Physikalisch-Technische Bundesanstalt

The venerable master kilogram has got itself talked about. Safely stored in a safe in Paris, this platinum iridium cylinder is occasionally brought out to compare it with its "offsets", the national kilogram prototypes. The result of these comparisons: The masses of the metal cylinder are changing! Relative to the Parisian master kilogram, most of the others are becoming heavier. An unsustainable condition for metrology which wishes to stand on firm ground with clearly defined units. Therefore, the metrological world has gone in search of a solution for this problem. In search of a new definition of what a kilogram should be.

At present, the most promising attempt to achieve a redefinition seems to be the so-called watt balance, a test which is above all promoted by the American sister of PTB, the National Institute of Standards and Technology. In this test arrangement, which extends over several floors, the weight force of a mass in the gravity field of the Earth is compensated by an electromagnetic force. If this test is successful, the "new mass" will, however - cheekily formulated - be "electrically" defined. (In that case, the next student generations cannot necessarily be envied)

But an attempt alone does not yet make a new and safer definition. And that is why everybody is also looking to the PTB in Braunschweig. Because it is there where the international Avogadro experiment is performed. An experiment, in which a "large mass" is to be fragmented into the sum of many "small masses" - more exactly: it is the aim of the experiment to find out how many atoms are contained in an almost perfect silicon sphere. If the scientists succeed in performing this "count work" (they must not miscount!), then this experiment could compete with the watt balance and two independent proposals would exist for the new kilogram.

However, things have not yet advanced that far. At present, the scientists hope to find a new silicon sphere which is composed of only one single silicon type (the sphere so far used is a mixture of the three naturally occurring silicon types). As to counting, there is no end in sight! 

Determination of the Avogadro Constant (Working Group 3.24)