Redefining the Kilogram, NIST-4 Components
1. The diamond-turned balance wheel is a distinctive feature of NIST-4. Other watt balances employ a balance beam. The aluminum wheel balances on a knife edge made from ultra-hard metal and can rotate as much as 10 degrees.
2. The mass and coil assembly is suspended from a cable attached to the wheel with a band of wire filaments. That configuration minimizes deviation from exactly vertical motion.
3. The three-armed "spider" holds the test mass and its tray and — by way of three black vertical rods — the suspended coil. For operation, three components must be precisely aligned vertically: the test mass, the coil center, and the magnetic field center.
4. The coil is completely immersed in the 0.55 T field created by a 1 ton permanent magnet system, and moves up and down in both weighing and velocity modes.
5. The permanent magnet system is situated at the bottom of the instrument and thus is not visible when the watt balance is viewed from the side. It was custom manufactured for NIST-4 with magnets made from samarium and cobalt in an iron enclosure.
6. Suspended from the opposite side of the wheel from the test mass and coil are the counter-mass and the electric motor that produces motion when the watt balance operates in velocity mode.
Created October 16, 2014, Updated January 19, 2017