Weighty Matter: The Kilo Is No Longer A Kilo!

The issue is weighty enough, no matter how you choose to define it. For the first time, it seems that the unit of mass is losing its proper definition. According to the SI system of units accepted around the world by scientists and the public alike, mass should be measured in the units of a kilogram. The definition of a kilogram is kept (yes, literally kept) at the Institute of Weights and Measures at Sevres, near Paris, France. It is a platinum-iridium rod, in the 90-10 (Pt-Ir) ratio, having a mass, which is defined as a kilo. The problem? It is losing weight.

Changing mass: The kilogram bar, kept at Sevres, near Paris.

Losing or Gaining? And How?

The metal cylinder is kept in a strongbox, which has enough security to make a politician jealous. The news of the mass loss came in 1992 and since then scientists have been scratching their heads trying to gauge the source of the mass loss. The situation is even worse, as Alain Picard describes:

Actually, we are not sure whether it has lost mass or gained it. The change may be due to surface effects, loss of gas from the metal or a buildup of contamination.

So, how much has the standardrod’s mass changed by? It has been measured to be 50 micrograms! (A microgram is one millionth of a gram!) Too small you say? Gigantic, say the various experimental labs, carrying out extremely precise tests.

How To Define The Units

Presently, kilogram is the only unit that is defined by a physical object. Earlier, the meter was also defined like this by a platinum rod, kept at Sevres. However, this was replaced by a more accurate (and more scientific) definition depending on the velocity of light it is the length travelled by light in vaccum in 1/299,792,458 th of a second. Then, what is a second you ask? As per NIST (National Standard of Standards and Technology), the second is the duration of  9 192 631 770  periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. Similarly, there are standard definitions for all the base units ampere (current), temperature (Kelvin), amount of substance (mole) and luminous intensity (candela). Check them out here.

Remedy? Just chuck it out!

The solution to this weighty problem is simple: just phase out the kilo cylinder. There is a proposal to replace it with a definition involving the ubiquitous and universal Planck’s constant or h’. It’s only fair that this get its place on the chart of definitions.

No official definition has been made as yet, but we expect the unit of mass to be defined in some multiple of definite energy(which is where the Planck’s constant will occur) over speed of light squared (in the standard way to define mass according to m=E/c2). The change will, however, not take effect any time before 2014.

For all daily matters, a kilo will be a kilo and you’ll never notice the difference when you go grocery shopping. However, the big high-precision labs should be looking forward to a fairly large revision of their numbers.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.