What is a lie detector and how does it work?

John A. Larson, a medical student from California University developed a machine in 1921 which can detect lies with a fair degree of accuracy. This machine is known as polygraph or a lie-detector. In 1972, the American inventor Allan Bell developed another improved model of a lie detector which was able to detect slight trembling in the voice which often occurs when a person tells a lie. Today, it is widely used in crime detection.

How does lie-detector machine detect lies?

When someone tells a lie, there occur certain physiological changes in his body. His heart begins to beat faster leading to rapid perspiration. His blood pressure and breathing pattern change. His voice also starts trembling. This machine is designed to record all these changes.

When a person undergoes a lie-detector test, various parts of the machine are attached to his body by wires. The interrogator asks the person all relevant questions. The physiological changes that take place while questioning are recorded by a pen recorder fitted in the instrument. Finally, these observations are analyzed.

However, the polygraph is not wholly dependable. Its accuracy is estimated to be about 80%. In fact, a lot depends on the skill of the examiner in this regard. Some people, who are truly unaware of the fact that they are lying, cannot be caught by the lie-detector. Moreover, the hardened criminals can control their emotions and render this machine ineffective. Because of these reasons the results of a lie-detector test cannot be accepted as evidence in the court of law. However, it is being used by the police departments in almost all the countries.