Why Was the Fahrenheit Scale Originally Based on Armpits and Salty Ice? A Deep Dive into Temperature History

Have you ever looked at a weather forecast and wondered why, in the Fahrenheit system, water freezes at the seemingly random number of 32 degrees? Why not a neat and tidy zero? The answer is a fascinating journey back to the early 18th century, involving a brilliant inventor, a quest for consistency, and a few surprisingly strange reference points. It turns out the original Fahrenheit scale was a product of its time, ingeniously based on what its creator could reliably measure—including salty ice and the warmth of the human body.

This blog post will unravel the history behind Daniel Gabriel Fahrenheit's iconic temperature scale. We will explore the three peculiar anchor points he chose and understand why these unconventional standards were, in fact, a revolutionary step forward for science.

The Man with a Plan: Daniel Gabriel Fahrenheit

Before the 1700s, measuring temperature was a chaotic affair. Thermometers existed, but they lacked a standardized scale. A "hot day" in London might register a completely different number than a "hot day" in Paris, even on instruments from the same maker. Daniel Gabriel Fahrenheit, a German-born physicist and instrument maker living in the Dutch Republic, sought to end this confusion.

His goal was to create a reliable and reproducible temperature scale that could be used by scientists everywhere. To do that, he needed fixed points—stable temperatures that he could consistently recreate in his laboratory to calibrate his high-quality mercury thermometers. This is where things get interesting.

Setting the Standard: Fahrenheit's Three Fixed Points

Unlike the modern Celsius scale, which is neatly based on the properties of water (0° for freezing, 100° for boiling), Fahrenheit based his system on three different reference points he could trust.

1. The Zero Point: Salty Ice (0°F)

Fahrenheit needed a definitive "zero" that was colder than freezing water. He found it in a "frigorific mixture"—a slurry of ice, water, and a salt (ammonium chloride or common sea salt). This mixture stabilizes at a temperature well below the freezing point of plain water. By defining this as 0°F, Fahrenheit established the coldest temperature he could consistently reproduce in his lab. It was a practical, reliable baseline for his entire system.

2. The Freezing Point: Pure Water (32°F)

Fahrenheit’s second fixed point was the temperature at which pure water freezes. Initially, he set this mark at 30 degrees, but he later adjusted it to 32. This small change allowed for a more convenient 180-degree interval between the freezing and boiling points of water, a feature that would become a defining characteristic of his refined scale.

3. The Warm Point: The Human Body (96°F)

For his upper reference point, Fahrenheit turned to the most consistent source of warmth he knew: the human body. According to historical accounts, he measured the temperature of a healthy person, either orally or under the armpit, and designated this point as 96°F.

Why 96? This number wasn't arbitrary. It was a highly divisible number (by 2, 3, 4, 6, 8, and 12), which made it easy for him to mark precise gradations on his thermometers. This mathematical convenience was a key consideration in the age before digital calculators. Of course, we now know the average human body temperature is closer to 98.6°F. Fahrenheit’s measurement was slightly off, likely due to the placement of the thermometer and the precision of his instruments, but it was consistent enough to serve his purpose.

From Armpits to a Global Standard

Fahrenheit’s scale was a massive success. For the first time, scientists had a standardized way to measure and communicate temperature. His system was eventually refined, and the two points now used to officially define it are:

• 32°F: The freezing point of water.
• 212°F: The boiling point of water at standard sea-level pressure.

This refinement locked the scale in place, cementing 98.6°F as the new normal for human body temperature. While most of the world has since adopted the Celsius scale, Fahrenheit’s system remains the standard in the United States and a few other countries.

Conclusion

So, the Fahrenheit scale wasn't based on random numbers. It was a pioneering effort to create a scientific standard from the most reliable reference points available in the 18th century. What seems odd to us today—a zero point of salty ice and a normal body temperature of 96—was actually a stroke of genius. Daniel Fahrenheit used the world around him to craft a system that brought order to the chaos of temperature measurement. The next time you check the weather in Fahrenheit, you can appreciate the bizarre but brilliant history of armpits and salty ice that made it all possible.