Electricity and Magnetism

Electromagnetism was discovered in 1819, by accident, by a Danish Physicist Hans Christian Oersted (August 14, 1777 - March 9, 1851). While h was preparing to do a lecture and to demonstrate that a wire carrying a strong electrical current becomes hot, each time he switched on the current through a voltaic pile which was an early form of the battery, he found that a nearby compass needle deflected from the north to south direction.

Upon hearing of Oersted's discovery, others began to investigate the strange effect and found ways of increasing the magnetism produced by an electrical current. A loop of wire was found to be much more effective that a straight wire and two loops were better than one. Coils with numerous turns were then constructed, and the magnetic effect was further increased by winding the coils on soft iron. By the 1820's,
electromagnets capable of lifting loads of over 1 ton had been built.

As Obersted discovered when his compass needle deflected, electricity can cause movement. In 1821, an English scientist named Michael Faraday found a way of using electricity to cause continuous movement. In fact, he invented the first electric motor. Ten years later, he demonstrated the opposite effect how movement could produce electricity. Moving a permanent magnet near a coil caused a current to flow through the wire. This discovery led to the development of powerful electricity generators.

Today the principle of electromagnetism is used in many devices. For example, the dynamic microphone, sound waves make a coil vibrate near a magnet the movement corresponds to the sound waves being picked up. Therefore, the current generated is a replica of the sound waves.
The electrical signals from the microphone can be amplified or strengthened and reproduced as sound by a loudspeaker.

They also can be recorded onto a magnetic tape. In a tape recorder, the amplified signals are passed through an electromagnet called the tape head. As signals fed to the head may vary, the magnetism varies also. A tape, coated with a substance that can be magnetized passes the head at a constant speed. As a result it acquires a magnetic pattern corresponding to the sound waves picked up from the microphone. On replaying the tape, its varying magnetization generates corresponding electrical signals in the tape head. The signals can then be amplified and then play back through a loud speaker.