Hi, When i Look at SI derived base units like Joule, Newton, pascal and base units like the Ampere which is defined as "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7newton per metre of length." Then 2 questions come to mind 1. How are theese units measured - With what equipement? How is the joule 2. How does it work - How does the measuring equipement measure it? What is i measuring? How is the Joule = kg⋅m2⋅s−2 Measured? How do you measure the force of 2 × 10−7newton of two currents one metre apart in vacuum? This seems so vague to me? Are there any good textbooks or websites which describe this in detail? I'm interested in understanding how things are done in physics.