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A six-foot long thermometer? The surprising history of a vital medical tool

From Hippocrates to Galileo and hot water to fever maps

By Glenn McDonald

Consider the humble thermometer, that little slice of technology so important in our current viral dilemma. For centuries, medical thermometers have given caregivers a critical early diagnostic advantage. Now, the ability to detect fever has become a front-line tool for quarantine and separation protocols in the COVID-19 crisis.

“When a health care provider triages you for COVID-19, the first question they will ask is: have you had a fever?” says Samuel Schotland, a medical student at the University of Michigan and a Ph.D. student in the history of science and medicine at Yale. The answer to that question informs the many vital decisions that come next.  

The understanding that a high body temperature is a potential sign of disease goes back to the first days of Western medicine, circa 400 BC. The Greek healer Hippocrates and other pioneering physicians used their hands to detect fever or chills. In the Middle Ages, when medicine was based on the theory of bodily humors — blood, phlegm, and bile — fever was considered an important indicator.

Around 1600, Galileo developed the first all-purpose temperature-measuring instrument, the thermoscope, a water-filled tube which used glass bubbles and principles of density to gauge temperature. In 1612, the Italian physicist Santorio invented the first thermoscope to measure oral temperature with a numerical scale. These early 17th-century systems were water-based; more accurate devices that used alcohol or mercury came later.

The familiar liquid-in-a-glass-style thermometer first started seeing medical use in the late 17th century. It was designed to be inserted under the tongue or in the armpit, but it could be an awkward clinical experience — early thermometers were sometimes several feet in length. In the 18th century, physicians at the Viennese School of Medicine established the practice of bedside thermometers to continually monitor a patient’s temperature. Researchers started recording observed temperatures as related to specific illnesses.

We’re not quite at the level of the Star Trek medical tricorder, but we’re closer than you might think.

In 1886, English physician and historian Sir Thomas Clifford Allbutt developed the first medical thermometer to more-or-less resemble our modern devices. A mere six inches long, Allbutt’s thermometer was able to discern a steady body temperature in under five minutes, as opposed to the 20 minutes of competing, foot-long designs. The Allbutt device, relatively fast and highly accurate, served as the go-to solution for several decades.

In the late 19th and early 20th centuries, body temperature became an increasingly widespread way for doctors and nurses to gauge the condition of hospitalized patients. “Thermometry — using a thermometer to measure body temperature — marks an interesting moment in the history of nursing,” says Amanda Mahoney, chief curator of the Dittrick Medical History Center in Cleveland, Ohio, and a retired nurse.

Since the era’s mercury thermometers required careful placement in the mouth, armpit, or rectum for up to five minutes, hospitals came to rely heavily on nurses to collect temperatures. “Using a thermometer required a nurse’s expert clinical knowledge and careful recording of data,” says Mahoney. “An accurate temperature was also dependent on gaining the cooperation of the patient.” Gaining patient trust and encouraging their participation in their own care became key nursing skills, says Mahoney, and they remain so to this day.

Over the centuries, physicians learned to find locations on and in the body where the temperature is stable. The sublingual method — under the tongue — was the first and most popular, although medical literature shows rectal thermometry goes back to the 18th century at least. The axilla (armpit) method was used early, too, but was considered less accurate.

In the 20th century, the clinical thermometer evolved in several directions thanks to advances in chemistry and material science. Digital thermometers used electronic components to directly record body temperature — no mercury required.

The ear thermometer was invented in 1964 by Theodor Benzinger, M.D. Also known as the tympanic thermometer, it uses infrared sensors to determine temperature in the ear canal’s blood vessels. Used properly, it gives the most accurate reading in relation to brain temperature.

New digital technologies have opened up faster, less invasive methods for taking body temperature. We’re not quite at the level of the Star Trek medical tricorder, but we’re closer than you might think. Temporal artery thermometers — or “forehead thermometers” — are most often encountered in triage situations. They use infrared sensors to detect heat radiating from the major artery in the head. Recently, airports have tested full-body scanners known as infrared thermal detection systems that can measure body temperature, and sometimes detect fevers, from several meters away.

As important as thermometry is in hospitals and doctor’s offices, it can be even more important in the home — especially during times of pandemic. Fever detection is one of our most powerful tools in the fight against COVID-19. And if you have a thermometer in the home, you can help the cause directly.

“For low-risk patients with COVID-19 symptoms who are in self-quarantine at home, having a medical thermometer on hand can help a health care provider monitor their condition remotely,” Mahoney says. “This helps keep our hospitals available for patients with severe symptoms or complications.”

A recent advance in thermometer technology is also emerging as a weapon against the pandemic. In 2014, the California company Kinsa introduced a line of Internet-connected “smart” thermometers for the home and clinic. The Kinsa system allows users to upload temperature readings and other symptoms, via smartphone app, to a central database. The anonymized aggregate data have proven useful for tracking flu outbreaks geographically.

Now, recent reports from Kinsa suggest that social distancing and stay-at-home orders and advisories are working to slow COVID-19 in the U.S. The company is sharing its constantly updated map of fever levels online. By providing such important tracking data, the thermometer is once again writing new chapters in the annals of medicine. Galileo would be proud.

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Glenn McDonald is a writer based in Chapel Hill, North Carolina.

Illustration by Verónica Grech


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