Resistance should be easy as well. I think the wikipedia page for a Wheatstone bridge explains this best.<p>If this sort of thing interests you, I&#x27;d encourage you to look at the quantum metrology triangle. Frequency, resistance, and current are all currently related to a josephson voltage standard, the quantum hall resistance, and the current generated by a single electron transistor. This allows us to standardize all these back to a frequency. Length can be related to frequency by wavelength of light. Everything has been related back to frequency. Basically the only thing not referred back to something absolute is mass, which I think is still referred back to a mass standard, which is a physical thing stored in a vault.<p>Okay well more than you ever wanted to know.