## B1 SI UNITS AND SYMBOLS

The following formulae are based on the International System of Units, known as SI (Systeme Internationale d'Unites) which is used throughout this book. SI was adopted in February 1969 by a resolution of the CGPM (Conference Generale de Poids et Mesures) as ISO Recommendation R1000.

A base unit exits for each of the dimensionally independent physical quantities: length, mass, time, electric current, thermodynamic temperature and luminous intensity. The SI unit of any other quantity may be derived by appropriate simple multiplication or division of the base units without the introduction of numerical factors.

The system is independent of the effects of gravity, making a clear distinction between the mass of a body (unit of mass = kilogram) and its weight, i.e. the force due to gravity (unit of force = Newton).

Example:

A force of 1 N acting on a mass of 1 kg results in an acceleration of 1 m s ?2.

Conversion factors from non SI units to SI units are to be found in Appendix C.

### SI BASE UNITS

Quantity

Unit symbol

Unit name

Length

m

meter

Mass

kg

kilogram

Time

s

second

Electric current

A

Ampere

Temperature

K

Kelvin

Luminous intensity

cd

candela

### DECIMAL MULTIPLES AND SUBMULTIPLES

Factor

Prefix

Symbol

10 12

tera

T

10 9

giga

G

10 6

mega

M

10 3

kilo

k

10 2

hecto

h

10

deca

da

10 ?1

deci

d

10 ?2

centi

c

10 ?3

milli

m

10 ?6

micro

?

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Density and Specific Gravity Instruments
Density and specific gravity instruments are meters used to determine density and specific gravity.
High Intensity Discharge Lamps
High intensity discharge lamps (HID) contain compact arc tubes, which enclose various gases and metal salts, operating at relatively high pressures and temperatures. HID lamps are often used as UV light sources.
Calibration and Test Weights

Calibration weights and mass standards have a certified mass that is traceable to national or international laboratory standards. Mass standards calibrate scales and weighing instruments and help evaluate material or mechanical properties.

Fluorescent Lamps
Fluorescent lamps are high-efficiency lamps that use electrical discharge through low-pressure mercury vapor to produce ultraviolet (UV) energy, which is then transformed into visible light.
Vacuum Pumps and Vacuum Generators
Vacuum pumps and vacuum generators provide sub-atmospheric pressure for a variety of industrial and scientific applications where a vacuum is required.

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