These possibilities occur when the measurements appropriate to a physical law, and the physical law itself , are more accurately true than the measurements needed to obtain the definition of a physical quantity. We can then obtain that physical quantity more accurately by application of the physical law than by application of the definition . In other words, the choice of the most accurate standard units depends both on the accuracy of physical law.
In a more modern context , the ac Josephson effect defines a voltage standard, not voltage itself , and the quantum Hall effect defines an electrical resistance standard, not electrical resistance itself. The efforts at laboratories like the National Institute for Standards and Technology are not merely practical, but are also epistemological, treating the very foundations of what we know and how we know it.