Why do electric plugs have holes?

You might have noticed that plugs for electric appliances have two holes. There are three reasons for these apertures -- find out what they are in this podcast from HowStuffWorks.com.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from house stuff works dot com
where smart happens. Hi Am Marshall Brain with today's question,
why do the two flat prongs on plugs for most
electrical appliances have holes in them? If you unplug any
appliance in your house, there's a nine percent chance that

(00:21):
the two flat prongs have holes in them. There are
three reasons for those holes. First, if you were to
take a part in outlet and look at the contact
wipers that those prongs slide into, you would find that
they have bumps on them. These bumps fit into the
holes so that the outlet can grip the plugs prongs
more firmly. This detenting prevents the plug from slipping out

(00:45):
of the socket due to the weight of the plug
in the cord. It also improves the contact between the
plug and the outlet. Second, electrical devices can be factory
sealed or locked out by the manufacturer or owner using
a plastic tie or a small padlock that runs through
one or both of those prong holes. For example, a

(01:07):
manufacturer might apply a plastic band through the hole and
attach a tag to it that says you must do
blah blah blah before plugging in this device. The user
can't plug in the device without removing the tags, so
the user is sure to see the instructions. Third, there's
also a small savings and raw materials for the manufacturer

(01:27):
of the actual plug prong. Every little bit helps. It's
been reported that really old outlets used captive ball bearings
in coil springs for the detent, but today it's done
with a bump on a springy copper contact. Do you
have any ideas or suggestions for this podcast? If so,
please send me an email at podcast at how stuff

(01:49):
works dot com. For more on this and thousands of
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