Exploring the Magnetic Effects of Electric Current: From Theory to Application

When Hans Christian Oersted accidentally nudged a compass needle with a live electric wire during a university lecture in 1820, he didn’t just stumble upon a curiosity – he unlocked one of the most consequential discoveries in the history of physics. The observation that electric current produces a magnetic field became the cornerstone of an entirely new scientific discipline: electromagnetism. Today, that same principle powers the electric motors in fans, the MRI machines in hospitals, and the speakers in your phone. Understanding the magnetic effects of electric current – from why they occur to how their direction is determined – is therefore not just textbook knowledge, but an insight into the workings of the modern world.

Oersted’s discovery: the moment electricity met magnetism
What is a magnetic field, and how does current create one?
Determining the direction: the right-hand thumb rule
Applying the rule to a circular loop
Applying the rule to a solenoid
Factors that affect the strength of the magnetic field
From theory to application: where this principle shows up
Electromagnets
Electric motors
MRI machines
Loudspeakers and microphones
Ampère and the mathematical framework

Oersted’s discovery: the moment electricity met magnetism

Before the 19th century, electricity and magnetism were studied as two entirely separate, unrelated phenomena. That view changed irreversibly on July 21, 1820. According to the American Physical Society, Danish physicist Hans Christian Oersted was using a voltaic pile during a university lecture when he noticed that a nearby compass needle deflected from its original position the moment he connected the battery. The needle came to rest perpendicular to the current-carrying wire – and returned to its original position when the current was switched off.

What made this observation especially significant was not just the deflection, but what it ruled out. As the Institute of Physics explains, earlier experimenters had placed compass needles at right angles to the wire, expecting the magnetic field – if any – to run parallel to the current. They saw nothing, because the field actually runs perpendicular to the wire, forming circular loops around it. Preconceived notions had prevented the discovery for decades.

Oersted spent the following months carefully verifying his findings. He confirmed that the effect persisted across different types of metal wires, and even when materials like cardboard, glass, stone, or water were placed between the wire and the compass – meaning the magnetic effect could not be shielded by non-magnetic materials. He concluded that an electric current produces a circular magnetic field around the conductor. His findings were published in a paper titled Experiments on the Effect of a Current on a Magnetic Needle, which caused an immediate sensation across Europe.

What is a magnetic field, and how does current create one?

A magnetic field is a region in space where a magnetic force can be detected. It is represented visually using magnetic field lines – imaginary lines whose direction at any point shows the direction the north pole of a compass would point if placed there. These lines never cross each other, and their density (how close together they are) indicates the strength of the field: the closer the lines, the stronger the field.

When electric current flows through a straight conductor, it produces a magnetic field in the form of concentric circles around the wire. These circular field lines are centred on the conductor and lie in planes perpendicular to it. The strength of this field depends on the magnitude of the current: a stronger current produces a stronger (denser) magnetic field. Crucially, if the current is turned off, the field disappears – unlike the field of a permanent magnet.

Two key properties govern this magnetic field:

The field is circular: Around a straight wire, the magnetic field lines form closed loops encircling the conductor.
The field reverses with current direction: If the direction of current through the wire is reversed, the direction of the magnetic field around it also reverses. This was directly demonstrated by Oersted, who reversed the current and observed the compass needle deflect the opposite way.

Determining the direction: the right-hand thumb rule

Since a magnetic field has both magnitude and direction, physicists needed a simple way to determine which way the field loops around a conductor. This is where the right-hand thumb rule (also called Maxwell’s Corkscrew Rule) comes in.

The rule states: hold the current-carrying conductor in your right hand with the thumb pointing in the direction of the conventional current (from positive to negative terminal). The direction in which your curled fingers wrap around the conductor gives the direction of the magnetic field lines around it.

This rule works for any straight conductor. For example, if the current flows vertically upward in a wire, pointing your right thumb upward and curling your fingers will show that the magnetic field circles the wire in an anticlockwise direction when viewed from above. Reverse the current, and the field circles clockwise. The right-hand rule was developed by André-Marie Ampère, who was inspired by Oersted’s experiment and went on to formulate a mathematical description of the relationship between current and magnetic force.

Applying the rule to a circular loop

When a conductor is shaped into a circular loop, the same right-hand thumb rule applies – but this time you curl your fingers in the direction of the current flowing through the loop, and the extended thumb points in the direction of the magnetic field at the centre of the loop. The field lines near the wire are concentric circles, but at the very centre of the loop, they add up to form a strong, nearly straight field perpendicular to the plane of the loop. The strength of this field increases with the current and with the number of turns in the loop.

Applying the rule to a solenoid

A solenoid is a long coil of wire wound into a helix. When current flows through it, each loop contributes a magnetic field, and these fields reinforce one another inside the coil. The resulting field inside a solenoid is strong and highly uniform – the field lines run parallel to the axis of the coil, just like the field inside a bar magnet. The field outside the solenoid is comparatively weak and dispersed. One end of the solenoid behaves as the north pole and the other as the south pole, depending on the direction of the current.

To find the north pole of a solenoid using the right-hand rule: wrap the right hand around the coil with the fingers pointing in the direction of conventional current through the loops. The thumb will point towards the north pole of the solenoid.

Factors that affect the strength of the magnetic field

The magnetic field produced by a current-carrying conductor is not fixed – several factors influence its strength:

Magnitude of current: A larger current produces a stronger magnetic field. The two are directly proportional.
Distance from the conductor: The magnetic field strength decreases as you move farther away from a straight wire. The field is strongest closest to the conductor.
Number of turns (for coils and solenoids): Adding more turns to a coil concentrates the magnetic field lines and increases the field strength proportionally.
Core material: Inserting a ferrous (iron-rich) core inside a solenoid can increase the magnetic field strength by several hundred times, as the iron becomes magnetised and adds to the overall field.

From theory to application: where this principle shows up

The magnetic effect of electric current is not a physics abstraction – it underpins a vast array of technologies we depend on daily.

Electromagnets

An electromagnet is essentially a solenoid with a soft iron core. Because the magnetism can be switched on and off with the current, electromagnets are used wherever controllable magnetic force is needed – from lifting cranes in scrapyards to sorting recyclable metals in waste processing facilities.

Electric motors

Electric motors exploit the force that a magnetic field exerts on a current-carrying conductor. When a current-carrying coil is placed inside a magnetic field, the interaction between the two fields causes the coil to rotate. This rotation is what drives fans, pumps, washing machines, electric vehicles, and countless industrial machines. The direction of this force is determined by Fleming’s Left-Hand Rule, a direct extension of the electromagnetism Oersted first observed.

MRI machines

Magnetic Resonance Imaging (MRI) uses extremely powerful electromagnets – often superconducting solenoids – to generate the strong, uniform magnetic fields needed to image soft tissue inside the human body. Such large field strengths can only be achieved by passing very large currents through thousands of tightly wound coils, illustrating the direct relationship between current, coil turns, and magnetic field strength.

Loudspeakers and microphones

Electric motors, microphones, loudspeakers, and transformers all trace their operating principles back to Oersted’s 1820 discovery. In a loudspeaker, a varying electric current through a coil placed near a permanent magnet creates a varying force, which makes the coil (and the attached cone) vibrate and produce sound.

Ampère and the mathematical framework

Oersted’s experimental discovery quickly inspired other scientists to build a rigorous mathematical framework around it. French physicist André-Marie Ampère built on Oersted’s findings to develop what is now known as Ampère’s Law, which relates the magnetic field around a closed loop to the electric current passing through it. Ampère also showed that two parallel wires carrying currents in the same direction attract each other, while those carrying currents in opposite directions repel – a finding that later became the basis for the international definition of the ampere, the SI unit of electric current.

Later, Michael Faraday demonstrated the reverse relationship – that a changing magnetic field can induce an electric current – and James Clerk Maxwell ultimately unified all of electricity and magnetism into a single mathematical theory, showing that light itself is an electromagnetic wave. All of this traces back to one compass needle deflecting during a lecture in Copenhagen.

What do you think? Given that Oersted’s discovery happened almost accidentally during a classroom demonstration, what does this tell us about the role of observation and curiosity in scientific progress? And if the direction of the current in a solenoid is reversed, how would the poles of the resulting electromagnet change – and why does that matter for the practical use of electromagnets?

References

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Table of Contents

Oersted’s discovery: the moment electricity met magnetism 🔗

What is a magnetic field, and how does current create one? 🔗

Determining the direction: the right-hand thumb rule 🔗

Applying the rule to a circular loop 🔗

Applying the rule to a solenoid 🔗

Factors that affect the strength of the magnetic field 🔗

From theory to application: where this principle shows up 🔗

Electromagnets 🔗

Electric motors 🔗

MRI machines 🔗

Loudspeakers and microphones 🔗

Ampère and the mathematical framework 🔗