There have been many different temperature scales invented throughout the history of science. The most popular ones are sufficient to quantify the weather forecast and cooking, and the rest are typically used in engineering or science disciplines.
Celsius - Used by the International System of Units, it is the most common temperature scale in the world. It is based on 0° for the freezing point of water and 100° for the boiling point of water at sea level air pressure.
Fahrenheit - Used in the United States and a few other countries in place of Celsius. The freezing point of water here is 32° and the boiling point of water is typically 212° at sea level. The Fahrenheit scale was invented by Dutch–German–Polish physicist Daniel Gabriel Fahrenheit in 1724.
Kelvin - The absolute thermodynamic temperature scale used in most scientific fields. Kelvin starts off with 0° as being "absolute zero," the temperature at which all thermal motion ceases in a thermodynamic sense - thus, there should be no such thing as a negative Kelvin temperature. On the Kelvin scale, the freezing point of water is still 273.15°.
Rankine - this is also an absolute thermodynamic temperature scale, like Kelvin, so zero is still "absolute zero." But the degree gradients mimic those of Fahrenheit, strictly to make conversion easier.
Delisle - This was invented by French astronomer Joseph-Nicolas Delisle in 1732. It originally started out in reverse, with the boiling point of water at sea level being 0° and 150° as water's freezing point. Delisle was among the first to calibrate a temperature scale on a mercury thermometer.
Newton - named obviously after Sir Isaac Newton, he devised this scale in 1701, although he didn't refer to temperature so much as "degrees of heat." Similar to Celsius, water freezes at 0° but the boiling point of water is set at only 33°. Give him a break, he was using linseed oil in his thermometer, as mercury wasn't widely used at the time.
Réaumur - Invented by René Antoine Ferchault de Réaumurm, a French entomologist (circa 1700s). He, too, set the freezing point of water at 0° and designed a thermometer to measure 80 points from here to the boiling point of water.
Rømer - Invented by a Danish astronomer, Ole Christensen Rømer (circa 1600s). He too devised a thermometer to measure form the freezing to boiling points of water, at 0° and 60°, respectively. His temperature scale was the basis for Fahrenheit, after Daniel Gabriel Fahrenheit built on his work.
Temperature is an important metric in most natural science fields, including physics, chemistry, medicine, and biology. Not to mention, we all check the outdoor temperature daily to determine whether or not we should bring a jacket.
What do we mean when we say "temperature"? We all know intuitively that it's a scale of measuring how hot an object or environment is, but what does that actually mean? We have to delve into technical details to examine the question.
Temperature is actually a measure of kinetic energy. As we remember from science class, the warmer something is, the more excited its atoms get. Called "thermal motion," this is the rate at which atoms in a substance vibrate. Water in its most excited state becomes steam. In its default state, it's a liquid. In its coldest state, it becomes ice, which is the lowest range of thermal motion for H2O molecules.
Any friction will end up creating heat, because you're deliberately agitating the molecules. Rubbing your hands together on a cold day is one obvious example. Another example would be ice skating: the blades of the ice skates make just enough friction with the ice surface to melt that tiny layer of water allowing the skate to glide.
But the electric coil on a stove top (assuming you're not cooking with gas) is another example: We're forcing a flow of electricity into a metal that is incapable of dispelling this energy at the same rate - like a dam holding water, electrons are held back by the resistant material. The friction of the flow against the electrical resistance of the material produces the heat.
This same method gets used over and over again in resistance coils, everywhere from hair dryers to portable floor heaters. Electricity plus resistant material equals heat energy. In the case of a stovetop heating element, that material is nichrome wire jacketed by ceramic insulation so the wire doesn't just melt.
It is notable that water becomes the basis for most temperature scale readings, simply because it's such a familiar substance to us. But water works as a poor standard for scientific work; there are many substances and states of matter throughout the universe which react at radically different heat levels. Mercury, for instance, freezes at -37.89° Fahrenheit and boils at 674.1° Fahrenheit. Hydrogen freezes at -434.5° Fahrenheit and at high enough pressure actually turns into a metal.
This may become relevant should we ever take a core sample of the planet Jupiter, which scientists think is made of metallic hydrogen.
Due to these extreme ends of the scale for discussing heat energy at different points in the natural sciences, we need a universal scale that makes sense for every possible scenario. But at the same time, substances like water and mercury are readily available and far easier to work with. And when you make a thermometer, you have to select a substance to put in the thermometer to serve as a standard unit.
We could theoretically make thermometers using liquid hydrogen to measure temperatures on Jupiter. Or we could use liquid tungsten, which has a melting point of 6192° Fahrenheit, to measure temperatures on the sun. But both of these theoretical thermometers would be difficult to work with on a daily basis in the average Earth-bound laboratory.
We've touched on a field of physics here known as "thermodynamics." It's a branch of physics most of us don't think about on a daily basis, but has application in literally every facet of existence. Knowing the immutable laws of thermodynamics is essential to understanding almost any action:
• The internal energy of an isolated system is constant. In other words, energy has to come from somewhere. Energy is never created or destroyed.
• Heat cannot spontaneously flow from a cold source to a hot source. Engineers shorten this to "energy always flows downhill."
• At absolute zero, all processes cease and the entropy of a system approaches a minimal value. This is a fancy way of saying we can't point to a source of absolute zero energy anywhere in the universe. Even the coldest vacuum of space has a few jiggling atoms in it.
Once you see the laws of thermodynamics laid out, they seem so intuitive that it's hardly worth the bother to state them. Yet consider the pseudo-science field of perpetual motion devices: Generations of quack inventors have attempted to engineer clever devices using magnets and gravity to "outsmart" the first and second laws of thermodynamics.
They always fail. These inventors are always stymied but persist anyway.
