Introduction to Magnetic Effect of Current

In 1820, Danish physicist Hans Christian Oersted discovered that a compass needle placed near a current-carrying wire gets deflected — proving for the first time that electricity and magnetism are related phenomena. This discovery laid the foundation for electromagnetism, later unified mathematically by James Clerk Maxwell.

Did You Know?

Oersted's discovery was accidental — he noticed the needle deflection during a lecture demonstration, not a planned experiment.

The phenomenon in which a current-carrying conductor produces a magnetic field around itself is called the magnetic effect of electric current.

Setup: A magnetic needle is placed inside a matchbox tray, around which a connecting wire is wound. This wire is connected in series with an electric cell, a plug key, and a bulb to complete the circuit. In the refined version, a thick straight copper wire is connected between two points (A and B) with a magnetic needle placed adjacent to it.

Procedure and observations:

• With the plug key open (no current flowing), the needle's position is noted — it aligns with Earth's magnetic field (north-south).
• A bar magnet brought near the needle causes it to deflect, confirming the needle behaves like a magnet.
• When the plug key is closed, the bulb lights up (confirming current flow), and the magnetic needle deflects from its original position.
• When the plug key is opened again, the needle returns to its original north-south position.
• When the connecting wires to the cell are interchanged (reversing current direction), the needle deflects in the opposite direction compared to before.

Conclusion: A current-carrying wire produces a magnetic field around it, since the needle deflects only when current flows and returns to normal when current stops. The direction of deflection depends on the direction of current flow, showing a direct relationship between current direction and magnetic field direction. This is the effect Hans Christian Oersted first observed.

Setup: A thick copper wire is passed vertically through a horizontal cardboard sheet, and the circuit is completed so that a large current (approximately 1A or more) flows through the wire.

Procedure and observations:

• A magnetic needle placed at various points on the cardboard around the wire orients itself in different directions at each point; these directions are marked with a pencil.
• When the needle is moved farther from the wire, the deflection becomes progressively weaker.
• Increasing the current increases the intensity of deflection, while decreasing the current reduces it.
• Replacing the needle with iron filings sprinkled on the cardboard, the filings arrange themselves into concentric circular patterns around the wire, tracing the magnetic lines of force.

Conclusion: A magnetic field exists in the space around a straight current-carrying conductor, forming concentric circular field lines. The field weakens with increasing distance from the wire (circles become larger and more spread out/rarefied), and strengthens when the current is increased. This experiment visually confirms both the existence and the spatial pattern of the magnetic field predicted by Oersted's original observation.