Oersted's Experiment

Jun 20 2026

Aim: To demonstrate that an electric current flowing through a conductor produces a magnetic field around it.

Apparatus Required

A thick straight insulated copper wire
A magnetic compass needle
A DC power supply or Battery eliminator
A plug key (switch)
Connecting wires
A rheostat (optional, for varying current)

Theory & Principle

In 1820, Hans Christian Ørsted discovered that a magnetic compass needle deflects when placed near a wire carrying an electric current. This established the fundamental connection between electricity and magnetism, known as electromagnetism.

Principle: A moving charge (current) creates a magnetic field in the surrounding space. The direction of the magnetic field lines can be determined using the Right-Hand Thumb Rule: If you hold the straight current-carrying wire in your right hand such that the thumb points towards the direction of current, then your folded fingers dictate the direction of the magnetic field lines.

The direction of deflection of the compass needle can also be remembered using the SNOW rule: If current flows from South to North in a wire placed Over a compass, the North pole of the needle deflects towards the West.

Procedure

Connect the thick copper wire in series with the battery, rheostat, and plug key using connecting wires.
Keep the copper wire straight and align it parallel to the magnetic meridian (North-South direction).
Place the magnetic compass needle exactly below the thick copper wire. Notice that when no current flows, the needle points North-South parallel to the wire.
Insert the plug key to allow current to flow through the circuit (from South to North). Observe the deflection of the compass needle.
Reverse the connections at the battery to reverse the direction of the current (from North to South) and observe the direction of deflection again.
Move the compass needle above the wire and repeat the observations.

Observation Table

S.No.	Position of Compass	Direction of Current	Deflection of North Pole
1	Below the wire	South to North	Towards West
2	Below the wire	North to South	Towards East
3	Above the wire	South to North	Towards East
4	Above the wire	North to South	Towards West

Result

The deflection of the magnetic compass needle indicates the presence of a magnetic field. Since the deflection only occurs when current flows, it is concluded that an electric current produces a magnetic field around the conductor. The reversal of deflection upon reversing the current shows that the direction of the magnetic field depends on the direction of the current.

Precautions

Ensure all electrical connections are tight and secure.
Do not pass a heavy current for a long duration, as the wire may heat up excessively.
Keep all other magnetic materials away from the apparatus to avoid interference with the compass needle.
Use a sufficiently thick wire so its resistance is low and heating is minimized.

Viva Questions & Answers

Q1: What happens to the deflection of the compass needle if the current in the wire is increased?

The deflection of the compass needle increases. This indicates that the strength of the magnetic field produced is directly proportional to the magnitude of the electric current.

Q2: How can you find the direction of the magnetic field produced by a straight current-carrying conductor?

By using the Right-Hand Thumb Rule. If the thumb points in the direction of the current, the curled fingers indicate the direction of the circular magnetic field lines.

Q3: Why does the compass needle not deflect when the circuit is open?

When the circuit is open, there is no flow of electric current. Consequently, no magnetic field is generated around the wire, and the needle remains aligned with Earth's magnetic field.

Q4: What is the shape of the magnetic field lines around a straight current-carrying wire?

The magnetic field lines form concentric circles around the straight wire.