Sunday, April 20, 2008

The Incredible Expanding Kilo

Over a hundred years ago, the clever-clogs at the French Academy of Science wanted a better standard kilgramme weight, so they got Johnson Mathey of the UK to knock them up a few.

They've been kept in controlled conditions in different locations all over the world. Every 50 years they have a reunion and compare them.

And they keep changing weight.

At the last major kilogram comparison done in and around 1990, some copies had gained as much as 132 micrograms. A few had lost up to 665 micrograms. The United States' No. 20 was 18 micrograms heavier.

There was no way to tell which was changing: Le Grand K, its copies or both.

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.

"Nobody has a really good idea why," said Davis of the International Bureau of Weights and Measures. "It's all speculation."


James Higham said...

That's amazing - never knew it varied.

Brian said...

Global warming?
My theory is that everything in the universe is constantly changing weight and this process will only cease when all objects weigh one ounce or multiples thereof. This theory is called Imperial Entropy and neatly reconciles science and religion because God is an Englishman.

Anonymous said...

If God is an Englishman, why does he speak with such a stern Scots accent?

Anonymous said...

First point, it wasn't the French Academy of Sciences. They certainly created an archive Kilogramme over 200 years ago but K - the current international standard and the national copies - including our primary national standard (since the late 19th century) were commissioned by an international convention of which the UK was a member. Though the international body governing the "metric system" remained by courtesy in Paris it has been an international body for over a century - and the director of its Paris bureau is currently a distinguished British metrologist, Dr Andrew Wallard - distinguished in particular for his work on the Iodine stabilised Helium Neon laser used in mise en pratique for the determination of the Metre.

The most likely explanations for the relative variations of the masses of the couple of dozen standard kilogrammes - K itself, the secondary standards maintained in Paris and the national copies, are (i) the absorbtion of mercury vapour (lots around in labs - and technicians have fillings too) or (ii) absorbtion of hydrocarbon vapours (eg petrol), when the standards are actually being weighed. Platinum has a capacity for absorbing hydrocarbons and mercury just loves amalgamating with other metals.

Brian said...
This comment has been removed by the author.
Brian said...

Because His Welsh accent sounds like a very poor attempt at Pakistani. And remember that the key to the Empire was getting Scots to do a very good job running it.