Losing Weight

The story of the kilogram has interesting ramifications for our concept of measurement to define performance or non performance.

A strong internal control system is one where it is possible to monitor and assess the quality of the system’s performance over time.

What happens to our concept of the measurement of performance, however, when the cornerstone of measurement – the kilogram – isn’t what it should be.


Forty feet underground, secured in a temperature- and humidity-controlled vault here, lies Kilogram No. 20.

It’s an espresso-shot-sized, platinum-iridium cylinder that is the perfect embodiment of the kilogram – almost perfect.

In the 120 years since No. 20 and several dozen other exact copies were crafted in France to serve as the world’s standards of the kilogram, they have been mysteriously drifting apart.

The difference is on average about 50 micrograms – the weight of a grain of fine salt.

But the ramifications have rippled through the world of precision physics.


In essence, no one really knows today what a kilogram is.

The kilogram is the last of seven base units in the International System of Units that is still based on a physical object

In the 18th century, hundreds of thousands of different weights and measures were in use around the world.

The French alone employed about 250,000 different units of measure.

The Enlightenment and French Revolution in late 18th century spurred the idea of standardization.

People could only be free if they could calculate for themselves the weight and cost of things they bought, philosophers reasoned.

The French government created the kilogram in 1795, defining it as the mass of a liter of distilled water at the temperature of melting ice.

A century later, the Treaty of the Meter established the kilogram as an international standard.

The foundation of the standard was a cylindrical ingot of 90% platinum and 10% iridium created in 1878 that became known as Le Grand K, or more officially the International Prototype.

Forty copies were made and distributed to governments around the world. Another 50 were made later.

These 90 copies serve as national standards, used to calibrate working weights in science and industry.

About every 50 years, the national prototypes are returned to the headquarters of the International Bureau of Weights and Measures in Sevres, France, to be compared with the International Prototype.

During the first major comparison about 1950, scientists noticed discrepancies between the average masses of Le Grand K and its copies. They were concerned, but could not discern a trend.

Science was already grappling with inconsistencies in other units and was trying to replace the pieces of metal and other artifacts that delineated the old world.

The meter, for example, was changed in 1960 from two scratches on a platinum-iridium bar to a certain number of wavelengths of light emitted from a particular kind of krypton. In 1983, it was changed again to the distance traveled by light in a specific fraction of a second.

At the last major kilogram comparison about 1990, some of the copies had gained as much as 132 micrograms. A few had lost up to 665 micrograms.

No. 20 was 18 micrograms heavier.

There was no way to tell what was changing: Le Grand K or its copies.

Perhaps the platinum in the cylinders was sopping up mercury from the atmosphere. Maybe dissolved gas was escaping from the cylinders. One idea was that cleaning the cylinders with distilled water and ether had altered their weights.


Two ideas have emerged as the leading contenders to redefine the kilogram.

One involves counting the trillion trillion atoms in the most perfect silicon sphere ever made.

The other attempts to measure the electrical current necessary to balance a one kilogram weight against Earth’s gravity.

After decades of work, both efforts have so far produced stunningly precise measurements – but still too inconsistently to prove the accuracy of their methods.

Based on Kilogram Question Weighs on World – Los Angeles Times – April 17, 2008

Subscribe to Receive Our Email Updates

  • This field is for validation purposes and should be left unchanged.