Quote by Phrak
Any instances where this replacement trick doesn't translate well should be the most interesting, such as a yardsticks measures d/dx.

I'm not sure what you mean. But yes, in the same way as a clock measures d/dt I suppose if you want to put it in terms of a differential.
http://en.wikipedia.org/wiki/Atomic_clock
Since 1967, the International System of Units (SI) has defined the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the ground state of the caesium133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard). Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions.[3]
