If you mean polygraph, or lie detector, it is an instrument that measures physiological changes in the body such as blood pressure, pulse, breathing rate and skin conductivity(which measures sweat on the skin) when a subject is asked a battery of questions. The subject is asked basic questions first, name, sex, etc. to establish a baseline. The idea behind this device is that when a person tries to lie, there will be physiological changes in the body that will be detected compared to when they are telling the truth. Most scientists believe this device has little credibility and in 1997, a survey of 421 psychologists estimated it to be only 61 percent accurate.
The lie detector has its origins in the early 1920s. A policeman with the Berkeley, California police department, John Augustus Larson, developed a device that measured a persons heart rate, breathing rate, and blood pressure as he/she was asked questions to determine if the subject was telling the truth. A small electrical current conducted over the skin was able to detect changes in perspiration, using a number called a "galvanic skin response". Theoretically, a person answering falsely on questions would experience changes in all these areas being measured. The typical procedure is to ask the subject a series of elementary questions and establish a base pattern, which is recorded by a device very similar to the seismogram recordings exhibited during an earthquake. The questions eventually get to the subject matter at the heart of the investigation. Wild, erratic swings exhibited on the recorder indicate the person is not telling the truth. Having experienced this procedure myself, I can vouch the procedure is fairly accurate, in the hands of a qualified administrator. As with all things, however, there is a small percentage of inaccuracy. Not all people register changes in the parameters measured when responding falsely.