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ELECTROMAGNETISM
The interaction of electrical and magnetic effect is known as electromagnetism. Electromagnetism is one of the most important concepts in physical science, and much of our current technology is directly related to this crucial intercourse. We can summarize the two basic principles of Electromagnetism as follows:
One of the most significant application of this principle is called an electromagnet. An electromagnet consists of a current-carrying coil of insulated wire wrapped about a piece of soft iron. At what time the current is turned on, a magnet field is created inside the coil. The soft iron is magnetized by this field and makes the magnetic field nearly 2000 times stronger. When the current is turned off, the iron loses nearly all of its magnetism.
The advantages of the electromagnet are numerous. First, it can be switched on and off. Second, the strength of the electromagnet can be controlled by the amount of current flowing in the wire. Third, reversing the current can reverse the poles of the magnet field.
Electromagnets are used in a variety of devices and appliances, such as doorbells and telephones, and in devices used for moving magnetic metals. The mechanism in a telephone receiver will serve as an illustration of one use.
 When a telephone number is dialed, a circuit is complete by another telephone's bell. When the ringing phone is picked-up, the circuit between the speakers and receivers of the two telephones is completed. A telephone conversation consists of converting the sound waves to varying electric current. This varying current travels along the wire to the other telephone's receiver where it is converted back into sound waves. The transmitter of a phone contains a diaphragm that vibrates because of the sound waves that are spoken. The diaphragm vibrates against a chamber that contains the carbon granules. As the diaphragm vibrates, the pressure on the carbon granules varies, that causes more or less electric resistance in the circuit. The resistance is low when the granules are presses together and increases while they spread apart. On account of Ohm's law this varying resistance causes a varying electric current to be transmitted. Upon the receiver end there is an electromagnet and a permanent magnet, which is attached to a disk. The varying of electric current gives the electromagnet varying magnet strength. The electromagnet attracts the permanent magnet by a variable force. The inconstant force on the permanent magnet and disk causes the receiver disk to vibrate. This vibration sets up the sound waves that closely resemble the original sound waves, and the voice is heard at the other telephone.
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