Length is established in two ways:

• Long distances are determined by measuring the time that light takes to travel over them in a vacuum, and multiplying that time by the speed of light (299,792,458 metres per second).
• Shorter distances are compared directly with the lengths of light waves, using a technique called 'interferometry'.

The kilogram is simply the mass of the international prototype.

The second is realised by using an atomic clock in which the ‘ticking’ of the clock is the vibration of an individual atom.

In one such type of clock, a group of caesium atoms is first cooled and then exposed to microwaves. The microwaves are then tuned to the value at which the electrons in the atoms vibrate most strongly. This is a frequency of 9,192,631,770 vibrations per second (hertz) – and this is used to define the second.

The kelvin is realised by using the freezing points of metals and the triple points of gases to define a temperature scale, often using an instrument called a 'platinum resistance thermometer'.

The candela, is realised by first determining the power of a laser beam using a cryogenic radiometer.

This measures the power by detecting the temperature rise of a cold object caused by the light. Once the beam power is known it can be used to adjust a photometer, an instrument which mimics the response of the human eye to luminous intensity. Finally, this photometer is used to measure the luminous intensity of a tungsten lamp in candelas.

The mole is realised by dividing the mass of a sample of a substance by the mass of one of its atoms.

Atomic masses can be measured by a mass spectrometer, and accurate weighing gives the mass of the substance sample.

Rather than realising the ampere, the unit of electric current, its value is determined from the watt, the SI unit of power. Power is related to current through the equation.

Power = Current x Current x Resistance

To realise the watt, electrical power is generated and its level determined by comparison with mechanical power.  An accurate measurement of resistance is then made and the value of the ampere is calculated.

Or

The ampere is realised in terms of the volt and ohm, both of which are quantum standards and so depend on fundamental constants.