Rather than calling a tonne a metric ton, we should have denoted ne as the first and only SI suffix, denoting a value of 1 imperial ton / 1 metric ton
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Why are the kilogram, ampere, kelvin, and mole still considered base units and not derived? Given their current definitions, they're purely defined in terms of other units and cannot be independently measured, right?
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It's no different to how the metre is defined by using the speed of light - it's taking a fixed value for a physical constant and then deriving the value of the base unit.
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Right, but the Planck constant isn't something you can independently measure. The kg is now defined by giving it an exact value
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Well, Kelvin is based on a known (if theoretical) value, absolute zero. And it is the same size per unit as Celsius. So I’d say that’s independently understood. Kilos are directly mapped to litres (1 litre of water weighs 1 kilo), but yeah, it’s strange that’s the base.
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Right, those are the historical definitions. But today the kg is defined in terms of the Planck constant, and Kelvin in terms of the Boltzmann constant.
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Base means here it cannot be derived from other units alone. kg is defined in terms of m and s AND h. If h weren't "injected" into the definition then it'd be a derived unit.
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h is a defined constant though, not a unit
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The kilogram at least is not derived it is defined. They have reference kilograms that they define all other kilograms by.
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That hasn't been true for nearly a year now
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