ICSE Revision Notes for Measurements and Experimentation Class 9 Physics
Chapter Name  Measurements and Experimentation 
Topics Covered 

Related Study 
Introduction
 An atom consists of a positively charged nucleus surrounded by electrons revolving around the nucleus in different orbits of some definite radii.
 The neutrons are uncharged particles.
 The atom is electrically neutral.
Need of Unit for Measurement
 Measurement is the process of comparison of the given physical quantity with the known standard quantity of the same nature.
 A unit is the quantity of a constant magnitude which is used to measure the magnitudes of other quantities of the same nature.
 The magnitude of a physical quantity is expressed as
Physical quantity = numerical value × unit  A unit can be chosen if it has appropriate properties.
Kinds of Units
 Fundamental or basic unit: A fundamental unit is that which is independent of any other unit or which can neither be changed nor be related to any other fundamental unit.
 Derived units: Derived units are those which depend on the fundamental units or which can be expressed in terms of the fundamental units.
Examples of Derived Units
Quantity 
Definition 
Derived unit 
Symbol 
Area 
length × breadth 
metre × metre 
m2 
Work or Energy 
force × displacement 
kilogram × metre/second^{2} × metre = joule 
kg m^{2} s^{2} = J 
Pressure 
force/area 
(kilogram × metre)/second^{2}/(metre)^{2} = newton/(metre)^{2} = pascal 
kg m^{1} s^{2} = N m^{2} = Pa

Resistance 
potential/current 
Kilogram(metre)^{2}/{(ampere × second^{3})}/ampere = volt/ampere = ohm 
kg m^{2} A^{2} s^{3} = V A^{1} = Î© 
There are three systems of units used worldwide. They are CGS, FPS and MKS.
 CGS system or French system: The unit of length is centimetre, the unit of mass is gram and the unit of time is second.
 FPS system or British system: The unit of length is foot, the unit of mass is pound and the unit of time is second.
 MKS system or Metric system: The unit of length is metre, the unit of mass is kilogram and the unit of time is second.
System Internationale d’unites or SI system
According to this system, there are nine fundamental units, including two angular units.
Quantity 
Unit 
Symbol 
Length 
metre 
m 
Mass 
kilogram 
kg 
Time 
second 
s 
Temperature 
Kelvin 
K 
Luminous
intensity 
candela 
cd 
Electric
current 
ampere 
A 
Amount of
substance 
mole 
mol 
Angle 
radian 
rd 
Solid angle 
steradian 
strd 
 For expressing large or small numbers/amounts, prefixes are used.
Units of Length
 The SI unit of length is metre (m).
 A metre is defined as the distance which light travels in 1/(299,792,458) of a second in air or vacuum.
Subunits of Metre
 Centimetre, millimetre, micrometre or micron and nanometre.
Nonmetric Units of Length
 Bigger Units
▪ Astronomical unit (AU): One astronomical unit is equal to the mean distance between the Earth and the Sun.
1 A.U. = 1.496 × 10^{11} m ≈ 1.5 × 10^{11} m
▪ Light year (ly): A light year is the distance travelled by light in vacuum in one year.
1 ly = 9.46 × 10^{12} km
▪ Parsec: One parsec is the distance from where the semimajor axis of the orbit of the Earth subtends an angle of 1” (one second) at the centre of the Sun. One parsec is 3.26 light years.
1 parasec = 3.08 × 10^{18} m
Smaller Units
Angstrom (Ã…) : 1 Ã… = 10^{10} m = 10^{8} cm = 10^{9} nm
fermi (f) : 1 f = 10^{15} m
Units of Mass
 The SI unit of mass is kilogram (kg).
 One kilogram is defined as the mass of a cylindrical piece of platinum–iridium alloy kept in International Bureau of Weights and Measures at Sevres near Paris.
The subunits of kilogram are gram (g) and milligram (mg).
The nonmetric unit of mass is atomic mass unit (amu) or the unified atomic mass unit (u).
1 a.m.u = 1/12th mass of one carbon1/12 atom
Units of Time
 The SI unit of time is second (s).
 A second is defined as 1/86,400 th part of a mean solar day.
1 s = 1/86400 × one mean solar day  In 1964, one second was defined as the time interval of 9,192,631,770 periods of a specified energy change in the cesium133 atom.
Common Units of Time
 Minute (min), Hour (h), Day, Month, Lunar month, Year, Leap year, Decade, Century and Millennium.
Guidelines for Writing Units
 The symbol of a unit, which is not named after a scientist, is written in lower case.
 The symbol of a unit, which is named after a scientist, is written with the first letter of his name in capital.
 The full name of a unit, even when it is named after a scientist, is written with a lower initial letter.
 A compound unit formed by multiplication of two or more units is written after putting a dot, cross or leaving a space in between the two symbols.
 A negative power is used for compound units, which are formed by dividing one unit by the other.
 A unit in its short form is never written in plural.
Least Count of a Measuring Instrument
 The least count of an instrument is the smallest measurement which can be taken accurately with it.
 The least count of an instrument is the value of the smallest division on its scale.
 Smaller the least count of an instrument, the more accurate the measurement
Measurement of Length with a Metre Scale
 A metre scale is a scale with length one metre which is graduated in cm and has subdivisions in mm. There are 10 subdivisions in each cm.
 The value of one small division on the metre scale is 1 mm. Thus, its least count is 1 mm.
 Precautions should be taken while measuring the length of an object.
 The limitation of a metre scale is that it can measure the length correctly up to one decimal place of a centimetre.
Principle of Vernier
 Two scales are used in this technique. One is the main scale, and the other is called the vernier scale.
 The least count of vernier is equal to the difference between the values of one main scale division and one vernier scale division. This is called the vernier constant.
 ‘n’ divisions of vernier are equal to n−1 divisions of the main scale. Therefore, the value of one division of vernier
= (n – 1)x/n
L.C. = Value of 1 main scale division (x)/(Total number of divisions on vernier (n)
Vernier Callipers
A vernier calliper is also called a slide calliper.
Part 
Function 
Inside jaws 
To measure the internal diameter of a hollow
cylinder or pipe. 
Outside jaws 
To measure the length of a rod, the diameter
of a sphere and the external diameter of a hollow cylinder. 
Main scale 
To measure the length correct up to 1 mm. 
Vernier scale 
To measure the length correct up to 0.1 mm. 
Strip 
To measure the depth of a beaker or a
bottle. 
 The least count of a vernier calliper is given as
L.C. = Value of 1 main scale division(x)/Total number of divisions on vernier(n)  Zero error in vernier callipers: The zero error is equal to the distance between the zero of the main scale and the zero of the vernier scale.
Kinds of zero error in Vernier callipers
There are two kinds of zero error—positive and negative zero error.
 Positive zero error: When the zero mark of the vernier scale is on the right of the zero mark of the main scale, the zero error is said to be positive.
 Negative zero error: When the zero mark of the vernier scale is on the left of the zero mark of the main scale, the zero error is said to be negative.
The correction due to zero error, that is, the correct measure of the length is:
Correct reading = Observed reading  Zero error
Positive zero error gets subtracted and negative zero error gets added to the observed reading.
Principle of Screw
 The pitch of the screw is the distance moved by the screw along its axis in one complete rotation of its head.
 There are graduations along the circumference of the head of the screw. There are 50–100 graduations. This is called the circular scale or head scale.
 The pitch of the screw is 1 mm. If there are 100 divisions on the circular scale, then the least count of the screw is 1/100 = 0.01 mm = 0.001 cm.
 The least count of a screw is the distance moved by it in rotating the circular scale by one division.
L.C. = (Pitch of screw)/(Total number of divisions on circular scale)
Screw Gauge
The main parts of a screw gauge and their functions are given below.
Part 
Function 
Circular scale 
To read length correct up to 0.01 mm. 
Main scale 
To read length correct up to 1 mm. 
Sleeve 
To mark the main scale and the base line. 
Thimble 
To mark the circular scale. 
Ratchet 
To advance the screw by turning it till the
object is gently held between the stud and the spindle of the screw. 
The pitch of a screw gauge is the linear distance moved by its screw on the main scale when the circular scale completes one full rotation.
The least count is the linear distance moved by its screw on the main scale when the circular scale is rotated by one division.
L.C. = (Pitch of screw)/(Total number of divisions on circular scale)
Kinds of zero error in Screw Gauge
There are two kinds of zero error—positive and negative zero error.
 Positive zero error: When the zero mark of the circular scale is below the base line of the main scale, the zero error is said to be positive.
 Negative zero error: When the zero mark of the circular scale is above the base line of the main scale, the zero error is said to be negative.
The correction due to zero error, that is, the correct measure of the length is:
Correct reading = Observed reading  Zero erro
Backlash error
Due to wear and tear of the threads of the screw, it is observed that on reversing the direction of rotation of the thimble, the tip of the screw does not start moving in the opposite direction at once, but it remains stationary for some part of rotation. The error caused due to this effect is called backlash error.