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The Relationship Between Electricity and Magnetism
At the beginning of the 19th century it was still thought that electricity and magnetism were two separate forces.
In 1820 Hans Christian Ørsted, while preparing for a scientific lecture, noticed that a compass needle was deflected from magnetic north by a nearby electric current passing through a wire. This deflection convinced him that there must be a fundamental relationship between electricity and magnetism.
This view was confirmed later in 1873 by James Clerk Maxwell in his Treatise on Electricity and Magnetism in which he established that an electric current in a wire creates a circular magnetic field around the wire and its direction depends on the direction of the current.
The relationship between electricity and magnetism is very close. A changing magnetic field induces an electrical current in a wire - the working principle behind electrical generators. And an electrical current flowing through a wire induces a magnetic field around it - the basic principle for most electric motors.
A compass needle (or magnetic needle) is deflected when placed nearby a wire with a current flowing through it.
For this, we use a simple series electrical circuit consisting of a battery, a switch and wires. When we close the switch a current flows through the wire and the needle of the nearby compass is deflected. When the direction of the current is reversed the deflection is also reversed.
The needle is deflected because instead of being attracted by the magnetic field of the earth, it is being attracted by the more powerful nearby magnetic field created by the current flowing through the wire.