The metric system determines everything from global trade to science experiments to the baking of perfectly gooey brownies, but its origins were surprisingly imprecise. When the system was adopted in the 19th century, base units such as the kilogram and candela were defined using hyperspecific objects—like a platinum-iridium cylinder and a one-sixth-pound pure spermaceti candle. That stuff’s not exactly easy to come by. Over the years, more rigorous measurements were developed. This month international delegates are gathering near Paris to formalize new standards for the metric system using fundamental constants of nature. There, representatives from around the globe will vote on methods for determining the absolute mass of a kilogram, the shock of a single amp, and more.
MASS
PREVIOUS STANDARD: Since 1889 the kilogram has been represented by a single platinum-iridium cylinder cloistered in a Parisian vault. However, like many of us, its replicas around the world have put on some weight with age. A 2013 study estimated one such cylinder had gained 250 micrograms from built-up debris.
NEW STANDARD: The Kibble balance generates an electromagnetic force to match the mass of any item placed on it. When the strength of the force reaches a specific value, that mass is 1 kg.
While your new bathroom scale will still register pounds, it will be calibrated to the standard kilogram.
LENGTH
PREVIOUS STANDARD: From 1889 to 1960, the meter was set as the distance between two markings on a platinum-iridium bar, propped up just so, at 0 degrees Celsius.
NEW STANDARD: Since the ‘60s, scientists have flirted with a gauge based on the properties of krypton gas. Most recently, the meter was set as the distance that light travels in a vacuum in 1/299,792,458 of a second.
In the 1790s, french revolutionaries decided that the meter would be 1 ten-millionth of the distance from the North Pole to the equator—then spent six years measuring it.
TIME
PREVIOUS STANDARD: In the 1940s, the second was pegged at 1/86,400 of an average day. But Earth’s rotation is imperceptibly slowing, lengthening each day by roughly a millisecond a century. That means 1.4 billion years ago, the day was only 18 hours long.
NEW STANDARD: In an extreme vacuum, scientists can make a cesium atom emit radio waves at a reliably steady frequency. A second equals 9,192,631,770 cycles of these waves.
To pinpoint your location, GPS satellites need to keep time to within 1 billionth of a second per day.
ELECTRIC CURRENT
PREVIOUS STANDARD: The old method for measuring electrical current called for placing two infinitely long wires perfectly parallel in a vacuum. Back in the day, they just settled for two really long wires; that kind of imprecision won’t cut it anymore.
NEW STANDARD:
It’s not about the length of the wire; it’s the charge of an electron. The new ampere is designated as some 6 quintillion charges flowing through a wire per second.
A shock of just 0.01 ampere will make you lose control of your muscles; 0.1 ampere could stop your heart.
TEMPERATURE
PREVIOUS STANDARD: On a thermometer, which starts at absolute zero, 273.16 kelvin equals the “triple point” of water: the temperature in which liquid water, ice, and gas can coexist at a specific pressure. But that measurement requires some seriously pure water.
NEW STANDARD: The thermometer still starts at absolute zero, but the value of the kelvin will now be calculated using what’s called the Boltzmann constant: the average kinetic energy of one atom.
No conventional freezer can achieve absolute zero, the coldest temperature physically possible. the closest A MOLECULE HAS ever come is within about a millionth of a Kelvin.
AMOUNT OF SUBSTANCE
PREVIOUS STANDARD: Previously, a mole was defined as the number of atoms in 12 grams of carbon-12. But chemists didn’t want the mole’s value to depend on the gram.
NEW STANDARD: No more carbon counting. The mole will now be precisely 6.02214076 × 1023 particles (usually atoms or molecules), a figure known as Avogadro’s constant.
A 12-ounce can of Bud Light contains about 1.5 × 1023 ethanol molecules, roughly a quarter of a mole.
LUMINOUS INTENSITY
PREVIOUS STANDARD: In the mid-20th century, standards were based on the glow of a small piece of freezing platinum. But as lighting tech progressed, it grew too taxing to compare platinum with tungsten and fluorescent bulbs.
NEW STANDARD: The new candela is roughly the brightness of a candle. That’s determined with a lamp that produces pure yellow-green light—the color our eyes are most sensitive to in daylight—calibrated using a device called a radiometer.
19th-century Brits adapted Moby Dick’s method as a lighting standard: a pure spermaceti candle weighing one-sixth of a pound burning at a specific rate.
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