by Chris Woodford. Last updated: February 28, 2019.
It was one of those moments when the
world changes forever. On March 10, 1876, Thomas Watson was staring at a strange
piece of electrical apparatus when he heard it speak the words that
made history: "Mr Watson! Come here! I want to see you!" Those three short
exclamations mark the moment when the telephone
properly came into being, thanks to Watson's brilliant colleague
Alexander Graham Bell (1847–1922). Since that moment, a little over a century
ago, the telephone has become one of the most commonplace
inventions in the world. Apart from handling voice calls, it helps us
send documents by fax and it's also the basic
infrastructure on which the Internet is built.
Telephones seem quite simple, but what exactly are they and how do they route our calls round
the world? Let's take a closer look!
Photo: Telephones used to be cumbersome and expensive pieces of equipment and, until
mobile cellphones became popular from the 1980s, were invariably
fixed in position. When you made a telephone call, the number you dialled reached a specific place;
often you had to ask to speak to the person you wanted and wait while they walked to the phone. Mobile phones
turned all of this on its head. Now phone numbers are linked not to places but to people, who take their phones and phone numbers
wherever they go, even from one country to another.
From telegraphs to telephones
Photo: Modern telephones scoop up their power from the phone line,
batteries, or a plug-in adapter; before electric power became so convenient and widespread,
telephone users had to generate some of their own power for a call using a built-in, hand-cranked
generator called a
This is an early example of a magneto phone in Buffalo Gap Historic Village, dating from
the late 19th century. Photo from The Lyda Hill Texas Collection of Photographs in Carol M. Highsmith's America Project, Library of Congress, Prints and Photographs
Have you ever tried
a tin-can telephone
out of two baked-bean cans and a length of string?
It really does work! Not only that, it gives
you a great insight into how a telephone carries people's voices from
place. Normally, sounds travel through the air as invisible waves,
transferring energy from something that
vibrates (like a drum skin or a
guitar string moving back and forth) to our eardrums. Sending sounds
through the air is fine when
the person we want to talk to is sitting in the same room. But if
they're in another building—or even another country—we need a different
form of communication.
Back in the 19th century, just before the telephone was invented,
another piece of electrical equipment called the telegraph
was the height of communication technology. A telegraph was a simple
electrical circuit stretching many miles between two towns, typically
alongside a railroad line. Messages could be sent back and forth down
the telegraph line as bursts of electricity.
How did that work? Think
of a flashlight. It's a very basic electrical circuit: a loop of cable
linking together a lamp, a switch, and a battery. Normally, when the
light is off, the switch is set so it breaks the circuit. The switch is
a kind of "drawbridge" that stops current flowing. When you flick
the switch, you lower the drawbridge: you remove the break in the
circuit, so electrons from the battery flow continuously around it, lighting the
Suppose you could make an absolutely gigantic flashlight hundreds of
miles long. If you put the switch part at one end, say in New
York City, and the lamp part at the other end, say in Detroit, you
send messages between those two places by flicking the switch on and
off. You'd stand in New York City clicking the switch and someone else
would stand in Detroit watching the light flash on and off. To send
messages, you'd need to agree a special code beforehand
so different kinds of flashes meant different things. If you wanted to
clever, you could have two of these gigantic flashlights side-by-side,
one to send messages from New York City to Detroit and the other to
send replies back the other way. What you'd have built would have been
a kind of telegraph. In real telegraphs, instead of a lamp, there's a
device that makes a clicking noise at one end every time the switch (which is
known as a key) is clicked on and off at the
other end. And the people at the two ends use a prearranged pattern of
short and long clicks ("dots" and "dashes") called Morse
Code to send their information.
Photo: A cordless telephone handset like this is like a cross between a landline and a cellphone. Like a cellphone, it uses radio waves to communicate with a base station plugged into a normal landline outlet. It has quite a low powered radio transmitter so it works only within a short range of your home and garden. You can see the wireless antenna extended in this photo. Push-button phones slowly began to replace rotary dial phones from the 1960s, following the development of tone dialing (DTMF, described in more detail below). Older phones had dials that sent pulses of current down the line to the exchange instead. Pulse dialing was much slower and it was quite easy to dial the
wrong number without realizing.
Telegraphs revolutionized communications, but they were slow and
rather laborious to use. One of the main difficulties was that people
had to learn Morse code before they could send and receive messages;
another problem was that messages had to be sent and received
at special telegraph offices: it was not possible to get them sent
directly to your own home. Telephones changed all that.
A real telephone is like a cross between a baked-bean can telephone
and a telegraph. When you "call" a friend on a baked-bean can
telephone, you speak into the can at one end and the sound of your
voice makes the can vibrate. The string then carries the vibrations to
the can your friend is holding, which vibrates too, and produces sounds
your friend can hear. Unlike a baked-bean can telephone, you can't
speak into a telegraph. Instead, you
send messages as coded pulses of electricity by flicking a switch on
and off. Suppose you could combine these two ideas: what if you
could turn the sound of your voice into an electrical signal that could
be carried down a wire of any length, then turned back into a sound
that someone else could hear at the other end? That was the idea that
occurred to Alexander Graham Bell—and it's the principle behind a
Parts of a telephone
A telephone is not just the thing that sits on your table at home.
It's a complete system: the handset at your end, the cable that runs
into the wall, a whole collection of communication apparatus
fiber-optics, microwave towers, and
satellites) that carries
telephone signals across country, some switching apparatus that makes
sure calls get to the right place, and a handset at the other end.
Let's think about a typical phone handset. At the top,
there's a loudspeaker you press against your ear. At the bottom,
there's a microphone you put near your mouth. Coming out of the
handset, wrapped inside
a single thick, coiled cable, are two pairs of copper wires. One pair
is an output: it takes outgoing electrical
signals from the microphone to the telephone system; the other pair is
an input: it takes incoming signals from the telephone system to the
Photo: You can see the microphone really
clearly when I unscrew the mouthpiece of this antique telephone.
Note that the microphone is connected by just two wires: one carries
electricity into the microphone; the other carries it back out again.
The loudspeaker and microphone work in similar but opposite ways.
The microphone contains a flexible piece of
plastic called a diaphragm with an iron coil attached to it and a
magnet nearby. When you speak into the mouthpiece, the sound energy in
your voice makes the diaphragm vibrate, moving the coil nearer to or
further from the magnet. This generates an electric current in the coil
that corresponds to the sound of your voice: if you talk loud, a big
current is generated; if you talk softly, the current is smaller. You
can think of a microphone as an energy converting device: it turns the
sound energy in your voice into electrical energy. Something that
converts energy from one form to another is called a transducer.
The loudspeaker in a phone works in the
opposite way: it takes an incoming electrical current and uses
magnetism to convert the electrical energy back into sound energy you
can hear. In some
phones, the loudspeaker and microphone units are virtually identical,
just wired up in opposite ways. (You can read more in our
articles about loudspeakers and
Making a call
Everyone knows what happens when you make a phone call: you pick up
the handset, dial, and wait for the person at the other end to answer.
But, just for a change, let's think about it from the phone's point of
1. Pick up the handset
When you pick up the handset, you switch on the telephone circuit:
lifting the handset is effectively the same thing as flicking a switch
that completes an electrical circuit between the handset and the local
telephone exchange (a building full of
telephone equipment in your local town or city that routes all the calls to and from your home).
I'll explain what happens in a telephone exchange later on in this article.
2. Dial the number
One important part of a phone we've not mentioned yet is the
push-button keypad. We still talk
about "dialing" phone numbers even though hardly any phones (except
antiques like the one described up above) have rotary
dials. On one of those old phones, you dial a number using a system
called pulse dialing. If you listen to the
handset as you dial, you hear lots of clicks going down the line as the
dial rotates. Actually the dial is temporarily interrupting the
electric current flowing down the line as it turns. The rapid
pulses it generates in this way indicate to the local exchange what
phone number you want to reach. A modern phone uses a different system
called tone dialing (also known as DTMF, or
dual-tone multi-frequency). As you press the numbers on the keypad, you
hear musical notes going down the line instead of clicks. The exchange
recognizes the number you want from the musical sounds your handset
makes. (Tone dialing is also useful for things like telephone banking.)
3. Make the connection
Photo: A female telephone operator sitting at a switchboard in 1965.
Photo by Martin Brown courtesy of NASA Glenn Research
You've picked up your handset and dialed the number. Now the
exchange has to route your call to another phone in someone
else's home. Imagine how this works. You can visualize an exchange as a
huge building with thousands of wires coming into it from people's
you wanted to connect Tom's phone to Ann's, in theory all you'd have to
do would be to take the two cables leading to their homes and
temporarily join them
together. In the late-19th century, when phones were still fairly new,
this is pretty much what happened at the exchange. There was someone
(typically a woman) called a switchboard operator.
She would take one person's phone line and physically connect it to another
by plugging it into a socket on a wooden board. She could connect any
person's phone to anyone else's by switching around the connections on
the board—which is why it was called a switchboard. Before long, these
manual switchboards were replaced by electromagnetic ones that switched
automatically using relays.
were invented in the late 1940s,
switchboards started to become smaller, quicker, and more efficient. Today,
switchboards are essentially just computers or digital
exchanges that perform all the telephone routing automatically—but they still work essentially the
same way as manual switchboards: they make a direct electrical
connection from the handset in your home to the one in the home you are
4. Talk into the phone
Once your call has been answered, you speak into the mouthpiece of
your phone. Your voice generates sound energy when the vocal chords in
your throat vibrate. The sound energy travels through the air into the
microphone and makes the diaphragm inside vibrate. The diaphragm
converts the energy from your voice into electricity, and this
electrical energy flows down the phone line. When it reaches the
handset at the other end, it flows into the loudspeaker in the
earpiece. There, the electrical energy is converted back into sound—and
your voice is magically recreated in the other person's ear.
When the other person speaks, the entire process runs in reverse. Since
are wires running in both directions, you can both speak and listen at
the same time.
To sum up what happens to energy when you use a phone to call a friend:
- The sound energy in your voice makes the air vibrate. Vibrating air carries the sound energy into the phone.
- The diaphragm in the mouthpiece microphone converts sound energy into electrical energy.
- The electrical energy travels from the phone, via exchanges, to your friend's phone.
- A diaphragm in the earpiece loudspeaker of your friend's phone converts the incoming electrical energy back to sound energy.
- The sound energy travels out from the earpiece into your friend's ear.
Making a telephone call, then, is all about converting energy from
sound to electricity, carrying the electricity down a very long wire, and then
turning the electricity back into sound. But if you want to make a call
to another country, a few more things are involved.
Making international calls
Once, all calls were carried down wires from one phone to another.
That's why long-distance (sometimes called "trunk") calls took longer
to route and were more expensive to make. International calls took so
long to route that there was a very noticeable (and quite confusing)
delay between you and the person at the other end, which was caused by
the time it took for signals to travel down the wire. Now, calls travel
in a whole variety of different ways. Most calls still go from homes to
local exchanges along old-style copper wires
(arranged in what's called a twisted pair). But calls can travel between exchanges down ultra-fast, high-capacity
fiber-optic cables. Longer-distance
calls are often beamed between urban centers
using microwave towers (like small satellite dishes mounted on high
buildings). International calls are typically bounced around the world
using space satellites. Fiber-optics,
microwave towers, and satellites
send and receive phone calls not as electrical signals but as pulses of
(light or radio waves) traveling at the speed of light. That's why modern international phone calls are much
faster, cheaper, and more reliable than they used to be—and why there's
hardly any time lag on calls anymore.
What does a telephone exchange do?
I've mentioned telephone exchanges a few times in this article without actually saying what
they do or how they go about it.
Suppose there are five people in your street and they all want
telephones so they can chat to one another. If you've got some baked
bean cans to hand, it's easy to wire them all up. Each person needs a
connection to all of the others, but that means quite a tangled mess:
four baked bean cans in each home and four lines stretching taut to the other buildings. It's not great, but we
could live with it. Now suppose there are five thousand people in
your village and they all want to talk to one another. Each house
would need 4999 lines running into it! Or what about if 10 million
people in a city wanted phone lines instead? Have you really got
room for 10 million phone lines in your home? You can see that it all
gets a bit much very quickly.
Artwork: What a telephone exchange does. Top: Without an exchange, we need a separate line linking every home to every other home: each of these nine homes needs eight incoming lines! Bottom: With a central exchange, each home needs only one line linking it to the exchange, which can route calls on to all the other homes.
That's where telephone exchanges come in. Instead of each person
being connected to everyone else, they're all connected to their
local exchange, and the exchanges themselves are connected together.
In our first example, the four people would need only one line
each—and the exchange would be able to connect any pair of them
together; in our last example, the 10 million people would still
need only one line each. But as the number of people increases, we
need to add more local exchanges, and now we're not just connecting
one person to another but one person, through their local exchange
(and potentially a series of exchanges) to another local exchange, until we reach the
receiver of the call. What we end up with is a kind of spider's web of interconnections quite like our
modern Internet. There are no permanent connections between any one line and
any other: just a series of circuits that can be switched about so they
connect together to make calls. This technology is called circuit switching.
When there were relatively few people with telephones, it was easy
enough to have operators at telephone exchanges who could manually
keep track of the calls by plugging leads in and out of switchboards.
But as the system rapidly scaled up in size, and people came to
expect faster calls, automated telephone exchange
equipment rapidly took over. Although you might think that telephones
were invented before exchanges, you'd be wrong. Exchanges were
invented for the telegraph some years before Bell patented his phone,
so the basic idea of central switching offices that could connect
places together to exchange electrical messages really came
Who invented the automated exchange?
Photo: One of the electromagnetic switching units in a typical Strowger telephone
exchange, c.1924. Photo by Harris & Ewing courtesy of US Library of Congress.
What Bell did for the telephone, Almon B. Strowger (1839–1902) of Kansas City,
Missouri did for the telephone exchange. Reputedly, he invented
the automated exchange switch because he was working as
an undertaker and couldn't understand why calls to his business
weren't getting through; someone at the switchboard was sending
them to a rival instead. There had to be a fairer way of handling
calls, he figured, and promptly decided to automate the process.
It was a good call—and earned him fame and fortune.
In his first exchange patent, US Patent 447,918: Automatic Telephone Exchange, granted March 10, 1891, Strowger described how a traditional
switchboard could be replaced by rotating cylinders with lots of connection
points on them, which could be turned automatically by
electromagnets to hook one telephone circuit to about 100 others.
This is called a step-by-step switch or Strowger switch.
The method it uses, which is known as rotary dialing, explains why telephones
themselves were fitted with rotating dials. As you dialed a number,
it sent little pulses of electricity down the line to the exchange.
This made a series of chattering Strowger switches rise up or rotate by a certain number of positions
so your call was automatically routed to its destination. (You can watch a video
of this happening in the links below.)
Artwork: How Almon Strowger's step-by-step switch worked. The incoming phone line from which the call is being made is connected to the blue part of the switch via the yellow stand. The outgoing phone lines (orange) are connected to the little holes in the red cylinder. When you dial a number, a series of electromagnets (green) operate levers that move the blue part of the switch up or down and rotate the red cylinder so many places to connect the incoming line to the appropriate outgoing circuit through the appropriate hole. A single Strowger switch can handle about 100 lines, but linking a series of switches together lets you connect any number. Artwork from US Patent 447,918: Automatic Telephone Exchange
by Almon Strowger, courtesy of US Patent and Trademark Office.
Although calls are routed electronically
these days along fiber-optic cables, Strowger's basic technology remained in
widespread use for almost a century, from the 1890s until about the 1980s. Though
relatively unknown compared to inventing giants like Edison, Morse, and Ford,
Strowger was, nevertheless, one of the most important and influential inventors of
the 20th century. We might not have the Internet without him;
the circuit-switching technology he invented ultimately evolved into a very different
way of sending information down lines, called packet switching, which
is how you're managing to read these words now! (You can find out more about that
in our article about how the Internet works.