About the Metric System
The modernized metric system is known as the International System of Units, usually abbreviated to SI, from the French, "Système internationald'unités".
Base on the number 10, SI is today the world's most widely used system of measurement in science, industry and commerce.
Units
Each SI unit has a specific symbol, which is the same in all languages. For example, in Irish the word for kilogram is 'cileagram', but the symbol is kg.
Base units
SI definitions are founded on seven base units, as set out in the table below.
Quantity
|
Unit
|
Symbol
|
Definition
|
| Length |
metre |
m |
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. |
| Mass |
kilogram |
kg |
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. |
| Time |
second |
s |
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. |
| Electric current |
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 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length. |
| Thermodynamic temperature |
kelvin |
K |
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. |
| Amount of substance |
mole |
mol |
- The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
- When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
|
| Luminous intensity |
candela |
cd |
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. |
Supplementary units
In addition, there are two supplementary units.
Quantity
|
Name of Unit
|
Symbol
|
| plane angle |
radian |
rad |
| solid angle |
steradian |
sr |
Derived units
These are derived with base and/or supplementary units. They are expressed by using the SI symbols with multiplication and/or division signs.
Quantity
|
Name
|
Symbol
|
Equivalent to
|
|
force
|
newton
|
N
|
kg.m/s2
|
|
pressure
|
pascal
|
Pa
|
N/m2
|
|
work, energy, quantity of heat
|
joule
|
J
|
N.m
|
|
power, heat flow rate
|
watt
|
W
|
J/s
|
|
quantity of electricity
|
coulomb
|
C
|
A.s
|
|
electric potential
|
volt
|
V
|
W/A
|
|
electric resistance
|
ohm
|
Ω
|
V/A
|
|
electric capacitance
|
farad
|
F
|
C/V
|
|
magnetic flux
|
weber
|
Wb
|
V.s
|
|
inductance
|
henry
|
H
|
Wb/A
|
|
magnetic flux density
|
tesla
|
T
|
Wb/m2
|
|
frequency
|
hertz
|
Hz
|
s-1
|
|
luminous flux
|
lumen
|
lm
|
cd.sr
|
|
illuminance
|
lux
|
lx
|
lm/m2
|
Prefixes
Decimal multiples and submultiples of SI units can be expressed by adding prefixes to the unit names. The prefix is directly attached to the unit name to form a new unit name, which is written as one word, e.g. kilometre; millimetre.
The prefix symbol is directly attached to the unit symbol to form a new unit symbol: km for kilometre; mm for millimetre.
Prefix
|
Symbol
|
Means multiply by
|
Or by
|
|
mega
|
M
|
1 000 000
|
106
|
|
kilo
|
k
|
1 000
|
103
|
|
hecto
|
h
|
100
|
102
|
|
deca
|
da
|
10
|
101
|
|
deci
|
d
|
0.1
|
10-1
|
|
centi
|
c
|
0.01
|
10-2
|
|
milli
|
m
|
0.001
|
10-3
|
|
micro
|
µ
|
0.000 001
|
10-6
|
SI Exceptions
Certain units have been used for many centuries and retain historical, cultural and practical importance.
Consequently, these units are recognized for use with SI.
Quantity
|
Name
|
Symbol
|
Value in SI Units
|
|
time
|
minute
|
min
|
1 min = 60 s
|
|
hour
|
h
|
1 h = 3 600 s
|
|
day
|
d
|
1 d = 86 400
|
|
year
|
a
|
|
|
plane angle
|
degree
|
º
|
1 º = (∏/180) rad
|
|
minute
|
'
|
1 ' = (∏/10 800) rad
|
|
second
|
''
|
1 '' = (∏/648 000) rad
|
|
area
|
hectare
|
ha
|
1 ha = 1 hm2 = 10 000 m2
|
|
volume
|
litre
|
L or l
|
1 L = 1dm3
|
|
mass
|
tonne
|
t
|
1 t = 1 000 kg = 1 Mg
|
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