Redefining the Kilogram

Fahad Paryani

The Problem

In a vault outside of Paris rests a small metal cylinder sitting inside three vacuum-sealed bell jars. The mass of the metal cylinder is known to be exactly 1 kg — the perfect kilogram. The weight was created in 1879 and is made from  an alloy of platinum and iridium. Since 1889 the perfect cylinder has been the standard against which every other kilogram on the planet has been judged.  However, the problem is the cylinder appears to be changing mass.

Despite it being in a very carefully controlled environment, contamination has built up on the surface of the object. The increase in mass is tiny but its effect on science is real. This tiny change in the kilogram won’t affect whether our apples are a bit heavier or lighter, but when the standard isn’t standard things get out of control.

The kilogram is used in many other units like the joule. The joule is the amount of energy required to move a one-kilogram weight one meter. The candela, a measure of the brightness of light, is measured in joules per second. These two units are very important in many industries and could cause more problems in the technology industry. As technology occurs on increasingly smaller scales, even minute errors like a faulty kilogram could cause huge problems. As a NIST physicist says, “The kilogram unreliability will start to be noticeable in the next decade or two in the electronics industry.”

By definition, the kilogram is the mass of the international prototype of the kilogram. This means mass is the last remaining base unit in the International System of Units that relies on a physical object. Today, whenever scientists need to verify something is precisely one kilogram, they turn to one of the replicas of the metal cylinderThis system sounds absurd, but not too long ago, lots of units relied on a similar method. That’s why scientist are working on defining the kilogram in terms of a fundamental constant. Constants are known to be universal and precise making them perfect for setting standards. All the other base unit in the SI are all defined by fundamental constants of nature such as the speed of light. For example, the meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,456 of a second.

The Solution (Kind of)

Right now, scientists are working on defining the kilogram by new means. There are two methods in the lead. The NIST team are currently attempting to use what’s called a watt balance. “It’s basically just a very highly calibrated bathroom scale,” says Steiner, who is in charge of the watt balance at NIST.

An ordinary beam balance works by adjusting the amount of mass on one side of the beam so, that its weight exactly balances the weight of a test mass on the other side. The normal beam balance uses gravity to balance the two object, but a watt balance uses the electromagnetic force to balance the two objects.  Scientist are then able to calculate how much voltage does it take to balance the object, and can define this in terms of Planck’s constant. The scale itself is extremely sensitive — so sensitive that it can detect changes as small as ten-billionths of a kilogram. In addition, lawnmowers, tides, and even earthquakes on the other side of the world are able to upset the balance.

Scientists in Australia, Italy, Japan, and Germany are hoping to redefine the kilogram by counting the number of atoms inside a very specially designed sphere. The sphere is created from pure Silicon-28 and weighs exactly one kilogram. So how will they count the number of Silicon atom? One of the unique things about Silicon is that when it crystallizes, it forms cubic cells that contain eight atoms each. The number of atoms can be calculated by examining the ratio between the total crystal volume and the volume occupied by each silicon atom. This, in turn, can be calculated by measuring the cubic cell.

With all these new techniques and technology, why haven’t scientists already redefined the kilogram? Well, the answer is quite simple. It still isn’t as accurate as the one in the International System of Units in Paris.

However, scientist are getting closer and closer. Just recently after years of discussion the kilogram will be defined in 2018 using the Planck constant (using the watt balance). The results from both the silicon sphere and the watt balance will be so similar they can be used to check on one another. Ultimately, the redefinition of the kilogram will make precision measurement more readily available to a greater number of labs. Peter Mohr, a theoretical physicist at NIST, says, “The new kilogram will be worth all the trouble. People have worried about mass standards for thousands of years.”

To learn more about the Silicone sphere click here

To learn more about the Watt balance click here

Picture credits to

For a great video about the kilogram problem click here

Editor: Robyn Sutter