CGPM set to update international system of weights and measures

Kilogram Has A Weight-Loss Problem!!

The international prototype of the kilogram is inside three nested bell jars at the Bureau International des Poids et Mesures in Paris.

More than a century ago, a small metal cylinder was forged in London and sent to a leafy suburb of Paris. The cylinder was about the size of a salt shaker and made of an alloy of platinum and iridium, an advanced material at the time.

In Paris, scientists polished and weighed it carefully, until they determined that it was exactly one kilogram, around 2.2 pounds. Then, by international treaty, they declared it to be the international standard.As it stands, the entire world's system of measurement hinges on this cylinder.

[ The Metre Convention was signed on 20 May 1875 and established the SI system, which since 1889 defines the magnitude of the kilogram to be equal to the mass of the international prototype kilogram,^[1] often referred to in the professional metrology world as the “IPK”. The IPK is made of a platinum alloy known as “Pt‑10Ir”, which is 90% platinum and 10% iridium (by mass) and is machined into a right-circular cylinder (height = diameter) of 39.17 millimeters to minimize its surface area.^[10] The addition of 10% iridium improved upon the all-platinum Kilogram of the Archives by greatly increasing hardness while still retaining platinum’s many virtues: extreme resistance to oxidation, extremely highdensity (more than twice as dense as lead and more than 21 times as dense as water), satisfactory electrical and thermal conductivities, and low magnetic susceptibility]

[ The IPK and its six sister copies are stored at the International Bureau of Weights and Measures (known by its French-language initials BIPM) in an environmentally monitored safe in the lower vault located in the basement of the BIPM’s Pavillon de Breteuil in Sèvres on the outskirts of Paris (see External images, below, for photographs). Three independently controlled keys are required to open the vault. Official copies of the IPK were made available to other nations to serve as their national standards. These are compared to the IPK roughly every 50 years.]

The official kilogram is kept locked inside a secured vault at the International Bureau of Weights and Measures near Paris. Scientists are so paranoid that they've only taken it out on three occasions: in 1889, 1946 and 1989. Each time, they've compared it to a set of copies. In 1889, the copies and the kilogram weighed the same, but by 1989, they had drifted apart. Based on the data, the kilogram appears to weigh slightly less than the copies.The real crux of this problem is that it's impossible to tell what has changed over the past 120 years. The copies may have grown heavier over time by absorbing air molecules. But it's equally possible that the kilogram is getting lighter.

Now The General Conference on Weights and Measures (CGPM) at its most recent meeting in Sčvres, France, has voted unanimously on a proposal to consider changes to at least some of the seven basic units (second, metre, kilogram, ampere, kelvin, mole and candela) of measurements used by most of the civilized world. This comes after years of debate concerning the kilogram, ampere, kelvin, and mole in particular. Of these, the kilogram has come under the most fire as it’s still based on a hunk of metal kept under lock and key in a vault in Paris. It’s been the subject of debate ever since it was discovered in 1948 that its mass had changed.

The seven basic units of measurements are used as standards all by themselves, but also as a means of deriving virtually all other measurements (grams, hours, etc.) As science has become more precise so too has the need for ever more precise measurements, and some of the old ways of doing so are just not holding up. The kilograms shortcomings are obvious, it’s the only one of the seven left still based on a manufactured object. A more accurate way to describe it would be to use a fundamental constant found in nature, such as the Planck constant. And this is exactly what a small group of meteorologists has recommended.

Peter Mohr, Terry Quinn, Barry Taylor and Edwin Williams recently banded together and published an article in the journal Metrologia that championed the switch to the Planck constant, and for changes to three other basic measurements as well: the ampere, kelvin, and mole. And it was this small group that lobbied for the proposed changes that have been addressed at the recent meeting of the CGPM.

At the meeting, it was stressed that any new changes to the way measurements are based, should not change the basic ways that they are now conventionally used, e.g. water would still freeze at 0° C, and boil at 100° and cooking recipes would still work and world records in Olympic events would still all hold, etc. What would change would be the degree of accuracy that could be used to describe a mass, distance, time, etc. that would be consistent regardless of where the measurements were being taken and by whom.

One good example is the metre (meter), a measurement not in need of change. It was originally based on an etching on a metal bar; now it’s based on the distance light travels in a vacuum.

This move by the CGPM means that proposals for the exact changes to be made will be considered and then voted on at the next meeting in 2014. If the changes pass, a timetable for their implementation will be set at that meeting as well.

The CGPM also known as the conférence générale des poids et mesures, is just one of three entities charged with maintaining the International System of Units (SI) by decree of an international convention held way back in 1875. It represents 52 member states and 26 other associates.