How Your Telephone Works

You may remember this scene in It’s A Wonderful life where George yells at Zuzu’s schoolteacher through his telephone.  Whenever I saw that scene as a kid, I wondered how that phone worked.  Today’s phones might appear more modern, but the basic technology behind the telephone hasn’t changed in almost 100 years.  George and Mary’s phone would work in your home today.  Here is a basic explanation of how it all works.

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Let’s say you pick up the phone to call a friend.  When you speak into the phone, your sound-waves enter your mouthpiece.  Your sound-waves strike a diaphragm in the mouthpiece and make it vibrate.  The diaphragm vibrates in patterns depending on what you are saying, how loud you are saying it, etc.  Meanwhile, your telephone company is constantly sending a low voltage electrical current to carbon grains that sit behind the diaphragm.  The low voltage current picks up your vibration patterns from the diaphragm, and your voice is now electricity and transmitted to your friend’s house. 

At your friend’s house, your voice is converted from electricity back into sound-waves that your friend can understand.  The ear piece of her phone (called a receiver) also contains a diaphragm. Behind this diaphragm are 2 magnets.  The electrical current from your voice causes her magnets to vibrate, which causes her diaphragm to vibrate.  As her diaphragm vibrates according to your speech pattern, it pushes and pulls the air in front of it.  The pressure on the air creates sound waves that sound exactly like you!

So,  how is your voice transmitted to your friend’s house?

In the olden days each phone was connected to a Local Office in town via its own set of copper wires.  A switchboard operator (let’s call her Myrtle) sat at the Local Office in front of a panel of little sockets – each connected to a phone in the town.  When someone lifted their phone receiver to call a friend, the low voltage current triggered the light above the caller’s socket to turn on.  Myrtle would plug a jack into the callers socket and ask who he wanted to talk to.  She would then plug her jack into the recipient’s socket, send a ring signal to the recipient’s phone, and tell the recipient that the caller wanted to talk to her.  Myrtle would then connect a cable from the caller’s socket to the recipient’s socket, and the conversation could begin.  When Myrtle saw that the lights above the sockets went out, she would pull out the cable.  Even when making a long distance call, the caller simply gave Myrtle the number, and she would physically connect him via long distance operators at a Central Office.  This was an incredibly simple, but incredibly expensive switching system.  All calls were manually patched together directly with separate physical wires.  Billing records were kept manually by timing long-distance calls.

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One way to look at “offices” of the telecom world is to break down the following phone number: 212-502-1234. 

  • 212 represents the Central Office
  • 502 represents the local office
  • 1234 represents your phone line

Today switchboard operators are computerized switches at Local Offices.  Long distance operators are computer switches at Central Offices.  Today copper wiring connects your phone in your house to a small 3 foot high box on the side of the road (I’m sure you’ve seen these on your street), then to a Local Office perhaps where Myrtle would have been.  Central and Local Offices are now connected to each-other via fiber-optic lines that carry digitized versions of your voice.  Calls are sent to other countries via underwater cabling or satellite systems.

In other words when you make a call today,  your voice travels from your phone to (1) a box or two on the side of the road (2) your local office (3) the recipients central office (4) the recipients local office (5) the recipients box(es) on the side of the road (6) the recipients phone.  Note that steps 3 and 4 aren’t needed, if you are calling someone in the same local office.  Step 3 is not needed if you are making a local call. 

Any questions?

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