- An air-filled rubber or flexible tube on your upper body, and one around your abdomen. These will detect changes in breathing.
- A fingertip plate, or galvanometer, that will detect perspiration (sweating)
- a blood pressure cuff to measure changes in blood pressure, particularly spikes (the blood pressure rises suddenly)
Here is a video of a Bloomington Police officer demonstrating how it works:
The only state allowing a polygraph test in court as evidence is New Mexico. Other states do not consider them reliable enough to prove a person's guilt or innocence and are aware that a false positive or negative result can jeopardize the wrong people.
This is why a polygraph is almost never allowed in evidence: It is possible to 'outsmart' a polygraph test. People have tried using sedatives before the test so they are not 'ruffled' by any questions asked of them. They have also tried to self-inflict painful stimulus during the test, so that they are always agitated; such as biting the tongue or putting something painful in their shoes. This could raise the blood pressure throughout the test. It is possible that someone with severe mental illness, such as a psychopath, would be able to lie about anything and feel no guilt or upset whatsoever and indicate no physical reaction. Hence, that person's test would not be a good source of the truth. It is also possible that an innocent person would be so upset by the test that they give 'false positive' reactions, which do not truly indicate guilt.
People who are expert at polygraph testing are known as Forensic Pschophysiologist. Training to be certified in performing the test can be completed by law enforcement workers as well.
What law enforcement careers might involve the use of a polygraph:
- Forensics Investigator
- Police Officer
- Homeland Security
- Fraud investigator