How The Lie Detector Works

A polygraph machine is really a combination of three machines, each designed to display results on a rolling graph. The three machines monitor pulse rate and blood pressure, respiration, and skin conductivity (sweatiness). Once the subject is hooked up, the tester asks them some control questions, some obviously true and some obviously false, to try and get a good idea of what the subject's normal responses look like. The real questions are then asked. Generally speaking, when a person lies, their pulse and blood pressure increase and their breathing becomes shorter. These effects show up on the graph directly after a lie has been told and this is how the test works. The polygraph is not considered faultless, however, and its results are not admissible in a court of law as evidence.

When a person takes a polygraph test, four to six sensors are attached to the person. A polygraph is a machine in which the multiple ("poly") signals from the sensors are recorded on a single strip of moving paper ("graph"). The sensors usually record:

1.The person's breathing rate
2.The person's pulse
3.The person's blood pressure
4.The person's perspiration


Sometimes a polygraph will also record things like arm and leg movement. When the polygraph test starts, the questioner asks three or four simple questions to establish the norms for the person's signals. Then the real questions being tested by the polygraph are asked. Throughout questioning, all of the person's signals are recorded on the moving paper.

Both during and after the test, a polygraph examiner can look at the graphs and can see whether the vital signs changed significantly on any of the questions. In general, a significant change (such as a faster heart rate, higher blood pressure, increased perspiration) indicates that the person is lying.

Useful links:

• Police candidates looking for a job need to take a look at this site. Here >>
   
• "Electronic Privacy Information Center.
  Here >>
   
• The polygraph as a truth detector Here>>