How Polygraph Machines Work

Also known as 'lie detectors,' polygraphs are used to record an individual's vital signs, such as breathing rate, pulse, and so forth. Check out this podcast from HowStuffWorks to learn more about polygraphs and the art of lie detection.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from how stuff works dot com
where smart Happens. Hi, I'm Marshall Brain with today's question
how does a polygraph machine work? You hear about lie
detectors all the time and police investigations, and sometimes a
person applying for a job will have to undergo a
polygraph test. The goal of a lie detector is to

(00:22):
see if the person is telling the truth or lying
when answering certain questions. Four to six sensors are attached
to a person taking a polygraph test. A polygraph was
originally a machine in which multiple or polly signals from
these sensors were recorded on a single strip of paper.
That's the graph part you get polygraph. Today, most of

(00:46):
the recording is done on a laptop instead of a
piece of paper. The sensors usually record the person's breathing rate,
the person's pulse, the person's blood pressure, and the person's
perspiration level. Sometimes a poly graph will also record things
like arm and leg movement. When the polygraph test starts,
the questioner asks three or four simple questions to establish

(01:09):
the norms for the person's signals. Then the real questions
begin throughout questioning all of the person's signals are recorded
both during and after the test. A polygraph examiner can
look at the graphs and can see whether the vital
sides change significantly on any of the questions. In general,
a significant change such as a faster heart rate, a

(01:32):
higher blood pressure, increased perspiration means that the person is lying. However,
because the examiner's interpretation is subjective and because different people
react differently to lying, a polygraph test is not perfect
and it can be fooled. Do you have any ideas
or suggestions for this podcast? If so, please send me

(01:54):
an email at podcast at how stuff works dot com.
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