The International Prototype of the Kilogram is an object whose mass was used to define the kilogram from 1889, when it replaced the Kilogramme des Archives, until 2019, when it was replaced by a new definition of the kilogram based entirely on physical constants. During that time, the IPK and its duplicates were used to calibrate all other kilogram mass standards on Earth. A Kibble balance, which is used to measure weight via electric current and voltage. With this instrument, the measurement of mass is no longer dependent on a defined mass standard and is instead dependent on natural physical constants.A diagram of the second-generation tabletop Kibble balance being built at NIST. In this version, movement is restricted to two dimensions using flexures, metal strips that only bend on one axis. The mass pan holds the physical object being weighed. An electromagnetic coil sits on each side of the device. Magnets provide a magnetic field through which the electromagnetic coils move. An optical encoder, which detects light and converts it into electrical signals, measures the movement of the coil. The whole device is roughly the size of a ream of printer paper.The circuit diagram of the Kibble balance in force mode (top) and velocity mode (bottom). A digital voltmeter (DVM) is used to measure the small difference between the programmable Josephson voltage standard (PJVS) and the voltage across the resistor (top) or the induced voltage across the coil (bottom).