An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

# SI Superheroes Return with Another Weighty Adventure

## Share

The nefarious Major Uncertainty has kidnapped Monsieur Kilogram, putting the world's measurements of mass in jeopardy. As the world spirals into "Mass Hysteria," the remaining SI Superheroes, champions of the metric system, leap into action to save the day, and hopefully Monsieur Kilogram as well.

This crisis kicks off the third and latest adventure from the League of SI Superheroes, the animated online series from the National Institute of Standards and Technology (NIST). "Mass Hysteria" touches upon a topic—how to redefine the basic unit of mass known as the kilogram—that represents a cutting-edge undertaking for researchers working to modernize the worldwide metric measurement system known as the International System of Units (SI).

From the very big to the very small, accurately measuring mass is important in the world around us. For example, many of the products you buy at the grocery store and other places are sold by mass or the related quantity of weight. Roughly speaking, mass is the amount and type of "stuff" there is in something, and weight is the force pulling on the mass by gravity. The masses of every ingredient in medications from aspirin tablets to cancer drugs are carefully measured to ensure that they are both safe and effective. In many cases, medical doctors consider the mass of the patient to determine the dosage of the medications they prescribe as well. And both the fuel and the amount of thrust produced by the huge engines that power airplanes and rockets depend on mass.

Small errors of even a few milligrams per kilogram may not sound like much, but they can be costly when measuring huge quantities of something like a tanker ship full of grain or oil. With medicines, slightly too little of a chemical could make it ineffective and slightly too much could be toxic.

Being the last standard unit of measure still based on an actual physical object, in this case a golf-ball-sized cylinder of platinum and iridium, the kilogram is vulnerable to damage, as well as being lost or even stolen. While the international prototype kilogram itself cannot change because it is the kilogram by definition, copies of the international prototype that many countries use as their standard of mass have been gaining or losing mass relative to it.

The SI Superheroes' latest episode briefly explores one of the efforts to redefine the kilogram in terms of natural forces called the watt balance, a complex machine that uses electric and magnetic forces to balance a 1-kilogram mass against the Earth's gravity. Precise measurements related to these forces can then be used to provide a consistent definition of the amount of mass in the kilogram.

While the superheroes' antics are not exactly representative of the efforts of actual researchers, super scientists at NIST and elsewhere have been working for years to reduce the errors in their measurement of this quantity to the point where the watt balance can take over for the international prototype kilogram.

They are closing in on their goal, and it is widely anticipated that the kilogram will be redefined in 2018.

Once this process is complete, the kilogram will have been freed from its dependence on a physical object, and anyone with the right technical expertise and equipment will be able to determine the mass of a kilogram for themselves.

Will the SI Superheroes finish the watt balance in time? Watch the next episode and find out! Modeled on the seven base units of the International System of Units, or SI, the League of SI Superheroes are:

• Meter Man: With his laser interferometer eyes, graduated arms and extendable body, no dimension is too big or too small for Meter Man to measure.
• The Mole: Able to sniff out and count the atoms of every element, the Mole is a master of chemistry.
• Professor Second: By reading the vibrations of her laser-cooled cesium atoms, Professor Second can measure any frequency and calibrate any clock.
• Monsieur Kilogram: Monsieur Kilogram loves lifting weights, and it shows. He is able to determine the mass of any object simply by holding it.
• Ms. Ampere: Ms. Ampere rules the flow of electrons—electrical current—and makes sure that the right amount gets where it needs to go.
• Dr. Kelvin: Dr. Kelvin heats up or cools down objects by speeding up or slowing down particles inside them. He can also measure the temperature of anything in the universe with his trusty thermometer.
• Candela: Don't let her small size fool you. Candela's power over light helps to brighten the whole world.

Catch up on their adventures at The League of SI Superheroes kids' page. Teachers can also request a classroom set of SI educational materials by submitting their contact information and grade level to TheSI [at] nist.gov (TheSI[at]nist[dot]gov).

Released February 2, 2016, Updated January 17, 2023