Transcript Document
How does telephone work Members: XIAN MAO(XQM5030) YANSHENG WANG(YVW5126) ZHONGYI LIU(ZVL5078) Telephones are actually one of the simplest devices in our house! How can telephone do such magic that enable communication in a long distance? • Have you tried making a tin-can telephone with two baked-bean cans and a length of string? Wondering, it really does work! • Not only that, this kind of simple telephone gives us a great insight into how a telephone carries people's voices from place to place. Normally, sounds travels through the air as invisible waves and transfers energy from something that vibrates to our eardrums. Sending sounds through the air is fine when the person we want to talk to is sitting in the same room. However, how can we communicate if we are in another building, or even another country? Then, we need a different form of communication. How simple(antique) telephone works? When a person speaks into a telephone, the sound waves created by his or her voice enter the mouthpiece. An electric current carries the sound to the telephone of the person he or she is talking to. A telephone has two main parts: the transmitter and the receiver. (1) The Transmitter of a telephone serves as a sensitive "electric ear." It lies behind the mouthpiece of the phone. Like the human ear, the transmitter has an 14 eardrum." The eardrum of the telephone is a thin, round metal disk called a diaphragm. When a person talks into the telephone, the sound waves strike the diaphragm and make it vibrate. The diaphragm vibrates at various speeds, depending on the variations in air pressure caused by the varying tones of the speaker's voice. (2) The Receiver serves as an "electric mouth." Like a human voice, it has "vocal cords." The vocal cords of the receiver are a diaphragm. Two magnets located at the edge of the diaphragm cause it to vibrate. One of the magnets is a permanent magnet that constantly holds the diaphragm close to it. The other magnet is an electromagnet. It consists of a piece of iron with a coil of wire wound around it. When an electric current passes through the coil, the iron core becomes magnetized. The diaphragm is pulled toward the iron core and away from the permanent magnet. The pull of the electromagnet varies between strong and weak, depending on the variations in the current. Thus, the electromagnet controls the vibrations of the diaphragm in the receiver. The telephone line diagram To be specific, we cannot avoid the contribution by Faraday, the electromagnetic induction! Faraday's Law is the basic principle behind the simple telephone. In a microphone there is a diaphragm, around which a coil is wrapped, which can move back and forth in response to sound waves. A stationary bar magnet, placed near the coil, induces current in the coil which can then be transmitted (with amplification) to the speaker of another telephone. Conversely, when the current reaches the speaker, which consists of another coil/diaphragm/magnet combination, the varying coil current causes the diaphragm to move and displace sound waves: What's inside an antique telephone? • 1.Dial mechanism: as the dial rotates, it interrupts the circuit between the phone and the exchange creating dialing pulses. If you dial number "9" you create nine pulses, dial "5" and you make five pulses, and so on. • 2.Bells: Old phones actually had real, shiny metal bells inside them to indicate an incoming call! Modern phones have electronic buzzers or bleepers, which don't sound nearly so nice. Going on... • 3.Electromagnet: When the phone rings, this electrically controlled magnet switches on and off rapidly, moving a clapper in between the bells at the front and making them ring. It's great to watch this happening when you take the cover off. It's like being back in the 19th century! • 4.Tilting switch mechanism: This detects when the handset is lifted or put back. If the handset rests on the switch, it cuts off the connection between the phone and the local exchange. The circuit is opened only when you lift the handset. • 5.Circuit board: All the little wires that control different parts of the phone (the dialing, the tilt switch, and so on) meet on this circuit board. Now, let's look at the nowadays telephone! •A switch to connect and disconnect the phone from the network - This switch is generally called the hook switch. It connects when you lift the handset. •A speaker - This is generally a little 50-cent, 8-ohm speaker of some sort. •A microphone - In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates. Sound waves from your voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone. A "real" telephone • Any "real" phone contains a device called a duplex coil or something functionally equivalent to block the sound of your own voice from reaching your ear. A modern telephone also includes a bell so it can ring and a touch-tone keypad and frequency generator. A "real" phone looks like this. • Still, it's pretty simple. In a modern phone there is an electronic microphone, amplifier and circuit to replace the carbon granules and loading coil. The mechanical bell is often replaced by a speaker and a circuit to generate a pleasant ringing tone. But a regular $6.95 telephone remains one of the simplest devices ever. Thank for you attention! • • • • References: http://www.physics.rutgers.edu/ugrad/205/ manuals/faraday.pdf http://www.explainthatstuff.com/telephone. html http://antiquetelephonehistory.com/telwork s.html http://electronics.howstuffworks.com/telep hone.htm