In the upcoming series of posts, I will deal with selected chapters from the fascinating history of thermodynamics. In the first post we opened with the basic question: What is temperature?
Here is a question for you, what is temperature? Maybe to some of you this sounds like a trivial question - temperature is what the thermometer shows, temperature measures how hot or cold something is, right? But where did the idea come from? How do you quantify the degree of heat? And why degrees?
Even before the thermometer was invented, temperature was a subjective feeling, and mostly unreliable. People thought that wells heat up on a cold day and cool down on a hot day, or that in winter the air is hotter than ice, but of course they had no idea that this was a purely physiological sensation resulting from the conduction of heat. In fact, the origin of the word temperature has nothing to do with heat or weather at all. Temperature comes from the Latin word tempera which means to mix! But mix what?
The connection between temperature and the degree of heat started as far back as Hippocrates, in the 5th century BC. The Greek doctor believed that there is a balance between the body's fluids and if the balance is violated, that is, if they are not mixed properly, diseases will emerge that will cause the body to heat up or cool down more than usual. Hundreds of years later, around the 2nd century AD, another Greek physician named Claudius Galanus developed Hippocrates' idea and took it a step further - if fluid imbalance causes body temperature to change, then why not the other way around? Galanus claimed that the weather outside can affect the mix of fluids in the body and these affect a person's character.
Weird idea: human temperature - mixing boiling water with ice
Glenus assumed that "temperament", as he called it himself, depended on a person's geographical location. People who live in cold areas will be barbaric and wild, those who live in warm areas will be relaxed and meek and those who live in the Mediterranean region, the area from which he comes, were considered the superior people. According to Glenus, the perfect weather makes the people of the Middle East intellectual and people with good judgment. To illustrate the desired degree of heat, he mixed boiling water with ice, let it equilibrate and said that this was the optimal degree of heat for a person, or the neutral temperature.
The strange idea was neglected in Europe during the Dark Ages, but doctors from Arab countries kept the embers and even defined 4 degrees of heat above the neutral measure and 4 degrees of cold below it. In the 16th century, with the end of the Dark Ages, the strange idea returned to Europe thanks to the Swiss doctor Johannes Hessler, who tried to quantify the degree of human temperament according to the latitude where he lived. Latitudes are measured in degrees, so the temperature of humans is tied to a certain height. The scale divided by the Arab doctors into nine years was very suitable for Hessler's world view. According to Hessler, who lives in the Mediterranean region, around 40-50 degrees, lives at a neutral temperature. Those living in the 90 degree area were barbaric and those near 0 degrees, that is in the desert or tropical areas, were lax people. Of course this is nonsense and he was wrong, after all, on average, the temperature of a healthy person does not change depending on his geographical location, but how could they know? After all, they didn't have a thermometer.
Who invented the first thermometer?
Historians of science differ on who invented the thermometer first, but one thing is certain, the one who overwhelmingly claimed to be the sole inventor was Galileo Galilei. Even at first, there was no agreement on how to calibrate the thermometer and the lack of precision made it difficult for scientists to compare the experiments they performed. The deep confusion reached the point where in the middle of the 19th century about 18 different units of measure were counted for temperature. The most famous of them was invented by Celsius who suggested setting 0 as the temperature at which water boils and 100 degrees as the temperature at which water freezes. Yes, I was not confused, he chose the scale to be the opposite of what we know to avoid negative numbers but after his death the scale was reversed again. The use of one scale or another was due to ego reasons and was used as a tool for political confrontation, but following a new concept that emerged, the fight in the scientific community was decided. At that time, scientists showed that the pressure and volume of a gas are related to its temperature as follows:
where P represents the pressure, V the volume, N the number of particles, kB the Boltzmann constant and T the temperature. This equation is known as the ideal gas equation.
If the temperature of the gas is small, then the volume will also decrease accordingly. If so, there is a temperature where the volume is equal to zero and according to this formula, the temperature is minus 273.15 degrees Celsius. Of course, there is no such thing as a negative volume, so this temperature was considered the lowest temperature.
At first people were not impressed by the idea, because they also knew that when a gas is cooled it turns into a liquid or a solid and then this formula is no longer valid. But at the same time, in small and slow steps, the scientific community began to be convinced that matter is made of molecules and atoms, and that temperature is the degree of vibration of those particles. If the temperature measures their average motion, it can explain the origin of the minimum temperature, which occurs when the particles are not moving at all. The discovery led William Thompson, later known as Lord Kelvin, to propose this temperature as the absolute zero temperature and calibrate the Celsius scale accordingly.
The fact that temperature derives from particle physics revealed the complexities in its definition, but for those who first played with the thermometer, it is less of an issue. They knew what they knew and that was enough for them. For them, temperature is what the thermometer showed when they put it in the tub or put it under a person's armpit. They were not sensitive to the scientific subtleties of the term temperature, apparently they thought these were trivial.
Who said that the measurement that the thermometer showed is the same as the temperature of the body being measured? But the fact that temperature is a uniform and continuous field makes it measurable and reliable. Those who deal with physical axioms call this law the zeroth law of thermodynamics, because it was established after the first and second laws were defined before it.
These laws developed following fascinating discoveries and breaking many scientific concepts, which I will tell in the next post.
Most of the information is taken from Ingo Muller's book:
A History of Thermodynamics
Do you have a question? Want me to write about a topic that interests you? Contact me at the email address: ppgad@pucrs.br
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