Resistance should be easy as well. I think the wikipedia page for a Wheatstone bridge explains this best. If this sort of thing interests you, I'd encourage yo

Resistance should be easy as well. I think the wikipedia page for a Wheatstone bridge explains this best. If this sort of thing interests you, I'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. Okay well more than you ever wanted to know.