Fever Before Numbers
For most of history, fever was judged by touch and by symptoms such as flushing, chills, thirst and a quick pulse. Physicians in ancient Greece and Rome wrote about fevers, usually describing patterns such as the days between attacks, but there was no way to attach a number to heat. A patient who felt hot to a doctor’s hand might not be feverish at all, and a fever could go unnoticed.
Thermoscopes and Santorio
The instrument that eventually became the thermometer began as a device for showing changes in heat, not measuring them. Around 1600, experimenters in Italy, with Galileo often named among them, used a glass bulb and tube standing in water, in which the liquid level rose and fell as air expanded or contracted. These devices, now called thermoscopes, had no standard scale and also responded to air pressure.
Santorio Santorio, a physician in Padua, is often credited with applying such a device to the human body in the early 1600s, having patients hold the bulb in the hand or mouth, and with adding a scale. Others also experimented in this period, so the exact credit is shared. Santorio’s work was part of a wider interest in weighing and measuring the body, but his instruments were large and slow, and clinical practice did not change much at the time.
Standard Scales
Practical thermometers depended on sealed tubes and agreed scales. The German instrument maker Daniel Gabriel Fahrenheit made reliable mercury and alcohol thermometers in the early 1700s and devised a scale that bears his name. In 1742 the Swedish astronomer Anders Celsius proposed a scale with fixed points at the boiling and freezing of water, which was originally the reverse of the modern direction and was later flipped. These advances made temperatures comparable between instruments and places, which is a precondition for using a thermometer in medicine.
Wunderlich and the Fever Chart
The person most associated with turning thermometry into a clinical routine is the German physician Carl Reinhold August Wunderlich. In Leipzig in the 1850s and 1860s he and his staff took temperature readings on very large numbers of patients, recorded them on charts and compared the curves across diseases. His 1868 book on temperature in disease argued that fever was a sign of illness to be tracked, not a disease in itself, and it suggested that a reading around 37 degrees Celsius was typical for adults. Because his figures are often quoted, it is worth noting that modern studies with better instruments have suggested that average normal temperature is a little lower and varies with the person and time of day. Wunderlich’s lasting influence was the habit of recording temperature as a number and plotting it over time.
The Problem of Size
Early clinical thermometers were not convenient. They could be a foot long, needed to remain in place for some 15 to 20 minutes, and had to be read while still in position, since the mercury fell as soon as the instrument was removed from the body. In 1867 the English physician Thomas Clifford Allbutt described a much shorter version, about six inches long, that could produce a reading in a few minutes. Short thermometers with a narrowing in the glass tube that held the mercury in place after removal, called maximum thermometers, followed, so the reading could be taken after the instrument left the body. The details of who first designed the constriction and when are told in several versions.
Together these changes made it practical for a nurse or doctor to take temperatures repeatedly through a day. Nursing routines in hospitals developed around the practice, and the temperature chart at the foot of the bed became a familiar sight.
Twentieth Century Changes
- Mouth, armpit, rectum and later ear: Different body sites give slightly different readings, and conventions about which to use varied by place and age group.
- Digital thermometers: Electronic versions with a sensor and a display appeared in the later twentieth century, giving fast readings without a glass tube.
- Infrared devices: Ear and forehead thermometers measure heat radiation from the skin or eardrum, and became common in clinics and homes.
- Mercury removal: Because mercury is toxic and glass can break, many health authorities encouraged a move away from mercury-in-glass fever thermometers, and sales were restricted in a number of places.
What the Thermometer Changed
The thermometer showed that bodies could be measured, which helped establish a more quantitative style of medicine. It sat alongside other bedside instruments such as the stethoscope and blood pressure cuff in the toolkit of the nineteenth and early twentieth century clinician.
A temperature reading is only one piece of information, and its meaning depends on context. This article is a history of the instrument, not a guide to interpreting a reading or treating an illness.