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Bell wire: Difference between revisions
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When a telephone is 'on hook' (a term dating back to candlestick phones) - i.e. the handset is on the base, then there is no current drawn from the 'A' and 'B' wires, and the voice circuit is disconnected from the line. When a telephone is taken 'off hook' - i.e. the handset lifted off the base, current is drawn on the line, connecting the voice circuit, and the current drawn (looping the line) signals the exchange to issue dial tone.
With the telephone back 'on hook', incoming calls are signalled by ringing current being applied to the line by the exchange. This is a large AC voltage - about 80V. In order to separate the ringing current from the rest of the telephone, a bell capacitor is used to route the ringing current to the bell. The bell capacitor is connected at one end to the 'B' wire, and to the bell at the other, the other end of the bell connects to the 'A' wire. This is all on the line side of the hook switch, so is connected to the line when the telephone is 'on hook'.
With pulse dialling (loop disconnect), the telephone is taken 'off hook', looping the line, and the dialled numbers 0-9 are signalled by the appropriate number of pulses briefly disconnecting the loop - except that 10 pulses are sent for number 0. This causes the audible clicks in the earpiece - in fact, you can count the pulses.
But there's an issue - the brief pulses of disconnection of the loop get seen by the bells of other telephones on the line, which can cause those bells to tinkle as another phone dials.
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