Difference between revisions of "Telephone"
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The principle behind the telephone is remarkably simple, and at
The principle behind the telephone is remarkably simple, and at most basic 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]].
Revision as of 13:04, 13 November 2009
A telephone is a device which allows instantaneous voice communication between two parties. Despite only being invented less than 150 years ago and having a relatively simple design and concept it has evolved into one of the most used pieces of equipment worldwide, and is a crucial component of the daily lives of most individuals, families and businesses.
The telephone was developed out of the telegraph, a system through which messages could be sent and received by send pulses of signal through an electrical circuit in the form of morse code. These required trained operators to both send and receive the message (which was then typed and delivered to the receiving party), however the basic concept led to the design of the telephone, and the extensive wiring provided infrastructure for early telephones.
The actual designer of the telephone is a subject of extensive debate. There are reports stating that it was developed as early as 1849 by Antonio Meucci, however it is unknown whether he built a telephone or a different type of electric device involving voice. The two main contenders for recognition as the inventor are Alexander Bell and Elisha Gray who designed similar products using electromagnets. Their patent applications were registered on the same day in 1876 (with Gray's being filed two hours before Bell's), however as Bell had actually built a working model it was his that was granted, Gray choosing to withdraw his on advice of his lawyers. The following month Bell transmitted his famous line, "Mr. Watson, come here, I want to see you", however he used components from Gray's patent in his final design.
Bell's struggles were not over after his patent application was confirmed. Thomas Edison had been developing a product which used a carbon microphone to transmit signals at a significantly cheaper price than the original design (which required permanent magnets and iron fittings), resulting in intense competition between the companies. In 1878 the first telephone exchanges were opened in both North America and Europe, however these were limited in that they required operators to monitor and connect lines. In 1879 David Connolly and JT McTighe patented an automatic dialing system, later developed by John and Charles Erickson and Frank Lundquist. This system was first implemented in 1919, however it was only in 1978 that the last manual phone was converted.
In 1927 the first transcontinental telephone call was made, however this was only possible through broadcasting radio waves. In 1935 the first international telephone call was made, leading to the running of wires along sea beds to allow long distance communication. Later developments in technology including satellite communications allowed for increased volume and higher quality in these calls.
The design of the telephone has allowed the development of other forms of technology, in particular the internet. Originally dial up modems connected directly through the telephone system, and more recently ADSL which uses partitions of telephone lines (this in turn has led to Voice over IP or VoIP in which calls can be placed through an internet connection). Cellular phones were also developed from the fixed line network, and utilise similar technology when transmitting calls between base towers and exchanges.
The principle behind the telephone is remarkably simple, and at its most basic 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 actual methodology behind the telephone network is slightly more complicated. Each line (or telephone circuit) from a building consists of a pair of thin copper wires. A number of these wires (eg all the wires on a street or in a region) are connected at an entrance bridge (typically a box on the side of a road) where they are packed into a thick wire. For small networks this wire may run directly to the central exchange, for larger networks several of this wires may meet at a local exchange, where their signals are converted into a digital signal which is transmitted to the main exchange.
At the exchange a call is connected to its destination phone. Originally this was done manually with human operators. When a phone was taken off the hook the circuit would be completed and an indicator light would switch on. The worker at the exchange would speak down the line and determine the intended destination phone before manually connecting the incoming call to a jack corresponding to the target phone, effectively completing the circuit. More recently this is achieved automatically using dial tones. The typical phone has twelve dials powered by the electrical circuit, each consisting of a different pair of fluctuating frequencies (each row has a sound frequency, each column has a sound frequency, these are added together). When the phone is taken off the hook the computer at the exchange "listens" to these dial tones and determines the target phone (hence a telephone number), and subsequently completes the circuit between the two phones.
Once the circuit is extended to the target phone it will start ringing, the actual ring being a fluctuation between two tones powered by the small current in the phone circuit and controlled by a capacitator. Although the speaker/microphone of the handset are not initially connected to the circuit the dial speaker is, when the phone is taken off the hook a switch takes the dial speaker off the circuit and connects the handset. When the handset is placed down at the end of the call the hook switch takes the handset off the circuit and places the dial speaker back on, however the computer at the exchange detects that the call is completed, and disconnects the circuit between the two phones.
When the handset is picked up again the dial tone is heard, indicating that there is a completed circuit between the phone and the exchange which can be extended to another phone. If a call is placed to a phone which is "off hook" the exchange will detect that no complete circuit can be established, and will return a busy tone down the line. In order to conserve power most telephone companies will limit the bandwidth, effectively limiting the pitch of tones that can be transmitted over the system (sounds of very high pitch are cut off because they have the highest frequency and hence require the most power to transmit). In the event of a power failure phone lines will continue to be functional (provided they themselves are not cut) because power is provided not through the mains lines but through the exchange, which itself usually has either a backup generator or a large battery supply or both.
Interstate and International Calls
International and interstate calls function roughly the same way as domestic calls, however rather than being connected through one main exchange two or more exchanges allow the circuit to be completed. If an incoming call to an exchange is detected to be outside of the exchange's jurisdiction (through area or country codes) the exchange will attempt to connect to the exchange in which the target phone is located. Over small distances this may be through a wire connecting the two centers, however over larger distances undersea cables or satellite technology may be used and an individual call may be routed through several exchanges (for example the original exchange dials and exchange connected to a satellite transmitter, which in turn sends the signals to an exchange connected to a satellite receiver, which may then connect to other exchanges in order to reach the target phone). Remarkably this entire process may take mere seconds, however it is more expensive and more prone to disconnections than local calls.