# What is temperature and what does it truly measure?

Everybody has used a thermometer at least once in their lives, but even without one, our bodies are decent sensors for measuring how hot or cold things are upon contact. We refer to this property as temperature which, in more technical terms, represents the average kinetic energy of the atoms and molecules comprising an object.

### Heat or temperature?

Before we go any further with our discussion, it’s important to get something out of the way.

Often heat and temperature are used interchangeably — this is wrong. While the two concepts are related, temperature is distinct from heat.

Temperature describes the internet energy of a system, whereas heat refers to the energy transferred between two objects at different temperatures.

But, as you might have noticed, heat can be very useful when describing temperature.

Imagine a hot cup of coffee. Before pouring the hot elixir of life, the cup had the same temperature as the air surrounding it. However, once it came in contact with the liquid, heat was transferred, increasing its temperature. Now, if you touch the cup, you can feel that it’s hot.

But, given enough time, both the cup and its contents will reach thermal equilibrium with the ambient air. Essentially, they all have the same temperature, which is another way of saying there is no longer a net transfer of energy. Physicists call this the “zeroth law of thermodynamics”. By this principle, heat can only flow from a body that has a higher temperature than another body with which it is in contact — and never the other way around.

### The dance of molecules

Everything in this universe is in motion, and motion begets kinetic energy. The faster a particle is moving, the more kinetic energy it has. In fact, kinetic energy increases exponentially with particle velocity.

Where does temperature fit into all of this? Well, temperature is simply an average measure of the kinetic energy for particles of matter. Another way of putting it would be that temperature simply describes the average vibration of particles.

Because the motion of all particles is random, they don’t all move at the same speed and in the same direction. Some bump into each other and transfer momentum, further increasing their motion. For this reason, not all particles that comprise an object will have the same kinetic energy.

In other words, when we measure an object’s temperature, we actually measure the average kinetic energy of all the particles in the object. However, it’s just an approximation.

Within this line of reasoning, the higher the temperature, the higher the motion of the particles. Conversely, when the temperature drops, the motion of the particles is slower. For instance, dyes spread faster through hot water than cold water.

This is why at a temperature of absolute zero, the motion of particles grinds to a halt. Absolute zero is just a theoretical construct and, in practice, it can never be achieved. However, physicists have been able to cool things to a fraction of a degree above zero, trapping atoms and molecules, or creating exotic phases of matter such as the Bose-Einstein condensate (BEC).

It’s important to note that temperature isn’t dependent on the number of molecules involved. A boiling cup of water has the same temperature as a boiling pot of water — both containers have water molecules with the same average kinetic energy, regardless of the quantity of matter involved.

### Temperature scales

There are various scales used to describe temperature. In the United States, the most commonly used unit for temperature is Fahrenheit, while much of the rest of the world uses Celsius (or Centigrade). Physicists often prefer to measure temperature in Kelvin, which is also the standard international unit for temperature.

For the Kelvin scale, zero refers to the absolute minimum temperature that matter can have, whereas in the Celsius scale, zero degrees is the temperature at which water freezes at a pressure of one atmosphere (273.15 Kelvin). At 100 degrees Celsius, water begins to boil at a pressure of one atmosphere, offering a neat, linear and relatable scale for describing temperature.

A worthy mention goes to the Rankine scale, which is most often used in engineering. The degree size is the same as the Fahrenheit degree, but the zero of the scale is absolute zero. Often just R for “Rankines” rather than °R is used for expressing Rankine temperatures. The zero of the Rankine scale is -459.67°F (absolute zero) and the freezing point of water is 491.67R.

### How temperature is measured

Because of our innate ability to sense how hot or cold things are, humans have had little use for precise measurements of temperature throughout history. However, there have always been mavericks bent on learning about things just for the sake of unraveling nature or getting a kick out of doing science.

Hero, a Greek philosopher and mathematician, is credited with the idea for the first thermometer, writing in the 1st century CE about the relationship between temperature and the expansion of air in his work Pneumatics.

The ancient text survived the degradation of the Roman Empire and the dark ages that followed, until it resurfaced during the Renaissance.

It is believed that Hero’s work inspired Galileo Galilei to invent the first device that precisely measures temperature. The Galileo thermometer is composed of multiple glass spheres each filled with a colored liquid mixture that often contains alcohol but can even be simply water with food coloring added.

Each bubble has a metal tag attached to it that indicates temperature, which also serves as a calibrated counterweight that’s slightly different from the others. These floating balls sink or float inside the surrounding water sinking or climb up the water column slowly and gracefully. People still use them to this day, mostly for decorative purposes.

For more precise measurements, there’s the traditional mercury thermometer whose fluid expands at a known rate as it gets hotter and contracts as it gets cooler. It’s then just a matter of reading the measurement indicated by where the column of liquid ends on the scale.

Robert Fludd, an English physician, is credited with designing the first thermometer in 1638 that had a temperature scale built into the physical structure of the device. Daniel Fahrenheit designed the first mercury-based thermometer in 1714 that ultimately went on to become the gold standard of temperature measurement for centuries.

# What’s a Galileo thermometer and how do you read it?

An assortment of Galileo thermometers of various sizes. The bigger the size, the more precise the instrument. Credit: Amazon.

There’s no more beautiful way to measure the temperature than using a Galileo thermometer. This fairly accurate instrument is based on the thermoscope invented by Galileo Galilei in the early 17th century. Unlike your typical mercury-in-glass thermometer that’s basically a narrow bulb made with mercury that expands and contracts, the Galileo thermometer is far more complex. It’s comprised of multiple glass spheres each filled with a colored liquid mixture which often contains alcohol but can even be simply water with food coloring added. These floating balls sink or float inside the surrounding water over time and temperature ever so slowly and gracefully.

### How a Galileo thermometer works

Each bubble has a metal tag attached to it that indicates temperature, which also serves as a calibrated counterweight. Each tag, in turn, has a weight that’s slightly different from the others. The bubbles themselves, typically hand-blown glass, are each different in size and shape but they’re all calibrated so they have the same exact density. It’s the weighted tags that make each bubble’s density vary slightly. Even so, the density of all the bubbles is very close to that of the surrounding water.

When the ambient air surrounding the thermometer increases or decreases in temperature, so will the temperature of the water that surrounds the glass bubbles. As the temperature of the water rises or falls, the liquid will either expand or contract, respectively, varying the density in the process as well. Specifically, water density decreases as its temperature increases. You can trust experience in this case. If you’ve ever dived into a lake, you must have felt how cold the water. That’s because closer to the surface, the water is warmer and lighter staying at the top. For our Galileo thermometer, for any given density, some of the bubbles will float while others will sink.

Each glass ball has a metal tag attached that serves to both indicate temperature and as a counterweight. Credit: Wikimedia Commons.

Essentially, the Galileo thermometer operates on the principle of buoyancy, the phenomenon by which objects of greater density than their surroundings sink and less-dense ones float. For instance, the ball marked at 78 degrees F will be just slightly less dense than tube liquid at that temperature, causing it to float. While the ball underneath it, marked and adjusted for 76 degrees will be denser than the tube liquid and will sink.

### How to read a Galileo thermometer

A small Galileo thermometer can have 6 degrees (F) of difference between the balls, which introduces a lot of error into the measurement. Larger such instruments have at least a couple of temperature difference between the diver globes. Most cover the temperatures from 68-84F degrees which makes them suitable only for heated/air conditioned indoor locations

To read the ambient temperature on a Galileo thermometer is very easy. What you have to do is simply look at the lowest ball that is floating while ignoring those tags that had sunk to the bottom of the container. It’s those balls that float or are neutrally buoyant that interest us. If there is no bulb floating in the gap between the rising and sunk bulbs, use the lowest bulb from the floating cluster to get the temperature.

The Galileo thermometer is certainly not the most precise instrument in the world but it is certainly clever. Really, I don’t think you’ll find a more attractive way to measure temperature which is why people use them for decorative purposes. You can find a great selection on Amazon.

# This Venetian doctor who invented the thermometer also helped shape modern chemistry

A book from the British Library reveals how Santorio Santorio, who lived between 1561 and 1636, came up with an explanation for how matter works two decades before Galileo Galilei.

Santorio’s marginal note to col. 406C-D, in Santorio Santori, Commentaria In Primam Fen Primi Libri Canonis Avicennae (Venice, 1625), British Library, 542.h.11. Courtesy of the British Library.

You might think your library is big, but it’s just peanuts to the British Library. The second largest library in the world holds over 150 million cataloged books, from all times and from all fields. Naturally within this gargantuan collection, also lie books which have not yet been properly explored. This was the case with 1625 edition of a book called Commentaria in primam Fen primi libri Canonis Avicennae (A Commentary on the First Fen of the First Book of Avicenna’s Canon), until it was discovered by Dr. Fabrizio Bigotti, from the Centre for Medical History at the University of Exeter.

Bigotti analyzed the handwriting and writing style and concluded that the book was almost certainly written by Santorio Santorio. Inside the book, Santorio (who is also credited with inventing the thermometer and other early medical devices) came up with a good explanation of how matter works. He did not share the elemental vision of nature, as most people presumably did at the time. To make it even better, he did this twenty years before Galileo, who is largely credited with this breakthrough.

“The notes show he did not see the world not made up of four elemental qualities – hot, cold, dry and moist – as Aristotle had suggested. This helped to start the process of getting rid of the idea that magic and the occult could be found in nature,” Bigotti explains.

The idea that the world is made from invisible ‘corpuscles’ is not really new. It was actually devised by the Greek philosopher Democritus, which is a stunning achievement in itself. But until Galileo Santorio, no one was able to prove that. He carried out a series of optical experiments, as well as urine distillation experiments, all of which indicated that all matter was made of tiny particles.

“This discovery makes the case for a deeper study of early modern chemistry in the Medical School of Padua, where Santorio taught, and the work carried out there between the end of the sixteenth and the beginning of the seventeenth century. Santorio’s true contribution to chemistry has been forgotten but, I hope, this new discovery means that will no longer be the case.”

It’s always thrilling to see just how far people could get with what we today would consider rudimentary instruments. The discovery further establishes Santorio as one of the most brilliant scientists in history.

“It was already known that Santorio laid the foundations for what is understood today as evidence-based medicine and the study of metabolism,” Dr. Bigotti said. “The new discovery shows he was he among the first scientists to suggest the body aims at preserving its own balance through discharge of invisible particles.”

# Scientists make the smallest thermometer from programmable DNA

Around the time DNA was first discovered more than 60 years ago, scientists also found these miraculous molecules that hold the blueprint of life can unfold when heated. Now, a team at the University of Montreal used DNA switches to build a thermometer that is 20,000 times smaller than a strand of human hair. This remarkable research could open the doors for biological thermometers at the nanoscale which might tell us a thing or two about how our bodies function at the smallest level.

Credit: Pixabay

Previously, scientists found that RNA and DNA act like the body’s nanothermometers triggering biological processes by folding and unfolding in the presence of temperature. This way, they act like molecular switches.

Prof. Alexis Vallée-Bélisle and colleagues devised their own DNA nanoswitches using the molecule’s simple chemistry to program them.

“Inspired by those natural nanothermometers, which are typically 20,000x smaller than a human hair, we have created various DNA structures that can fold and unfold at specifically defined temperatures,” Prof. Vallée-Bélisle said.

“DNA is made from four different monomer molecules called nucleotides: nucleotide A binds weakly to nucleotide T, whereas nucleotide C binds strongly to nucleotide G,” explains David Gareau, first author of the study published in the journal Nano Letters.

“Using these simple design rules we are able to create DNA structures that fold and unfold at a specifically desired temperature.”

“By adding optical reporters to these DNA structures, we can therefore create 5 nm-wide thermometers that produce an easily detectable signal as a function of temperature,” adds Arnaud Desrosiers, co-author of this study.

The DNA thermoswitches offer a precise ultrasensitive response over a desired, small temperature interval (±0.05 °C). Using a combination of thermoswitches of different stabilities, the researchers made extended thermometers that respond linearly up to 50 °C in temperature range. This is more than enough considering the human body is maintained at a constant temperature of 37 °C. However, it’s very likely that there are large temperature variations at the nanoscale within each cell. Nano-sized machines, switches, sensors or motors have been developed by nature over the course of billions of years worth of evolution. Soon, scientists will be using these mechanisms to explore the slightest variations in the smallest parts of our bodies. This way, we’ll learn how these small blocks work together to build a large structure — and also what happens when it comes crumbling down.