INTRODUCTION:
Science, Engineering and Technology are based on experimental facts. Experiments need measurement. Naturally measurement requires instruments.
Engineering needs precision and precise control. Unless a variable is measured accurately, nothing can be done to control the variable accurately either manually or by automatic means.
Measurement of any quantity requires units and standards. The fundamental quantities are length, mass and time. There are derived quantities, like area, pressure etc. To measure any physical quantity a standard of the same quantity is essential and that is called the UNIT. Measuring any thing is to compare it with the standard. A standard is arbitrary and its virtue lies in its international agreement.
The unit of any quantity should be definite, constant and convenient for measurement. The measure of any quantity is the number of times the unit is contained in it. Two kinds of units are used in science and engineering. They are the fundamental units and derived units.
The fundamental unit is the unit of a physical quantity which is independent of any other quantity. The derived unit is the unit of a physical quantity which can be derived. Three standardized systems of units were used to measure the fundamental quantities. They are
(i) F.P.S.
(ii) C.G.S.
(iii) M.K.S systems.
Their units are as shown in the table below.
System

UNITS
 
Length

Mass

Time
 
1. FPS

foot

pound

second

2. CGS

centimetre

gram

second

3. MKS

metre

kilogram

second

In selecting suitable standards one has to bear in mind the following:
1. It should be possible to define them unambiguously.
2. They must be easy to copy.
3. They must be invariable with time and place.
4. Methods must be available for multiplying or subdividing each one of the standards.
A coherent, rational and comprehensive International system of units was accepted at the Eleventh International Conference of weights and measures in 1960 A.D. This system is S.I. system. System International d' Units system. We follow this system for all measurements. Other systems of units are becoming obsolete. The six basic S.I., quantities and their units of measurement and symbols are given below : (Sometimes S.I. units are called International M.K.S. system of units).
It is realised that only the three fundamental quantities are not sufficient to derive physical quantities. Four more physical quantities are taken as fundamental quantities. They are Thermodynamic temperature, Illuminating power (Luminous intensity), Strength of electric current and Quantity of matter. Additionally to these fundamental quantities, plane angle and solid angle are taken as supplementary quantities. Table shows the units and symbols.
Fundamental Quantity

Unit

Symbol

Length

metre

m

Mass

kilogram

kg

Time

second

s

Thermodynamic temperature

kelvin

K

Illuminating power (luminous intensity)

candela

cd

Strength of electric current

ampere

A

Quantity of matter

mole

mol

Plane angle

radian

rad

Solid angle

steradian

sr

ELECTRICAL AND MAGNETIC UNITS:
Originally the electrical and magnetic units like volt, ampere, ohm, henry etc. were derived in CGS system of units. Having thought that the establishment of the practical units from the definitions of CGS system will be too difficult for laboratory work, practical units were defined. It is fairly simple to establish them.
The ampere was defined in terms of the rate of deposition of silver nitrate solution by passing a current through that solution.
The ohm was defined as the resistance of a specified column of mercury.
The above units and those derived from them were called International units. Improvement of measuring techniques has shown that there is small difference between the CGSm derived practical units and the international units. The differences are specified as follows.
The differences between CGSm and International units
1 int. ohm = 1.00049 (practical CGSm unit)
1 int. ampere = 0.99985 A
1 int. volt = 1.00034 V
1 int. coulomb = 0.99985 C
1 int. farad = 0.99951 F
1 int. henry = 1.00049 H
1 int. watt = 1.00019 W
1 int. joule = 1.00019 J
The electrical and magnetic units and their defining relationships, multiplication factors for conversion of SI units are shown below.
Quantity and Symbol

SI Unit

Conversion factors
 
Name, Symbol

Defining Equation

CGSm

CGSe***
 
Electric current, I

Ampere, A

Fz = 10^{7}I^{2}dN/dz

10

10/c

Electromotive force, E

Volt, V

P** = IE

10^{8}

10^{8}c

Potential, V

Volt, V

Vp** = IV

10^{8}

10^{8}c

Resistance, R

Ohm, Ω

R = V/I

10^{9}

10^{9}c

Electric charge, Q

Coulomb, C

Q = It

10

10/c

Capacitance, C

Farad F

C = Q/V

10^{9}

10^{9}/c^{2}

Electric field strength. E

V/m

E = V/l

10^{6}

10^{6}c

Electric flux density, D

C/m^{2}

D = Q/l^{2}

10^{5}

10^{5}/c

Permittivity, Ɛ

F/m

Ɛ = D/E



10^{11}/4πc^{2}

Magnetic field strength, H

A/m

∮ H dl = nI

10^{3/4}



Magnetic flux, Φ

weber, wb

E = dΦ/dt

10^{8}



Magnetic flux density, B

tesla, T

B = Φ/l^{2}***

10^{4}



Inductance L, M

henry, H

M = Φ/I

10^{9}



Permeability, μ

H/m

μ = B/H

4π x 10^{7}



Note for table : * N denotes Neumann's integral for two linear circuits each carrying the current I. Fz is the force between the two circuits in the direction defined by coordinate z, the circuits being in a vacuum.
** p denotes power
** l^{2} denotes area.
∮c = velocity of light in free space in cm/s = 2.997925 x 10^{10}
DEFINITIONS OF FUNDAMENTAL UNITS IN SI SYSTEM
1. Metre: Metre is 1,650,763,73 times the wavelength of the orange light in vacuum emitted by _{36}Kr^{86} (krypton) in the transition 2p_{10} to 5 d_{5}.
In 1983 A.D. a new definition was given to metre. Metre is 1 in 299,762,458th parts of the distance travelled by light in vacuum in 1 second.
2. Kilogram: Kilogram is the mass of a platinumiridium cylinder kept at Serves.
3. Second: One second is the time taken by 9, 192,631,770 cycles of radiation from the hyperfine transition in Cesium when unperturbed by external fields.
4. Kelvin: This is 1/273, 16 of temperature at the triple point of water measured on thermodynamic scale.
5. Candela: Candela is the luminous intensity in a direction normal to the surface of 1/6 x I0^{5}m^{2} of a black body at the temperature of freezing platinum at a pressure of 101325 newton per square metre.
6. Ampere: Ampere is the current which when flowing in each of two parallel conductors of infinite length and negligible crosssection and placed one metre apart in vacuum causes each conductor to experience a force exactly 2 x 10^{7} newton per metre of length.
7. Mole: Mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon  12.
8. Radian: Radian is the angle projected at the centre of a circle by an arc whose length is equivalent to the radius.
2p radians = 360° therefore
1 radian = 360/2p = 57°17'44"
9. Steradian: The solid angle projected at the centre of the sphere of radius 1 metre by its surface of area 1 square metre.
Table of fundamental, supplementary and derived units.
Quantity

Symbol

Dimension

Unit

Unit Symbol

Fundamental length

l

L

meter

m

Mass m

M

Kilogram

kg



Time t

T

Second

s



Electric current

I

I

ampere

A

Thermodynamic temperature

T

θ

Degree, Kelvin

^{o}K

Luminous Intensity Supplementary





candela

cd

Plane angle

α, β, γ

(L)^{ o}

radian

rad

Solid angle



(L^{2})^{o}

steradian

sr

Derived Area

A

L^{2}

square meter

m^{2}

Volume

V

L^{3}

Cubic meter

m^{3}

Frequency

f

T^{1}

hertz

Hz(1/s)

Density

ρ

L^{3}M

kg per cubic meter

Kg/m^{3}

Velocity

v

LT^{1}

meter per second

m/s

Angular Velocity

ω

(L)^{o}T

radian per second

rad/s

Acceleration

a

LT^{2}

meter per second squared

m/s^{2}

Angular acceleration

α

(L)^{o}T^{2}

radian per second squared

rad/s^{2}

Force F

LMT^{2}

newton

N (kgm/s^{2})



Pressure, stress

p

L^{1}MT^{2}

newton per square meter

N/m^{2}

Work, energy

W

L^{2}MT^{2}

Joule

J/Nm

Power Quantity of Electricity

P

L^{2}MT^{3}

watt

W(J/s)

Potential difference, electromotive force

V

L^{2}MT^{3}I^{1}

volt

V (W/A)

Electric field strength

E, Ɛ

LMT^{3}I^{1}

Volt per meter

v/m

Electric resistance

R

L^{2}MT^{3}I^{2}

ohm

Ω (V/A)

Electric capacitance

C

L^{2}M^{1}T^{4}I^{2}

farad

F(As/V)

Magnetic flux

Φ

L^{2}MT^{2}I^{1}

weber

Wb(vs)

Magnetic field strength

H

L^{1}I

ampere per meter

A/m

Magnetic flux density

B

MT^{2}I^{1}

tesla

T(Wb/m^{2})

Inductance

L

L^{2}MT^{2}I^{2}

henry

H (Vs/A)

Magnetomotive force

U

I

ampere

A

Luminous flux





lumen

lm (cd sr)

Luminance





candela per sq. meter

cd/m^{2}

Illumination





Lux

1 x (1m/m^{2})

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