a demonstration showing the end of a closed tube partially filled with air in a vessel of water. The expansion and contraction of the air caused the position of the water/air interface to move along the pipe.
Such a mechanism was then used to indicate the temperature and coldness of the air with a tube in which the water level was controlled by the expansion and contraction of the gas. These devices were developed by several European scientists in the 16th and 17th centuries, notably Galileo Galilei and Santorio Santorio. As a result, devices were shown to produce this effect reliably, and the term thermoscope was adopted because it reflected changes in sensible heat (the modern concept of temperature had not yet emerged). The difference between a thermoscope and a thermometer is that the thermometer has a scale. Although Galileo is often said to be the inventor of the thermometer, there is no surviving evidence that he actually produced such an instrument.
The first clear diagram of a thermoscope was published in 1617 by Giuseppe Biancani (1566 – 1624); The first to show a scale, thus forming a thermometer, was made in 1625 by Santorio Santorio. It was a vertical tube closed at the top with an air bulb, with the lower end opening into a container of water. The water level in the tube is controlled by the expansion and contraction of air, so this is what we now call an air thermometer.
The word thermometer (in its French form) was first coined in 1624 by J. Leurechon describing an 8-degree scale in La Récréation Mathématique. The word comes from the Greek words θερμός, thermos meaning “hot” and μέτρον, metron meaning “measure”.
The disadvantage of the above instruments is that they are also barometers, that is, they are sensitive to air pressure. In 1629, Joseph Solomon Delmedigo, a student of Galileo and Santorio in Padua, published the apparently first description and illustration of a liquid thermometer in a sealed glass. It is described as having an ampoule at the bottom of a sealed tube partially filled with brandy. The tube had a numbered scale. Delmedigo did not claim to have invented this instrument. He did not name anyone else as its inventor.
In about 1654, the Grand Duke of Tuscany, Ferdinando II de' Medici (1610-1670), produced such an instrument, the first modern-style thermometer dependent on fluid expansion and independent of air pressure. Many other scientists experimented with various fluids and thermometer designs.
However, every inventor and every thermometer was unique – there was no standard scale. Early attempts at standardization added a single reference point, such as the freezing point of water. Although the use of two references to grade the thermometer is said to have been introduced by Joachim Dalence in 1668, Christiaan Huygens (1629-1695) had already proposed the use of degrees based on the melting and boiling points of water in 1665.
In 1694, Carlo Renaldini (1615–1698) proposed using them as fixed points on a universal scale. In 1701, Isaac Newton (1642-1726/27) proposed a 12-degree scale between the melting point of ice and body temperature.