Sorry if this if off-thread julian403, but the units we use are fascinating. I always meant to put these down in one place in a manner that each is derived from the previous ones. Some of you will know all of these, some will be a surprise, some have changed very slightly due to convenience, but here goes:

Metre = circumference of earth/40,000,000

Second = length of day/ 24/ 60/60

Degree C = temperature of boiling point-temperature of freezing point of water/100

Joule = energy required to raise the temperature of 1 cubic cm of water by one degree

Newton= force required to create one joule of energy per metre the force moves though

Ampere= current flowing though 2 parallel infinite wires one metre apart that creates a force of one Newton between them

Coloumb = charge passing per second when a current of one ampere flows.

Volt = potential difference required to create power of one joule per second if one ampere is flowing.

Henry =If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry.

Magnetic permeability= Henry's per metre.

Farad= One farad is defined as the capacitance of a capacitor across which, when charged with one coulomb of electricity, there is a potential difference of one volt

Electric permittivity= farads per metre

It is striking how all our units are related to the properties of water and the earth.