Invention of the Clinical Thermometer

Locale Padua and Venice (now in Italy)

Date 1612

The clinical thermometer was invented by Sanctorius, an Italian physician and physiologist, who described in his 1612 book his use of the heat-measuring device. Sanctorius’s thermometer, although not the first to observe heat, was the first to measure it

Key Figures

• Santorio Santorio (Sanctorius; 1561-1636), Italian physician, physiologist, and inventor of medical instruments
• Giovanni Alfonso Borelli (1608-1679), Italian physicist and physiologist
• Robert Boyle (1627-1691), Irish mathematician, physicist, and chemist
• Cornelis Drebbel (1572-1633), Dutch glassblower, chemist, and inventor
• Gabriel Daniel Fahrenheit (1686-1736), German physicist
• Ferdinand II de’ Medici (1610-1670), grand duke of Tuscany, r. 1621-1670, scientist, and inventor
• Robert Fludd (1574-1637), English physician, chemist, and mystic
• Galileo (1564-1642), Italian physicist and astronomer
• Otto von Guericke (1602-1686), German architect, engineer, physicist, and inventor
• Robert Hooke (1635-1703), English mathematician and general scientist
• Athanasius Kircher (1601-1680), German Jesuit priest, general scientist, and inventor
• Jean Leuréchon (1591-1624), French Jesuit priest and mathematician
• Jean Rey (1583?-1645?), French physician and chemist
• Evangelista Torricelli (1608-1647), Italian mathematician and physicist

Summary of Event

Before the last decade of the sixteenth century, there was no known way to quantify heat. All means of measuring it were subjective. About 1593, Galileo began experimenting with possible solutions to this problem, and by 1596, he had invented a device that showed differences in temperature. His device showed a rising or falling column of water in an unsealed glass tube, rather like a thin Florence flask inverted in a bowl of water. As air in the bulb at the top of the tube heated and expanded, it pushed the water down; as it cooled and contracted, it allowed the water to rise.

This instrument, commonly called the air thermometer, was sensitive but inaccurate. Its inaccuracy mainly came from the unrecognized effects of barometric pressure on the unsealed system. Barometric pressure was unknown until Evangelista Torricelli described it in the 1640’s and Otto von Guericke, Robert Hooke, and Robert Boyle investigated it in the 1650’.

Galileo did not invent the familiar, decorative Galileo thermometer, which indicates temperature with liquid-filled glass balls rising and falling in a liquid sealed inside a glass column. This was developed later, probably by Athanasius Kircher, from a principle Galileo discovered. The principle stated that the density, and therefore the buoyancy, of a liquid varies according to its temperature.

In the second and third decades of the seventeenth century, Sanctorius (Santorio Santorio ) invented several variations of the air thermometer, at least three of which had clinical applications. The first mention of a clinical thermometer, and one of the earliest published mentions of any thermometer, is in Sanctorius’s 1612 book Commentaria in artem medicinalem Galeni (commentary on the art of medicine of Galen). He did most of his work on thermometry when he was professor of theoretical medicine at the University of Padua from 1611 to 1624, and he published his results shortly thereafter. His 1625 book Commentaria in primam fen primi libri canonis Avicennae (commentaries on the first part of the first book of the Canon of Avicenna) describes and illustrates one uncalibrated and three calibrated clinical air thermometers.

The first of these four woodcut illustrations shows a Florence flask with an unusually long and thin neck inverted in an open vessel of water. The patient’s hand would enclose the bulb of the flask and within a few minutes the thermometer would react. Even though it had no scale, two threads tied around the neck could be slid up or down to record temperatures. The second woodcut is the most remarkable, as it illustrates the world’s first oral thermometer. It had a tiny bulb at the top that was placed in the patient’s mouth. The scaled neck was even longer and had four switchback curves. The length improved accuracy and the curves saved space. The third woodcut depicts a similar instrument, but with a larger bulb at the top to take the temperature of the patient’s hand. The fourth woodcut shows the least successful of the three calibrated models, with the bulb at the top replaced by a funnel into which the patient would exhale.

Sanctorius believed that quantifying the heat of healthy as well as diseased bodies could help medical diagnoses. Keener instruments were required because correlating temperatures and other symptoms with specific ailments was the key to clinical progress in this area and because differences in body heat could be quite small. Sanctorius was one of the originators of the idea that, to be effective, medicine and its instruments must be precise. He was an iatromechanist in medicine. Iatromechanism is a theory that states the phenomena of human physiology, health, and disease can be quantified and are best understood numerically. Iatromechanism reduced physiology to physics, mechanics, and mathematics, while its main ideological rival, iatrochemism, reduced physiology to chemistry and alchemy.

Sanctorius’s greatest achievement in thermometry was being the first to attempt to quantify heat by adding a scale. A Latin manuscript called Mathematica maravigliosa, written in Rome and dated 1611, contains the earliest known illustration of a calibrated air thermometer. This instrument, attributed to Sanctorius, consists of two volumetric flasks, the thinner-necked flask inverted as far as possible into the thicker-necked flask, with the column of liquid at about the midway point of the two necks. The outer neck is scaled in eight divisions of six degrees each.

Sanctorius’s experiments with calibration led to the first genuine thermometer. Strictly speaking, what Galileo invented was the thermoscope, not the thermometer, because it allowed its user to see heat but not to measure it. In 1624, Jean Leuréchon coined the word thermomètre from two Greek words, thermos (hot) and metron (measure), to name the calibrated device and distinguish it from the noncalibrated variety.

Robert Fludd independently invented a calibrated air thermometer in London around 1615. By the late 1630’, this “weather glass” was a popular commercial item in Britain. Cornelis Drebbel developed a calibrated J-shaped air thermometer in the Netherlands before 1625. Other early experimenters with air thermometers include Kircher in the 1640’s and Guericke about 1660.

Thermometry entered an important new phase when Jean Rey invented the first practical liquid thermometer between 1630 and 1632, a Florence flask with a long calibrated neck and a small bulb at the bottom. Heating or cooling the liquid in the bulb would expand or contract it in the neck, but since the top was open the instrument was still subject to air pressure. The next step was to seal the thermometer and close the system. Duke Ferdinand II de’ Medici of Tuscany built the first sealed liquid thermometer in 1641 and by 1654 had much improved its design. Hooke’s Micrographia (1665) describes his sealed, four-foot, wine-filled thermometer, for which he attempted to standardize a scale with one fixed point, the freezing point of water.

Significance

By 1709, Gabriel Daniel Fahrenheit had invented the sealed alcohol thermometer, followed in 1714 by the sealed mercury thermometer. He published his familiar temperature scale in 1724, setting five key points: zero as the temperature of a mixture of equal weights of snow and dehydrated sea salt, 32 as the temperature of the equilibrium of ice and water, 96 as the axillary temperature of the human body, 212 as the boiling point of water, and 600 as the boiling point of mercury. Normal healthy human body temperature is between 98.2 and 98.6 degrees (now called degrees Fahrenheit) and the boiling point of mercury is really 675.1, but the Fahrenheit scale remains acceptable for many applications, especially in the United States, even though the Celsius and Kelvin scales have been the standards in science since the nineteenth century.

Developing the means to measure changes in body temperature paved the way for rapid progress in physiology and in the clinical methods that depend upon accurate functional knowledge of physiology. Sanctorius’s contribution led directly to better understanding of the respiratory and cardiovascular systems. Iatromechanism made further progress when Giovanni Alfonso Borelli’s clinical and animal experiments in the 1660’s challenged and overturned the ancient Galenic idea that the heart heats the body and the lungs cool it, thus maintaining ideal body temperature within a healthy balance of the four humors.

Bibliography

Castiglioni, Arturo. “Life and Work of Sanctorius.” Medical Life 38 (1931): 730-785. The standard biography and the most detailed treatment of Sanctorius in English.

Chang, Hasok Chang. Inventing Temperature: Measurement and Scientific Progress. New York: Oxford University Press, 2004. A philosophical and historical analysis of the several theories of measuring temperature.

Major, Ralph H. “Santorio Santorio.” Annals of Medical History, n.s. 10 (1938): 369-381. A eulogistic biography that highlights the development of the thermometer and other instruments. Major’s photograph collection at the Clendening History of Medicine Library, University of Kansas Medical Center, Kansas City, contains images and descriptions of several seventeenth century thermometers, including three from Santorio.

Middleton, William Edgar Knowles. A History of the Thermometer and Its Use in Meteorology. Baltimore: Johns Hopkins University Press, 2003. A reprint of the well-reviewed 1966 edition, which, despite its title, also contains much information on the history of clinical thermometers and three chapters on the seventeenth century.

Mitchell, S. Weir. The Early History of Instrumental Precision in Medicine. New York: Burt Franklin, 1971. A reprint of a groundbreaking medical historiography that considers the contributions of Sanctorius to the development of accurate diagnosis.