page_title: Kibble balance
section_title: Implementation
hierarchical_section_title: Kibble balance / Implementation
caption_attribution_description: English: Schematic representation of a kibble balance in weighing mode
context_page_description: A Kibble balance is an electromechanical measuring instrument that measures the weight of a test object very precisely by the electric current and voltage needed to produce a compensating force. It is a metrological instrument that can realize the definition of the kilogram unit of mass based on fundamental constants.
  It was originally called the watt balance because the weight of the test mass is proportional to the product of current and voltage, which is measured in watts. In June 2016, two months after the death of its inventor, Bryan Kibble, metrologists of the Consultative Committee for Units of the International Committee for Weights and Measures agreed to rename the device in his honor.
  Prior to 2019, the definition of the kilogram was based on a physical object known as the International Prototype of the Kilogram.
  After considering alternatives, in 2013 the General Conference on Weights and Measures agreed on accuracy criteria for replacing this definition with one based on the use of a Kibble balance.
context_section_description: The Kibble balance is constructed so that the mass to be measured and the wire coil are suspended from one side of a balance scale, with a counterbalance mass on the other side. The system operates by alternating between two modes: "weighing" and "moving". The entire mechanical subsystem operates in a vacuum chamber to remove the effects of air buoyancy.
  While "weighing", the system measures the "I" component and the "v" component. The system controls the current in the coil to pull the coil through a magnetic field at a constant velocity "v". Coil position and velocity measurement circuitry uses an interferometer together with a precision clock input to determine the velocity and control the current needed to maintain it. The required current is measured, using an ammeter comprising a Josephson junction voltage standard and an integrating voltmeter.
  While "moving", the system measures the "U" component. The system ceases to provide current to the coil. This allows the counterbalance to pull the coil (and mass) upward through the magnetic field, which causes a voltage difference across the coil. The velocity measurement circuitry measures the speed of movement of the coil. This voltage is measured, using the same voltage standard and integrating voltmeter.
  A typical Kibble balance measures U, I, and v, but does not measure the local gravitational acceleration "g", because "g" does not vary rapidly with time. Instead, "g" is measured in the same laboratory using a highly accurate and precise gravimeter. In addition, the balance depends on a highly accurate and precise frequency reference such as an atomic clock to compute voltage and amperage. Thus, the precision and accuracy of the mass measurement depends on the Kibble balance, the gravimeter, and the clock.
  Like the early atomic clocks, the early Kibble balances were one-of-a-kind experimental devices and were large, expensive, and delicate. As of 2019, work is underway to produce standardized devices at prices that permit use in any metrology laboratory that requires high-precision measurement of mass.
  As well as large Kibble balances, microfabricated or MEMS watt balances (now called Kibble balances) have been demonstrated since around 2003. These are fabricated on single silicon dies similar to those used in microelectronics and accelerometers, and are capable of measuring small forces in the nanonewton to micronewton range traceably to the SI-defined physical constants via electrical and optical measurements. Due to their small scale, MEMS Kibble balances typically use electrostatic rather than the inductive forces used in larger instruments. Lateral and torsional variants have also been demonstrated, with the main application (as of 2019) being in the calibration of the Atomic Force Microscope.
caption: Weighing mode
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