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===Basic Principle===
===Basic Principle===
The principle behind the telephone is remarkably simple: it consists of an electrical circuit between two handsets (each containing a speaker and a microphone). The circuit is maintained at about 30 milliamps (provided by the telephone exchange), and is connected when the handset of the phone is taken off the hook. As one party speaks into the telephone the microphone vibrates, resulting in small changes in resistance of the circuit. These changes are tiny, however since the circuit is operating at such a low current they are proportionally large, resulting in a fluctuating current. This changing current results in the [[speakers]] in the circuit vibrating at changing frequencies, effectively reproducing the original input sound. In order to prevent one's own speaker to vibrate at one's input (in other words to stop a person hearing their own voice) many phones are fitted with a [[duplex]] coil which splits the incoming current and prevent [[foldback]].
The principle behind the telephone is remarkably simple: it consists of an electrical circuit between two handsets (each containing a [[Audio speaker|speaker]] and a [[microphone]]). The circuit is maintained at about 30 milliamps (provided by the telephone exchange), and is connected when the handset of the phone is taken off the hook. As one party speaks into the telephone the microphone vibrates, resulting in small changes in resistance of the circuit. These changes are tiny, however since the circuit is operating at such a low current they are proportionally large, resulting in a fluctuating current. This changing current results in the speakers in the circuit vibrating at changing frequencies, effectively reproducing the original input sound. In order to prevent one's own speaker to vibrate at one's input (in other words to stop a person hearing their own voice) many phones are fitted with a [[duplex]] coil which splits the incoming current and prevent [[foldback]].
===Telephone Networks===
===Telephone Networks===