How Everything Works
How Everything Works How Everything Works

Question 794

Can you explain how the telephone wiring in my home works for the telephone? My touch-tone phone has 4 wires, but I understand that only 2 wires are used. Does the phone use the other 2 wires for the light on the phone pad, etc.? — DS, Larkspur, CA
Your telephone performs all of its functions using only those 2 wires. The 2 extra wires are virtually never used by a single-line telephone. The only exception that I'm aware of is the old "Princess Telephone," which had a special light powered by the extra pair of wires. In most telephones, even the power for the lighted keys comes from the 2 main wires. While the telephone is off the hook, the telephone company sends a constant DC current through those two wires. This current powers the telephone's electronics and its lights. When you talk, the microphone causes the telephone's electric impedance to fluctuate up and down and this variation causes sound to be reproduced in your friend's earpiece. Pressing the dialing buttons causes similar fluctuations in impedance and the telephone company uses these tones to make the proper connections. When the telephone company rings your telephone, they send a higher voltage AC current through the two wires and the telephone's bell rings.

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