The Kibble balance, seen in last issue, is a more accurate version of the ampere balance, an early current measuring instrument in which the force between two current-carrying coils of wire is measured and then used to calculate the magnitude of the current. The Kibble balance operates in the opposite sense; the current in the coils set very precisely by the Planck constant, and the force between the coils is used to measure the weight of a test kilogram mass. Then the mass is calculated from the weight by accurately measuring the local Earth’s gravity (the net acceleration combining gravitational and centrifugal effects) with a gravimeter. Thus the mass of the object is defined in terms of a current and a voltage— allowing the device to “measure mass without recourse to the IPK (International Prototype Kilogram) or any physical object”. Basic principles of the Kibble balance, a device that can be used to measure mass.The International Prototype Kilogram, stored in a vault in Paris, was replaced in 2019 by a formula that uses the Planck constant, since the IPK was losing mass.Prior to 2019, the definition of the kilogram was based on a physical object known as the International Prototype of the Kilogram (IPK),It’s the mass of a hunk of platinum-iridium alloy that’s been housed at the International Bureau of Weights and Measures in Sèvres, France since 1889. It’s called the International Prototype Kilogram (a.k.a. Big K, or Le Grand K), and it has many copies around the world . After considering alternatives, in 2013 the General Conference on Weights and Measures (CGPM) agreed on accuracy criteria for replacing this definition with one based on the use of a Kibble balance. After these criteria had been achieved, the CGPM voted unanimously on November 16, 2018 to change the definition of the kilogram and several other units, effective May 20, 2019, to coincide with World Metrology Day. One important reason for the change is that Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it’s still exactly one kilogram as per the current definition.