Communication: How Telephones Work

Learn how simple telephones worked yesterday and today, and how phone signals travel from home to home, and from one phone to another.

In today's world, we have many devices in our homes that make our lives easier, or more entertaining, such as televisions, computers, and kitchen appliances. One of the oldest devices still in use today is the telephone.

The first working telephone, or "electrical speech machine" as it was then called, was built in 1876 by inventor Alexander Graham Bell. Other versions had been created previously, but Bell is usually credited with the invention, since his was the first to function properly.

Today's basic telephones work in essentially the same way as the originals. Most modern phones have a lot of extra bells and whistles, but the parts that make it function as a phone are very similar.



The earliest and simplest telephones had three working parts: a speaker, a microphone, and a hook switch. The speaker, which was located in the earpiece, allowed the user to hear the caller. The microphone, located in the mouthpiece, was used to transmit the user's voice. Finally, the hook switch was a device that connected and disconnected the phone from the telephone network (the series of wires and telephone lines that connect telephones around the world) each time the receiver was lifted or replaced. The number of times this connection was broken and re-established determined what number was "dialed."

Modern telephones contain the same workings, with a few additions. Many early telephone users found it frustrating that they could hear their own voices echoing back to them as they spoke. To prevent this, devices called duplex coils were added. Most phones today also contain a bell or other type of ringer and an amplifier. Touch-tone keypads were added, too, although many phones still use pulse-dialing, which is simply an automated way of repeatedly tapping the hook switch.

Now that we know what is inside the actual phone, we need to determine how it connects us to the outside world. First, a wire connects from the telephone to a special outlet in the wall called a jack. From there, a pair of copper wires (usually red and green--a second set for an extra phone line would most likely be black and yellow) run to a box called an entrance bridge at your home. The wires then continue to another box at the street, or to boxes attached to poles if the wiring is above-ground.

These boxes can typically be found near every to or three houses. Within the boxes, the wires for several houses are spliced together into a larger cable; the cables from several of these boxes then lead to a larger box, about four to five feet high. Once again, the cables are spliced into larger cables, which now contain 25 to 50 pairs of copper wires.

The larger cables come together at a large box called a multiplexer. This box usually also contains an electrically-powered device known as a digitizer, which converts the signals so they can be carried on smaller wires. These wires then lead to small buildings called switches, or to more boxes known as digital concentrators.

Once a signal reaches the switch or concentrator, it is digitized and sent on one cable to the telephone company. For a local call, a loop is formed between both parties, and the signal travels in both directions via this network. If the call is long-distance, the signal will travel over a fiber-optic line, by satellite, or by microwave tower to the receiving party's local phone company office, and vice versa.

The telephone system may sound complex, but it is still one of the simplest communication devices we have in today's world.

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