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How does a Lie Detector Test Work?
A polygraph machine consists of multiple sensors whose response is recorded in the form of a graph on a single strip of paper. These sensors are attached to the person taking the Lie Detector Test. These sensors usually record the following: The person's breathing rate, pulse, blood pressure and perspiration i.e. sweat. Some polygraphs also record arm and leg movement of the person.
Now, when the test starts, a number of questions are asked by the examiner to the person. During the test, all the signals coming from the person are recorded on the moving paper. Now, while examining the paper the examiner can see whether any vital signs changed significantly during any of the questions. Generally, a significant change in vital signs indicates that a person may be lying.

Reference:
https://science.howstuffworks.com/que...

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