Fever is a common symptom of an illness. It is a perception of warmth, often coupled with a mix of feeling cold, shaking and an urge to cover oneself with a blanket. As fever leaves our body, sweat takes over, leaving us drenched and weak. Doctors started giving fever a number only in later half of 19th century, as there was no clinical thermometer before then.
The earliest thermometers
While perception of either hot or cold is a daily occurrence, there was no device to measure it, till 1610 when Galileo invented first “thermoscope”. Prior to him scientists had tried to use a water column to measure heat. Galileo developed a glass tube filled with alcohol, and as temperature rose, the column became higher.
Two years later in 1612, another Italian physiologist Santorio Santorio calibrated the tube and was first to attempt measurement of human temperature. This Sanctorius thermoscope was long, cumbersome and hence difficult to use. By 1650s, we realized that barometric pressure also influenced column of a thermoscope. In 1654, Grand Duke of Tuscany in Florence, Italy designed earliest sealed, ‘spirit-in-glass’ thermometers. While these ‘Florentine thermometers’ gained popularity, they did not have a scale. There was no unanimity about assigning numbers for a temperature. What should be the number when ice melts, or water boils, and under which conditions, was an unsettled question.
Fahrenheit and his scale
Daniel Gabriel Fahrenheit a German speaking Dutch instrument maker solved the calibration or scale problems with thermometers. Born on May 24, 1686 he was an unlikely physicist. He was orphaned as both is parents died of mushroom poisoning on the same day in 1701. Devoid of funds, he fell into a debt trap and fled Amsterdam. Meanwhile, fate introduced him to Florentine thermometers, and he realized that alcohol is not a good medium to scale temperature. He discovered that mercury was better as it expands without boiling, within the range of measurable temperatures. Further, unlike alcohol it doesn’t wet the walls of glass tubes. This meant greater accuracy and reliability over time.
In 1714 Fahrenheit, suggested a scale for temperature. He initially chose freezing point of water as 30, human temperature as 96, and boiling water as 212. Later freezing point was made 32, so that he could count 180 degrees between boiling and freezing. Both 180 and 32 are numbers that can be divided into smaller whole numbers. This was a modification of Rømer scale, wherein brine freezes at 0, water at 7.5, body temperature is 22.5, and water boils at 60 degrees. Despite his groundbreaking work, Fahrenheit died in poverty in 1736. Probably poisoned by the same mercury he extensively worked with.
Celsius brings a metric system
Six years later in 1742, a Swedish astronomer Anders Celsius, proposed a scale from 0 to 100. While in this scale 0 was boiling point of water, and 100 was its freezing point – this 100-point or centigrade scale made sense. It reduced dependence on negative numbers for most winter temperatures in Sweden.
In 1745, a year after Celsius’s death, another biologist Carl Linnaeus reversed the scale, and 0 became melting point and 100 a boiling point. In 1789 French overthrew monarchy and adopted democracy. They also adopted metric system, and found Celsius scale to be attractive.
Measuring human temperature
While the two scales – Fahrenheit and Celsius came together, it was still tough to measure human temperature. While in normal humans it was 96 on Fahrenheit and 37 on Celsius scale, the thermometers were more than a foot long, and took about 20 minutes to measure.
The first physicians to use thermometer measurements in clinical practice were a Dutch physician Herman Boerhaave and Gerard L.B. Van Swieten, founder of the Viennese School of Medicine. His student Anton de Haen at Vienna taught to all his students that a physician’s touch was inadequate, and temperature needs to be measured, the measurement was so tough, that these efforts were given up.
In 1867, an English physician Sir Thomas Allbutt invented the first practical medical thermometer. This was 6 inches long, and took in 5 minutes to record. By next year, in 1868 a German physician Reinhold Wunderlich took over a million temperature records in 25000 patients. With this we were able to conclude that in humans temperature is between 36.5 to 37.5 °C
As science expanded in 20th century, thermometer became an integral medical instrument. Temperature charts were drawn, patterns described, and were linked to diseases. Fevers could now be defined both as a pattern as well as in numbers.
In 1960s world adopted metric system, and hence a scale of 100 (centigrade or Celsius scale) was a natural choice. US however has persisted with Fahrenheit. This is because, it was felt to be more intuitive. In clinical world we continue to use this scale, where 99 is fever, and above 104 it is really high.
Importance of these numbers is carried on to this day. During COVID pandemic, many entry-points were equipped with temperature measurement devices. Thermometer was elevated to the status of a virus-stopper and a tool that could protect others !!!
