"SI" redirects here. For other uses, see Si (disambiguation).
Cover of brochure The International System of Units
The older metric system included several groups of units. The SI was developed in 1960 from the old metre-kilogram-second system, rather than the centimetre-gram-second system, which, in turn, had a few variants. Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improves.
The system has been nearly globally adopted with the United States being the only industrialized nation that does not mainly use the metric system in its commercial and standards activities. The United Kingdom has officially adopted metrication, but not with the intention of replacing customary measures entirely. Canada has adopted it for all legal purposes but imperial/US units are still in use, particularly in the buildings trade.
Ampere - A
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 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 Newton per meter of length.Electric current is the same as electric quantity in movement, or quantity per unit time, expressed like
I = dq / dt
where
I = electric current (ampere)
dq = electric quantity (coulomb)
dt = time (s)
Coulomb - C
The standard unit of quantity in electrical measurements. It is the quantity of electricity conveyed in one second by the current produced by an electro-motive force of one volt acting in a circuit having a resistance of one ohm, or the quantity transferred by one ampere in one second.Farad - F
The farad is the standard unit of capacitance. Reduced to base SI units one farad is the equivalent of one second to the fourth power ampere squared per kilogram per meter squared (s4 A2/kg m2).When the voltage across a 1 F capacitor changes at a rate of one volt per second (1 V/s) a current flow of 1 A results. A capacitance of 1 F produces 1 V of potential difference for an electric charge of one coulomb (1 C).
In common electrical and electronic circuits units of microfarads μF (1 μF = 10-6 F) and picofarads pF (1 pF = 10-12 F) are used.
Ohm - Ω
The derived SI unit of electrical resistance - the resistance between two points on a conductor when a constant potential difference of 1 volt between them produces a current of 1 ampere.Henry - H
The Henry is the unit of inductance. Reduced to base SI units one henry is the equivalent of one kilogram meter squared per second squared per ampere squared (kg m2 s-2 A-2).Inductance
An inductor is a passive electronic component that stores energy in the form of a magnetic field.The standard unit of inductance is the henry abbreviated H. This is a large unit and more commonly used units are the microhenry abbreviated μH (1 μH =10-6H) and the millihenry abbreviated mH (1 mH =10-3 H). Occasionally, the nanohenry abbreviated nH (1 nH = 10-9 H) is used.
Joule - J
The unit of energy work or quantity of heat done when a force of one Newton is applied over a displacement of one meter. One joule is the equivalent of one watt of power radiated or dissipated for one second.In imperial units the British Thermal Unit (Btu) is used to express energy. One Btu is equivalent to approximately 1,055 joules.
Siemens - S
The unit of electrical conductance S = A / VWatt
The watt is used to specify the rate at which electrical energy is dissipated, or the rate at which electromagnetic energy is radiated, absorbed, or dissipated.The unit of power W or Joule/second
Weber - Wb
The unit of magnetic flux.The flux that when linking a circuit of one turn, produces an Electro Motive Force - EMF - of 1 volt as it is reduced to zero at a uniform rate in one second.
- 1 Weber is equivalent to 108 Maxwells
Tesla - T
The unit of magnetic flux density the Tesla is equal to 1 Weber per square meter of circuit area.Volt
The Volt - V - is the Standard International (SI) unit of electric potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance.Reduced to SI base units,
1 (V) = 1 (kg m2 / s3 A)
The tables below list the systems of electrical and magnetic units. They only include units of interest in the field of Radio.
The older systems were the CGS and Gaussian systems.The Gaussian system being based on a mix of Electrostatic units (ESU) and Electromagnetic units (EMU). The current standard is the International System of Units (SI) and is sometimes referred to as rationalised MKS units.
Conversions from one system to others are given in two ways. Firstly numerically by multiplying factors. Note that c stands for the velocity of light in space and its value is exactly 299792458 metre/second exactly (by definition of the metre).
The other method of conversion allows you to change formulas given in old books into the modern SI form. This will be particularly useful to you if, like me, you have books by Terman, Scroggie, etc. or the Admiralty Handbook of Wireless Telegraphy. There were lots of useful formulas in these old books, which can now be given new life.
I have tried to make the tables as complete and accurate as possible and have checked lots of different sources. Nevertheless there may be errors and I will be grateful for any corrections or additions. Please email me at the address given on the home page.
SI Units
| Quantity | Symbol | Unit & (Abbr.) | Dimensions | ESU | EMU |
| Mass | m | kilogram (kg) | M | 1000 | 1000 |
| Length | l | metre (m) | L | 100 | 100 |
| Time | t | second (s) | T | 1 | 1 |
| Power | P | watt (W) | ML2T-3 | 107 | 107 |
| Electric Current | I | ampere (A) | I=M½L1½T-2 | 10c | 0.1 |
| Charge | Q | coulomb (C) | TI | 10c | 0.1 |
| Electric Potential | V | volt (V) | ML2T-3I-1 | 106/c | 108 |
| Resistance | R | ohm ( ) | ML2T-3I-2 | 105/c2 | 109 |
| Conductance | G | siemens (S) | M-1L-2T3I2 | 10-5c2 | 10-9 |
| Inductance | H | henry (H) | ML2T-2I-2 | 105/c2 | 109 |
| Capacitance | C | farad (F) | M-1L-2T4I2 | 10-5c2 | 10-9 |
| Electric Field Strength | E | volt/metre (V m-1) | MLT-3I-1 | 104/c | 106 |
| Electric Charge Density |  | coulomb/cu.m. (C m-3) | L-3TI | c/105 | 10-7 |
| Electric displacement Electric flux density | D | coulomb/sq.m. (C m-2) | L-2TI | 4 10-3c | 410-5 |
| Magnetic Potential |  | ampere (A) | I | 410c | 410-1 |
| Magnetic Field Strength | H | ampere/metre (A m-1) | L-1I | 4c/10 | 410-3 |
| Magnetic flux |  | weber (Wb) | ML2T-2I-1 | 106/c | 108 |
| Magnetic Induction Magnetic Flux Density | B | tesla (T) | MT-2I-1 | 100/c | 104 |
| Magnetic susceptibility |  e | none | none | 10-4c-2/(4) | 1/(4) |
| Magnetic Moment | m | ampere metre2 (A m2) | L2I | 105c | 103 |
| Magnetisation (Magnetic moment/unit volume) | M | ampere/metre (A m-1) | L-1I | 10-1c | 10-3 |
| Magnetic Polarisation | J | tesla (T) | MT-2I-1 | 102/(4c) | 104/(4) |
| Magnetic Pole Strength | P | ampere metre (A m) | LI | 103c | 10 |
| Magneto Motive Force | Fm | ampere (A) | I | 410c | 410-1 |
| Magnetic Reluctance | S | ampere/weber(A Wb-1) | I2M-1L-2T2 | 410-9 | 410-9 |
| Permittivity of space | farad/metre (F m-1) | M-1L-3T4I2 | |||
| Permeability of space |  | henry/metre (H m-1) | MLT-2I-2 |
