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.
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. As more “thermoscopes” were designed, 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.
In 1714 Daniel Gabriel Fahrenheit, a German speaking Dutch instrument maker suggest 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. While this was an awkward scale, it surely became popular. There were other scales that were in use then. For instance Rømer scale, wherein brine freezes at 0, water at 7.5, body temperature is 22.5, and water boils at 60 degrees.
In 1742, two years before his death, 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.
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.
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 !!!