Polygraph Machine – How Does A Lie Detector Work?


Human beings are conditioned to believe that lying is morally wrong. They do not like to lie but do so consciously or unconsciously when they are under pressure. Lying produces nervousness and anxiety. These two conditions in turn produce distinct physiological effects that are sometimes visible and most often measurable in a lie detection machine such as a polygraph. The physiological effects of lying include:

  • An increase in breathing rate accompanied by a decrease in the depth of each breath, also known as shallow breathing. The pneumograph aspect of the polygraph measures these effects.
  • An increase in blood pressure and pulse rate. The cardiograph aspect measures these vital signs.
  • An increase in sweating. The galvanograph aspect measures the difference in the electrical resistance of the skin as the subject sweats under pressure. Electrical conductivity increases with an increase in sweating due to the electrolyte concentration found in perspiration. It does not measure the amount of sweat directly.

The lie detector machine records the measurements on an analog graph. Over the last half century or so, analog polygraphs have traced out their measurements on moving paper rolls which look similar to fax paper rolls on early 90’s fax machines. These papers are then annotated by the examiner during the polygraph test and read after the completion of the test.

More recently, polygraph machines have been hooked up to and its measurements recorded onto laptop or desktop computers thus making it easier for the examiner to administer the test without having to fumble around with lengthy paper rolls. Computers have revolutionized the way polygraphs are recorded. Computers have made analog polygraphs obsolete and make it efficient in storing the results of multiple subjects in one place.

The most important aspect of digital polygraph machines is the software program. The software program includes chart analysis capabilities designed to help the examiner in interpreting the results.

Again, the results of digital polygraph tests can be inconclusive provided that the test subject has had previous surgeries, illnesses, or is taking drugs, medications, or alcohol.

Was it worth reading? Let us know.