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Polygraph Electronics Circuits
The following circuits (suitable for science fair projects) are based on the fact that a person's skin resistance changes when they sweat (sweating because they're lying). Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more. A. Here's a simple lie-detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie. Al you need are three resistors (R1 to R3), a transistor (Q1), a capacitor (C1), a analog meter (M1). When the subject is lying the meter changes. http://www.aaroncake.net/circuits/lie.htm B. This circuit consists of three transistors (TR1 to TR3), a capacitor (C1), two lights or LEDs (L1 & L2), five resistors (R1 to R5), and a variable resistor (VR1). When somebody lies, his sweat lowers the skin resistance and a red LED comes on whereas the green one comes off. http://www.hackcanada.com/ice3/wetware/lie_detector_circuit.html Polygraph Background 
A polygraph or lie detector is a device which measures and records several physiological variables such as blood pressure, heart rate, respiration and skin conductivity while a series of questions is being asked, in an attempt to detect lies. A polygraph test is also known as a psychophysiological detection of deception (PDD) examination. A typical polygraph procedure starts with a pre-test interview designed to establish a connection (or find a control) between the tester and the testee and to gain some preliminary information which will later be used for "Control Questions " or C (see below). Then the tester will explain the polygraph, emphasizing that it can detect lies and that it is important to answer truthfully. Then a "stim test" is often conducted: the testee is asked to deliberately lie and then the tester reports that he was able to detect this lie. Then the actual test starts. Some of the questions asked are "Irrelevant " or IR("Are you 35 years old?"), others are "probable-lie" Control Questions that most people will lie about ("Have you ever stolen money?") and the remainder are the "Relevant Questions " or R the polygrapher is really interested in. The different types of questions alternate. The test is passed if the physiological responses during the probable-lie control questions are larger than those during the relevant questions. If this is not the case, the tester attempts to elicit admissions during a post-test interview ("Your situation will only get worse if we don't clear this up"). These admissions are the main goal of the test. The accuracy of polygraph tests is a matter of considerable controversy. While some claim the test to be accurate in 70% - 90% of the cases, critics charge that rather than a "test", the method amounts to an inherently unstandardizable interrogation technique whose accuracy cannot be established. Polygraph tests have also been criticized for failing to trap known spies such as Aldrich Ames, who passed two polygraph tests while spying for the Russian government. Several countermeasures designed to pass polygraph tests have been described, the most important of which is never to make any damaging admissions. Additionally, several techniques can be used to increase the physiological response during control questions. In an interview, Ames was asked how he passed the polygraph test. His response was that when told he was to be polygraphed he asked his Soviet handlers what to do, and was quite surprised that their advice was simply to relax when being asked questions, which he did. The polygraph machine was tested for the first time on February 2, 1935 when Leonard Keeler conducted the experiment in Portage, Wisconsin. They were often used by employers in an attempt to screen out dishonest job applicants, but this practice was outlawed for most private employers in the US in 1988. Many US Government agencies still apply routine lie detector tests to screen all employees. Admissibility of polygraphs in CourtWhile lie detector tests are commonly used in police investigations in the US, no defendant or witness can be forced to undergo the test. In United States v. Scheffer (1998) [1], the US Supreme Court left it up to individual jurisdictions whether polygraph results could be admitted as evidence in court cases. In most European jurisdictions, polygraphs are not considered reliable evidence and are not generally used by police forces. In Canada, the use of a polygraphs are sometimes employed in screening employees for government organizations. However, in the 1987 decision of R. v. Béland, the Supreme Court of Canada rejected the use of polygraph results as evidence in court. Related techniquesA related technique called the bogus pipeline involves connecting a person to a non-functioning polygraph (or other sophisticated looking device), and convincing him or her that the device can detect deception. One example might be a metal colander placed on the subject's head, with non-functional wires leading to a Xerox photocopier. When a lie is suspected, the copy button could be pushed - thus spitting out a piece of paper with the words "LIE DETECTED". There have not been any confirmed examples of this actually being used by a police department (although it was depicted in the TV series "Homicide"1). Studies have shown that, compared to control conditions, individuals connected to a bogus pipeline, who believe the pipeline is able to detect lies, are more likely to provide truthful responses2. If the subject is aware of the bogus nature of the pipeline, the test is useless. See also
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This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Polygraph" |
