Stop that interference! [Basic Metrology]

The esoteric world of setting up and propagating the base SI units with measurements made by relatively simple apparatus to incredible parts-in-a-billion accuracy seems remote and irrelevant to ordinary everyday measurement and control, but the measurements rely on strict adherence to simple basic principles which are relevant right down the measurement chain. For example, many years ago one of us had to set up, maintain and disseminate the SI impedance units: the farad, henry, and ohm.