First invented in the 1600s, thermometers measure temperature using a sensor (the bulb tip) and a scale. Temperature is a way of talking about, measuring, and comparing the thermal energy of objects. We use three different kinds of scales to measure temperature. Fahrenheit, Celsius, and Kelvin. (The fourth, Rankine, which is the absolute scale for Fahrenheit, is the one you’ll learn about in college.)

Mr. Fahrenheit, way back when (18th century) created a scale using a mercury thermometer to measure temperature. He marked 0° as the temperature ice melts in a tub of salt. (Ice melts at lower temperatures when it sits in salt. This is why we salt our driveways to get rid of ice). To standardize the higher point of his scale, he used the body temperature of his wife, 96°.

As you can tell, this wasn’t the most precise or useful measuring device. I can just imagine Mr. Fahrenheit, “Hmmm, something cold…something cold. I got it! Ice in salt. Good, okay there’s zero, excellent. Now, for something hot. Ummm, my wife! She always feels warm. Perfect, 96°. ” I hope he never tried to make a thermometer when she had a fever.

Just kidding, I’m sure he was very precise and careful, but it does seem kind of weird. Over time, the scale was made more precise and today body temperature is usually around 98.6°F.

Later, (still 18th century) Mr. Celsius came along and created his scale. He decided that he was going to use water as his standard. He chose the temperature that water freezes at as his 0° mark. He chose the temperature that water boils at as his 100° mark. From there, he put in 100 evenly spaced lines and a thermometer was born.

Last but not least Mr. Kelvin came along and wanted to create another scale. He said, I want my zero to be ZERO! So he chose absolute zero to be the zero on his scale.

Absolute zero is the theoretical temperature where molecules and atoms stop moving. They do not vibrate, jiggle or anything at absolute zero. In Celsius, absolute zero is -273 ° C. In Fahrenheit, absolute zero is -459°F (or 0°R). It doesn’t get colder than that!

As you can see, creating the temperature scales was really rather arbitrary:

“I think 0° is when water freezes with salt.”
“I think it’s just when water freezes.”
“Oh, yea, well I think it’s when atoms stop!”

Many of our measuring systems started rather arbitrarily and then, due to standardization over time, became the systems we use today. So that’s how temperature is measured, but what is temperature measuring?

Temperature is measuring thermal energy which is how fast the molecules in something are vibrating and moving. The higher the temperature something has, the faster the molecules are moving. Water at 34°F has molecules moving much more slowly than water at 150°F. Temperature is really a molecular speedometer.

Let’s make a quick thermometer so you can see how a thermometer actually works: