Instruments are devices that use basic principles of physics to perform useful work.
Some instruments help us lift heavy loads, some help us see distant objects, some ensure safety, and others help in communication.
Most of these instruments are based on:
- Pressure
- Motion
- Heat
- Sound
- Magnetism
- Optics
Understanding their principle, construction, and working is more important than memorising names.
Table of Contents
Simple Pendulum
A simple pendulum consists of:
- A small heavy bob
- A light, inextensible string
- A fixed support
When displaced and released, it oscillates to and fro.
Principle
A simple pendulum works on periodic motion due to gravity.
Uses
- Measuring time (old clocks)
- Studying oscillatory motion
Key Point
Time period depends on:
- Length of string
- Acceleration due to gravity
It does not depend on mass of bob.
Simple Pulleys
A pulley is a wheel with a groove through which a rope passes.
Types
- Fixed pulley
- Movable pulley
- Block and tackle system
Principle
Pulley works on change of direction of force and mechanical advantage.
Uses
- Drawing water from wells
- Lifting loads at construction sites
Siphon
A siphon is a bent tube used to transfer liquid from a higher level to a lower level.
Principle
Works on:
- Atmospheric pressure
- Gravity
Uses
- Draining water from tanks
- Transferring liquids safely
Levers
A lever is a rigid bar that rotates about a fixed point called fulcrum.
Parts
- Fulcrum
- Load
- Effort
Types of Levers
- First class (see-saw)
- Second class (wheelbarrow)
- Third class (human arm)
Principle
Levers work on principle of moments.
Balloon
A balloon rises in air when filled with hot air or light gas.
Principle
Based on Archimedes’ principle.
Explanation
Hot air is lighter than cold air, so buoyant force pushes the balloon upward.
Pumps
Pumps are devices used to lift liquids.
Types
- Hand pump
- Lift pump
- Force pump
Principle
Pumps work on pressure difference and atmospheric pressure.
Uses
- Drawing groundwater
- Supplying water to houses
Hydrometer
A hydrometer is used to measure density of liquids.
Principle
Based on Archimedes’ principle.
Uses
- Testing purity of milk
- Measuring density of liquids
Pressure Cooker
A pressure cooker cooks food faster.
Principle
Boiling point of water increases with pressure.
Explanation
High pressure raises boiling point, so food cooks faster.
Thermos Flask
A thermos flask keeps liquids hot or cold for long time.
Principle
Prevents heat transfer by:
- Conduction
- Convection
- Radiation
Uses
- Storing hot beverages
- Preserving temperature
Gramophone
A gramophone reproduces sound from records.
Principle
Works on vibration of diaphragm.
Explanation
Grooves on record cause needle vibrations which produce sound.
Telegraph and Telephone
Telegraph
Used to send coded messages using electric signals.
Telephone
Used to transmit voice signals.
Principle
Based on conversion of sound into electrical signals and back.
Periscope
A periscope is used to see objects not in direct line of sight.
Principle
Works on reflection of light.
Uses
- Submarines
- Military observation
Telescope
A telescope is used to observe distant objects.
Principle
Based on refraction of light through lenses.
Uses
- Astronomy
- Observing distant objects
Microscope
A microscope is used to see very small objects.
Principle
Uses magnification by lenses.
Uses
- Biology labs
- Medical diagnosis
Mariner’s Compass
A mariner’s compass helps determine direction.
Principle
Based on Earth’s magnetism.
Uses
- Navigation
- Sea travel
Lightning Conductors
A lightning conductor protects buildings from lightning.
Principle
Provides a safe path for electric discharge to ground.
Uses
- Protecting tall buildings
- Preventing fire hazards
Safety Fuses
A fuse protects electrical circuits.
Principle
Based on heating effect of electric current.
Explanation
Excess current melts fuse wire and breaks circuit.
FAQs – INSTRUMENTS (Exam-Focused)
Which principle does hydrometer work on?
Archimedes’ principle.
Why does a pressure cooker cook food faster?
Because boiling point increases with pressure.
Which lever is human arm?
Third class lever.
Why does siphon need outlet lower than inlet?
For gravity to act.
What prevents heat loss in thermos flask?
Vacuum and reflective surfaces.
Which instrument uses Earth’s magnetism?
Mariner’s compass.
Why does fuse melt during overload?
Due to heating effect of current.
Which device changes direction of force?
Pulley.
Which instrument uses reflection twice?
Periscope.
Which instrument magnifies small objects?
Microscope.
Last-Moment Notes (Cheat Sheet)
INSTRUMENTS
- Pendulum → periodic motion
- Pulley → change direction of force
- Lever → principle of moments
- Siphon → atmospheric pressure
- Balloon → buoyant force
- Pump → pressure difference
- Hydrometer → density measurement
- Pressure cooker → high pressure, high boiling point
- Thermos flask → prevents heat transfer
- Gramophone → sound vibrations
- Telephone → sound to electric signal
- Periscope → reflection
- Telescope → refraction
- Microscope → magnification
- Compass → Earth’s magnetism
- Lightning conductor → safe discharge
- Fuse → heating effect of current