Watt balance

The watt balance is a precision scale, in future the kilogram could be defined with by the Planck 's constant is given a fixed numerical value.

Background

Kilogram is the only SI base unit, which can not be realized with the aid of a measuring procedure. It is defined on the prototype kilogram kept in Paris since 1889. Comparative measurements between this prototype and national copies show a divergence of about 50 ppb over 100 years. For decades, physicists strive therefore, the reproducibility of experiments, with which the unit mass can be attributed to natural constants to improve to <10 ppb. One approach is to 1975 by BP Kibble at the British National Physical Laboratory ( NPL) proposed watt balance.

Principle of measurement

On a coil in a magnetic field, two measurements are performed, a weighing, and movement. When weighing, the current I is determined, which is necessary for the compensation m the weight of the composition:

This, g is the gravitational acceleration, the measured very accurately by case experiments, ie to the set by natural constants base units meter and second can be recycled.

The interferometrically controlled motion with velocity v induces a voltage U

Which is measured at zero current. The proportionality constant, 1/BL or BL, from induction and length of the coil wire, cut out in the multiplication of the equations:

On both sides of this equation, a power unit is connected to the Watt. This gave the process the name. A direct electrical power measurement would be distorted by the Joule heat. To obtain a measurement result of the mass, this equation will be transformed into:

Is the voltage U is measured as a multiple n of a Josephson voltage Uj

Which is precisely adjusted via the microwave frequency fJ. h is the Planck constant and e is the elementary charge.

The current I is also determined by means of the quantum Hall effect of a voltage:

This n 'and r more dimensionless factors and is the von Klitzing constant.

Of the natural constants occurring in both quantum effects and h e is shortened out the latter:

Experimental

The measurement takes place in a very complex structure in a high vacuum. Interfering magnetic fields must be excluded at greater distances. As well as other deformations and vertical movements of the coil.

At the International Bureau of Weights and Measures (BIPM ) a copy with a superconducting coil is currently being established, which allows the simultaneous measurement of current and voltage without measurement error by a coil resistance. This reduces the requirements for the stability of the magnetic field and coil geometry.

For the test mass in the watt balance various alloys are discussed, for example, a gold -platinum alloy. The material must not only, as usual for mass - measuring standards, be abrasion and corrosion resistant, but also the lowest possible magnetic susceptibility ( magnetization ) have.

Competing processes

In addition to the watt balance is working on further procedures by which the kilogram can be redefined. In addition to the watt balance while the Avogadro project is the most promising.

The Avogadro project

In this project, the kilogram should be defined as a multiple of the atomic mass of a particular isotope. For connection to ponderable masses, a large number of these atoms must be precisely determined. The approach of the Avogadro project is the indirect counting from the volume and the lattice constants of a single crystal, isotopically pure silicon sphere. The volume interferometrically and the lattice constant is determined by X-ray diffraction.