Details of operation

Details of operation

The telephone consists of a switchhook (A4) and an alerting device, usually a ringer (A7), that remains connected to the phone line whenever the phone is "on hook" (ie. the switch (A4) is open), and other components which are connected when the phone is "off hook". The off-hook components include a transmitter (microphone,A2), a receiver (speaker,A1) and other circuits for dialing, filtering (A3), and amplification.
A calling party wishing to speak to another party will pick up the telephone's handset, operating a lever which closes the switchhook (A4), which powers the telephone by connecting the transmitter (microphone), receiver (speaker) and related audio components to the line. The off-hook circuitry has a low resistance (less than 300 ohms) which causes a direct current (DC), which comes down the line (C) from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, and sends a dial tone to indicate readiness. On a modern push-button telephone, the caller then presses the number keys to send the telephone number of the called party. The keys control a tone generator circuit (not shown) that makes DTMF tones that the exchange receives. A rotary-dial telephone uses pulse dialing, sending electrical pulses, that the exchange can count to get the telephone number. (Most exchanges are still equipped to handle pulse dialing.) If the called party's line is available, the exchange sends an intermittent ringing signal (about 90 volts alternating current (AC) in North America and UK and 60 volts in Germany) to alert the called party to an incoming call. If the called party's line is in use, the exchange returns a busy signal to the calling party. However, if the called party's line is in use but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.


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