How much does a kilogram weigh? The unit is due to be redefined in 2018, and now physicists have new tools for moving masses in a vacuum, a step vital to the work. The kilogram is the only base unit in the International System of Units still defined with reference to a material artefact. One way to redefine it is in terms of Planck's constant, which relates the energy and frequency of a photon. A practical method involves a watt balance, a device that converts a measurement of mass into current and voltage.
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