Changes

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.

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.

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 [[capacitor]]. 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.

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.