How is lie detecter --polgraph -carried out i.e the method and how is the result of the test read?

Question

How is lie detecter --polgraph -carried out i.e the method and how is the result of the test read?

Answer 1

it measures heartbeat, brain activity and tension. when these levels increase, it means the body is undergoing stress, which is what the body does when it lies. we cannot always see this stress, hence the job of the machine

Answer 2

A polygraph does not measure brain activity.

Most polygraphs will measure respiration, heart rate and galvanic skin response (electrical skin conductivity). These measures reflect sympathetic nervous system activity and emotional arousal. Most people are emotionally aroused when they lie, and therefore their SNS is more active.

the examiner will ask control questions that the subject will answer truthfully, questions that the subject will probably answer untruthfully but are unrelated to the purpose of the test, and questions relevant to the purpose of the test. The person is not lying on the relevant questions if their physiological response is less than on the probable lie questions.

For more info, see sources