The Nist Watt Balance is an interesting and beautiful experiment housed in the nonmagnetic building behind the reactor. The underlying physical principles of the experiment are very simple: a current in a magnetic gradient will produce a force that is quantifiable in terms of fundamental electrical units. Also, the electrical units can be related to mechanical units by comparing the magnetic force to the weight of a known mass -- a mechanical force. By these principles, the experiment determines the value of Plank's constant, and fixes the SI unit of the volt. If the accuracy of the experiment can be improved sufficiently, it will allow us to remove the dependence of the SI on the artifact mass standard which defines the kilogram. This is our long term goal.
Although the Watt Balance concept is straightforward, its implementation requires the consideration of many important and subtle details. The balance is tied to many diverse fields of physics including: absolute gravimetry, voltage and resistance comparison and standards (quantum Hall effect and the Josephson junction), mass standards, high precision laser interferometry, and magnetic measurements. This breadth of aspect implies a door opened to many sources of error; we are working hard to close this door. In the newest incarnation of the experiment, now in the finishing stages of construction, we are incorporating many features designed to reduce uncertainty and improve our ability to test and monitor experimental parameters.