5 Weirdly Nonstandard Things About the Metric System

The modern metric system - known as the International System of Units, or SI - is a model of consistency and logic. But in some cases, the logic seems to break down. In this blog post, we'll explore some of these quirks and why they (mostly!) make sense at the end of the day (or about 86,400 seconds, as we sometimes like to say at NIST).

1. Why is the kilogram (a thousand grams) a basic unit of measurement instead of just the gram?

In the metric system, we express quantities using individual units, such as length in meters and time in seconds. But the unit of mass is a kilogram - a thousand grams (kilo = thousand).

Why isn't the gram itself a base unit in the SI? Well, for starters, it was once hard to make objects that were exactly one gram. Kilograms, which are equal to about 2.2 pounds, were much easier to make accurately and more useful in day-to-day trade.

During the French Revolution, the new republic introduced the metric system, which used uniformly sized natural objects rather than, say, the length of an important person's foot or a random grain of barley. The founders of the metric system defined the gram as the mass of a 1-centimeter cube of water at 4 degrees Celsius (about 39 degrees Fahrenheit). But that's about the size of a pea and the weight of a raisin. It doesn't make for a very practical definition for commerce or the classroom.

So the French created an object a thousand times heftier - a kilogram - which was a much more useful reference. By 1875, the kilogram took hold internationally. Seventeen countries, including the United States, signed the Treaty of the Meter, which established agreed-upon references for length and mass.

In 1879, an international prototype for the kilogram, known as "Le Grand K," was made in London and housed in France. In 1889, Le Grand K was declared the international standard. It served as the international standard of mass until May 20, 2019, when the world agreed to redefine the kilogram in terms of fundamental constants of nature. This means that now anyone who needs a highly precise measurement can use a device called a Kibble balance to get an accurate measurement of a kilogram - no trip to France required.

While the kilogram is still the official international unit of mass, NIST has been revisiting how to measure the gram, with precise mass-measurement devices called tabletop Kibble balances. Improving our ability to measure the gram is useful for pharmaceutical manufacturers, who want to dispense drug doses accurately, and for the military and companies that need to make precision parts for electronics, aerospace and other critical applications.

German researcher Beatrice Rodiek prepares "the sphere" - one of the roundest objects on Earth - to be measured on a highly accurate weighing machine, known as the Kibble balance, at NIST's campus in Gaithersburg, Maryland.

2. What is unusual about the way we count minutes and hours?

The inventors of the metric system designed it to be easy to use, with multiples of 10 representing different quantities. For example, 10 millimeters equals a centimeter, 100 centimeters is a meter, and 1,000 meters is a kilometer. It's what mathematicians call a base-10 system.

But we count time a bit differently. We still express fractions of a second the metric way. For example, a thousandth of a second is a millisecond.

But on the scale of seconds and minutes, timekeeping becomes a base-60 system, counted in chunks of 60. When we reach 60 seconds, it becomes one minute, and 60 minutes equal one hour. Minutes and hours are not official SI units, but they are accepted for use within the metric system.

Humans had good reasons to use base-10 and base-60 systems to count. Scientists believe that our affinity for base-10 has to do with the number of fingers we have. And whereas older measurement systems often had complicated conversions involving different units - such as 6 feet in a fathom - metric stays with the same unit, with simple prefixes indicating higher or lower quantities.

For example, when you have 1,000 watts, it's called a kilowatt, while a thousandth of a meter is a millimeter. The origin of our base-60 system is more complicated. People living in one of the earliest civilizations, the Sumerians, first developed base-60 around 3000 B.C.

While the reasons they chose 60 are unknown, scientists speculate that 60 is a convenient number for arithmetic. Sixty is evenly divisible by many numbers, including 1-6,10, 12, 15, 20 and 30. And going back to human fingers, we can count to 60 pretty easily by using our knuckles. Knuckles split each finger (excluding the thumb) into three parts, so four fingers add up to 12 parts. If you use the thumb and fingers of your other hand to count by 12s, the total is 60.

Starting around 2000 B.C., the Babylonians then used base-60 for astronomical observations, splitting the sky into that many sectors. To this day, fractions of a degree in the sky are still expressed in minutes and seconds. And in the second century B.C., Greek astronomer Hipparchus used this system to split the hour and minute into 60 parts.

3. Why is the official definition of the meter linked to the unit of time, instead of the length of an object?

The meter was originally defined as one ten-millionth of the distance from the equator to the North Pole through Paris. In the 1790s, French scientists created the first metal bar that was officially a meter long. Their successors forged a new, more stable International Prototype Metre in 1889. This metal artifact served as the official meter stick against which all other meters were measured.

But there are problems with using actual objects to be the official meter. For one, it's virtually impossible for anyone to make two bars of exactly the same length. Heat and other factors can also cause the bar to slightly change size.

Today, the meter is no longer defined in terms of an inevitably imperfect physical object. Since 1983, the meter has been based on the distance that light travels in 1/299,792,458 of a second through empty space. Nothing is more dependable than the speed of light - it's the speed limit for the universe and one of our most accurately measured quantities.

But there's a tiny issue: The 1983 definition doesn't specify exactly what a "second" is. So in 2019, the official meter was refined to include the official definition of the second, which is equal to about 9 billion "ticks" of a cesium atom (see our atomic clock site for more information).

Like any unit of measurement, the kilogram needs to be the same for everyone, all the time. But different countries have their own ways of defining the kilogram (or "realizing," as researchers call it). NIST researchers and their German counterparts are working on experiments to see how they can bridge the very tiny gap in the two countries' measurement approaches.

4. Which metric unit might extraterrestrials find to be the strangest (and most human) one of all?

In the modern metric system, all the base units are defined by constants of nature, such as the speed of light and the charge of the electron. As best as we can tell, these constants appear to be the same everywhere in the universe. And as far as we know, extraterrestrials would measure the same values for these constants as we do.

But there is one base unit that is very specific to humans. That unit is called the candela, which means "candle" in Latin, Spanish and Italian. Originally derived from the estimated brightness of a single wax candle, it's the base unit for "luminous intensity," the intensity of an object as perceived by the human visual system.

Note that we say "intensity of an object as perceived by the human visual system." Because of the quirks of human vision, our eyes have different levels of sensitivity for different colors. For example, green laser pointers appear brighter to us than red laser pointers of the same power. For this reason, the candela is very useful for measuring the intensity that we perceive from car headlights. Candelas are also used for taking photographs with proper exposure, creating stage lighting with the intended impact on the audience, and ensuring optimal lighting levels in an operating room.

The precise definition of the candela specifies a reference color, a shade of yellow-green to which the human eye is most sensitive, corresponding to a light with a frequency of 540 THz. (See our page on the candela for a full breakdown of the definition.) From this reference color, scientists can calculate the luminous intensity of other colors.

Scientists believe our eyes adapted to this yellow-green color because it is the most abundant shade of light from the Sun that strikes the Earth's surface. This is also the color that plants absorb most strongly. It stands to reason that humans' sensitivity to this color helps them spot different types of vegetation when foraging for food.

Extraterrestrials might be more sensitive to another color in the spectrum, based on the star (or stars) that shines over their home planets. If we ever make contact, measurement scientists from our planet will likely be very interested in knowing how aliens might define their own candela.

5. Why isn't there a metric unit for rotation or angles?

If you think about it, the metric system is very straight-edged. There's a unit for length, namely the meter. However, there is no fundamental base unit for geometrical angle, which would be very useful for measurements involving objects rotating in a circle. In our round world and curved universe, why isn't there a base unit devoted to rotation?

This is not just a philosophical question: It creates real-world issues. For example, measurements of torque, or twisting force, have the same units as the physics quantity known as "work." Work describes the expenditure of energy when, for example, you lift a weight from the ground. The units for both are "Newton meters." If you see a measurement in Newton meters, you may not know if it's a measure of torque or work.

To remedy this, NIST's Peter Mohr and Bill Phillips (building upon suggestions from others) have proposed that the radian become a base unit of measurement. Like the degree, the radian indicates the amount of an angle or rotation in an object. The two units are analogous to Fahrenheit and Celsius - both measure the same thing but in different ways.

A full rotation around a circle is equal to 360 degrees, which is equivalent to 2 π radians (this has to do with the fact that the circumference of a circle, as you may remember from school, is equal to 2 π times the circle's radius). But degrees and radians are only communicating the amount of an angle; they are really just numbers without a base unit.

Making the radian a base unit would take things to the next level. It would bake in the idea of a dimension along the arc of a circle. By expressing an angle in terms of radians as a base unit, you'd be specifying not only the amount of the angle but also that the dimension is circular.

Having the radian as a fundamental unit would solve the real-world problem that we mentioned earlier. The twisting force could now be defined in terms of Newton meters per radian. With this new definition, you'd be able to show torque as the amount of energy per rotation of an object, perhaps a more physically accurate concept than what we had before.

And more generally, making the radian a unit would allow the metric system to more fundamentally communicate concepts such as rotation and the angle at which an object is oriented. This idea is still at the proposal stage, but it is being discussed in the measurement science community.

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I had to really plumb the depths (or many fathoms, so to speak) to identify unusual things in and around the metric system. As systems go, it's remarkably straightforward and consistent. Whether doing science in the lab or brewing beer in your cellar, metric really can make your life easier, even on those nonstandard occasions.