Exact Value
Since 1948, the SI unit ampere has been defined by choosing the numerical value of μ0 to be exactly 4π×10−7 H/m. Similarly, since 1983 the SI metre has been defined by choosing the value of c0 to be 299 792 458 m/s. Consequently
- exactly,
or
- . This situation may change if the ampere is redefined in 2015. See New SI definitions.
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