Can cans and string really be used like a telephone?

You've probably heard of this trick before: Connecting two cans with a piece of string in the bottom of each can will supposedly allow people to speak over a distance to one another. Tune in as Marshall Brain explains how it works in this episode.

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Episode Transcript

Speaker 1 (00:00):
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(00:22):
dot com slash brain Stuff. Welcome to brain Stuff from
how Stuff works dot com where smart happens. I am
Marshall brand with today's question. Can two cans and a
string really be used to talk over a distance? The

(00:44):
old two cans and a string technique that you see
in cartoons really does work. The key is to make
sure that the string is tight between the two cans,
and this normally means that the distance is limited and
the two people have to be connected each other by
a straight line. But as long as the string is tight,
it does work. It's also a great way to understand

(01:06):
how telephones work. To try this technique, take two large
cans or paper cups and punch a small hole in
the center of the bottom of each with a sewing
needle or something similar. Take a piece of non stretchable
thread or kite string about a hundred feet long, and
thread each end through each hole. Either not or tape

(01:29):
the strings so it cannot go back through the hole
when the string is stretched. Now with two people, have
each one take one of the cups and spread apart
until the string is tight. If one of you talks
into one of the cups while the other listens, the
second person should be able to hear what the first
person says. Here's why it works. When one person talks

(01:51):
into his or her cup, the bottom of the cup
vibrates back and forth with the sound waves. Imagine the
bottom of the cup moving back and forth very quickly
thousand times per second or more with the sound waves
of the speaker's voice. The vibrations travel through the string
by pulling the string back and forth. Therefore, the bottom

(02:13):
of the second cup should start to vibrate back and forth,
just like the bottom of the first cup is vibrating,
producing sound waves. The second person can hear the sound
waves and can therefore hear what the first person says,
this is not much different from how a telephone works,
except that electric current replaces the string and the telephone.

(02:33):
In an old style telephone, the person speaking vibrates a
metal diaphragm. The diaphragm's vibrations rapidly compressed and uncompressed carbon granules,
changing their resistance. A current passes through the granules, and
it's strengthened or lessened by the changing resistance. At the

(02:54):
other end, the rapidly changing current runs through a speaker
and causes it's diaphragm to vibrate back and forth. So
the second person here's the first person speaking. Whether you're
using strings or current, you can see that the same
basic idea is at work. A voice is vibrating something
at one end, and at the other end, something else

(03:17):
is made to vibrate in the same way so that
the second person can hear what the first person is saying.
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