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The watt balance relates electrical and mechanical power to determine the SI watt, which, in conjunction with an SI ohm, can determine the SI ampere or volt. It can also be used to measure Planck's constant h in SI units, and if the SI is altered to fix the value of Planck's constant rather than the value of the mass of the prototype kilogram, the last artifact standard will be eliminated from the SI. An ensemble of watt balances, which is run by laboratories around the world, could then provide a way of maintaining the SI unit of mass. At present, there are three operational watt balances, with more planned or under construction. Two laboratories, namely, the National Institute for Science and Technology (NIST) and the National Physical Laboratory (NPL), have published results from their recent watt balance measurements. These results disagree by more than their combined standard uncertainties, and further work is required to discover the source of the discrepancy.