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Types of Polygraph Machines

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Polygraph tests are often called "lie detector" tests. While they can't read your mind to see if you're lying, they measure physiological changes, such as heart and perspiration rate, to determine if you may have become nervous when being asked a certain question. This may indicate that you aren't telling the truth, or are covering up something. Modern polygraph specialists use two types of machines to conduct their tests.

General Operation

All polygraph tests operate in the same general way. The specialist will attach a cuff to the upper arm to record blood pressure, and will affix two small metal devices to the fingers to monitor perspiration activity. There may be straps placed around the abdomen to measure breathing rate as well. Testing is comprised of three sections: pre-test, chart collection, and test data analysis. No polygraph testing instruments include the use of voice stress analysis.

Conventional Instruments

Conventional test machines, also known as analog instruments, are still sometimes used for testing, although they are diminishing in popularity compared with computerized versions. The analog instruments used a complex "box" with a series of switches and dials, a roll of chart paper, and moving pens to record changes in physiological measurements. The examiner would then need to read all of these papers, recording which questions were asked when, in order to determine whether the noted changes indicated a lie. Problems with this system included ink shortage, paper tears, and discrepancies resulting from initial pen placement.

Computerized Instruments

New technology has allowed for the development of computerized polygraph tests, rendering the analog equipment nearly obsolete. Rather than wasting paper and time fiddling with the conventional equipment, examiners now use programs such as those created by Axciton Systems and Stoelting Company to record and decipher all measurements on their portable computer screens. The system will usually have options to view the test results on-screen or printed out, and may have composite options with chart-scoring as well. While polygraph enthusiasts argue that the computerized versions have immense improvements over their older analog counterparts, anti-polygraph activists maintain that the same and other issues are still there, such as ease of "beating" the test using physiological modifiers.


Jennifer Simon has been a copywriter since 2007, a copyeditor since 2004 and currently teaches English Composition at Full Sail University. Her edited articles have appeared in "The Washington Post," "The Huffington Post" and "The Network Journal." Simon has a Master of Arts degree from Duquesne University with a focus in modern English grammar, linguistics and editing.

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