The ampere ( SI unit symbol: A ), often shortened to amp, is the SI unit of electric current ( quantity symbol: I, i ) and is one of the seven SI base units.

* A short history of the SI units in electricity

Category: SI base units

In the IERS numerical standards, the speed of light in a vacuum is defined as c < sub > 0 </ sub > = in a vacuum, in accordance with the SI units.

Neither G nor M < sub >☉</ sub > can be measured to high accuracy in SI units, but the value of their product is known very precisely from observing the relative positions of planets ( Kepler's Third Law expressed in terms of Newtonian gravitation ).

Only the product is required to calculate planetary positions for an ephemeris, which explains why ephemerides are calculated in astronomical units and not in SI units.

As seen from the definition, the derived SI units of angular momentum are newton meter seconds ( N · m · s or kg · m < sup > 2 </ sup > s < sup >− 1 </ sup >) or joule seconds ( J · s ).

It is also not consistent with the SI convention that only units named after persons should be capitalized.

The International System of Units does not define units for digital information but notes that the SI prefixes may be applied outside the contexts where base units or derived units would be used.

The most commonly used units for molarity are mol / L ( the official SI units are mol / m < sup > 3 </ sup >).

Like other SI base units, the candela has an operational definition — it is defined by a description of a physical process that will produce one candela of luminous intensity.

Category: SI base units

Within each table, the units are listed alphabetically, and the SI units ( base or derived ) are highlighted.

For each physical quantity, a number of different units ( some only of historical interest ) are shown and expressed in terms of the corresponding SI unit.

In SI units, the current density is measured in amperes per square metre.

* SI electromagnetism units

These are Kelvin ( SI units ) or Rankine scale ( Imperial & US units ).

The SI unit of charge, the coulomb, " is the quantity of electricity carried in 1 second by a current of 1 ampere ".

In the International System of Units ( SI ), the standard unit of area is the square metre ( written as m < sup > 2 </ sup >), which is the area of a square whose sides are one metre long.

The SI unit of area is the square metre, which is considered an SI derived unit.

The meter is defined to be a unit of proper length, but the SI definition does not specify the metric tensor to be used in determining it.

The SI unit of measurement of electric current, the ampere, is named after him.

* aw, or attowatt, an SI unit of power

In the International System of Units ( SI ), B is the symbol of the bel, a unit of logarithmic power ratios named after Alexander Graham Bell.

Considerable confusion exists about the meanings of the SI ( or metric ) prefixes used with the unit byte, especially concerning prefixes such as kilo ( k or K ) and mega ( M ) as shown in the chart Prefixes for bit and byte.

( Note that K is the SI unit for temperature ( kelvin ) and should not be confused with k, the SI prefix for kilo.

In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule, though it may be used as a measure of agricultural energy production ( Btu / kg ).

The candela ( or ; symbol: cd ) is the SI base unit of luminous intensity ; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ( a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency function ).

Although the candela is now defined in terms of the second ( an SI base unit ) and the watt ( a derived SI unit ), the candela remains a base unit of the SI system, by definition.

The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, where one hertz is equal to one oscillation per second.

Caesium clocks are the most accurate commercially produced time and frequency standards, and serve as the primary standard for the definition of the second in SI ( the metric system ).

When the independent variable x represents time ( with SI unit of seconds ), the transform variable ξ represents frequency ( in hertz ).

In SI units, angular frequency is normally presented in radians per second, even when it does not express a rotational value.

In electromagnetic transmission devices, especially those dealing with telecommunications, effective radiated power or equivalent radiated power ( ERP ) is a standardized theoretical measurement of radio frequency ( RF ) energy using the SI unit watts, and is determined by subtracting system losses and adding system gains.

* Cycle ( s ) per second, now known as hertz, which is the SI unit for frequency

The angular frequency ( ω = 2πf ) of this cyclotron motion for a given magnetic field strength B is given ( in SI units )< ref > In SI units, the elementary charge e has the value 1. 602 × 10 < sup >- 19 </ sup > coulombs, the mass of the electron m has the value 9. 109 × 10 < sup >– 31 </ sup > kilograms, the magnetic field B is measured in teslas, and the angular frequency ω is measured in radians per second .</ ref > by

Spectral radiance expresses radiance as a function of frequency ( Hz ) with SI units W · sr < sup >− 1 </ sup >· m < sup >− 2 </ sup >· Hz < sup >− 1 </ sup > or wavelength ( nm ) with units of W · sr < sup >− 1 </ sup >· m < sup >− 2 </ sup >· nm < sup >− 1 </ sup > ( more common than W · sr < sup >− 1 </ sup >· m < sup >- 3 </ sup >).

The SI term hertz has replaced the term cycles per second as a unit of frequency.

* Hertz, the SI unit of frequency

The majority of the clocks are caesium clocks ; the definition of the SI second is written in terms of caesium.

To express even smaller angles, standard SI prefixes can be employed ; in particular, the milliarcsecond, abbreviated mas, is used in astronomy.

Industry practice, more thoroughly documented at Timeline of binary prefixes and continuing today, is to specify hard drives using SI prefixes and symbols in their SI or " decimal " interpretation.