John A Larson, a medical student from California University developed a machine in 1921 which can detect lies with a fair degrees of accuracy. This machine is know as ‘Polygraph’ or a Lie Detector. In 1927, the American inventor Allan Bell another improved model of a Lie Detector which was able to detect slight tremblings in the voice which often occur when a person tells a lie. When someone tells a lie there occur certain physiological changes in his body. His heart begans 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 such 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 taking place while questioning are recorded by a pen recorder fitted in the instrument. Finally these observations are analysed. However, the Polygraph is not wholly dependable. It accuracy is estimated to be about 80%. Infact 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, can not be caught by the Lie Detector. Moreover the hardened criminals can control their emotions and render this machines ineffective. Because of these reasons the results of a Lie Detector Test cannot be accepted as an evidence in the court of Law. However it is being used by the Police departments in almost all the countries.