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Scientists at the Measurement Standards Laboratory of New Zealand (MSL) have developed a novel research approach to the international development of a new mass standard. This approach has attracted the attention of fellow researchers from Canada, China, France, Switzerland, USA and at the BIPM (the home of the SI system).

Research at MSL on the new mass standard is focused on redefining the kilogram in terms of a fundamental physical constant – the Planck constant – instead of depending on a manufactured block of metal to serve as the mass standard.

In order to achieve this, research is centred on relating mechanical power to electrical power by using a device known as a watt balance.

What is a watt balance?

A watt balance relates mechanical power to electrical power by comparing the gravitational force on a mass with the electromagnetic force on a current-carrying coil in a magnetic field.

A watt balance experiment has two parts:

  • A static or weighing experiment in which the gravitational force on a given mass is balanced by an electromagnetic force developed on a coil.
  • A moving or calibration experiment in which the same coil is moved through a magnetic field to produce a voltage.

In the weighing experiment, a mass (m) and a coil of length (L) are suspended from a balance. The coil is placed in a magnetic field of strength (B), and when a current (I) is sent through it, an electromagnetic force is generated, pushing the coil downwards. This downwards force balances the gravitational force on the mass suspended on the other arm of the balance. When balanced, the weight force is equal to the electromagnetic force.

In the moving experiment, the coil is moved at a constant speed (v) through the magnetic field. This action generates a voltage (U) across the coil.

If the properties of the coil (L) and the magnetic field (B) do not change between the two experiments, by combining the results from both experiments, it can be shown that mechanical power equals electrical power. The unit of power is the watt, so the device is called the watt balance.

When the electrical quantities U and I are measured using special quantum electrical effects (known as the Josephson effect and the quantum Hall effect), a link can be established between mass and a fundamental constant in physics known as the Planck constant.

MSL’s novel approach

A watt balance is relatively simple in principle but the real challenge is to perform the experiment with an accuracy of 1 part in 100,000,000 (108).

The hardest parts of the experiment are:

  • moving the coil in a strictly vertical line
  • aligning the coil axis with gravity and the magnetic field at right angles to gravity.

Existing watt balance experiments require correction systems, which add complexity and potentially introduce ‘noise’ to the system.

Dr Chris Sutton from MSL has developed a novel approach to overcoming these design problems.

A long involvement with gas-operated pressure balances led Chris to consider its use as the basis for a watt balance in both the weighing and moving experiments. Chris and his colleagues have developed pressure balances so that two of them can be used as a very precise mass comparator. This eliminates some of the difficulties encountered with conventional watt balances.

Chris’s previous experience with developing a system for calibrating seismometers led him to the idea of the coil moving in an oscillatory manner rather than at a constant speed in the moving experiment. A laser measuring system can track the oscillatory motion of the coil with very high accuracy.

A key enabling technology for this watt balance is the ability to measure the oscillatory voltage (U) in the moving mode with high speed and accuracy. Dr Laurie Christian from MSL, an expert in voltage measurement, will develop a quantum sampling voltmeter based on the Josephson effect for this measurement. He has established a collaboration with the National Institute of Standards and Technology (NIST) – one of the few laboratories in the world that can make the superconducting Josephson integrated circuits required for this sampling voltmeter.

The combination of these ideas along with successful preliminary trials has attracted international recognition. Both Chris and Laurie and their colleagues are developing and refining this novel approach.

Nature of science

From expertise in mass and pressure metrology, Chris saw the match between the need for a watt balance and the features and performances of pressure balances. Unexpected discoveries like this are not uncommon in scientific advances.

    Published 17 August 2011 Referencing Hub articles