A polygraph works by measuring multiple physical characteristics of a person while they’re asked questions to which – so the theory goes -they’ll answer either truthfully or deceptively, with deceptive answers detectable by fluctuations of the aforementioned signs.
The polygraph doesn’t detect if someone is telling a lie, however, it only tells you if they are exhibiting ‘deceptive behaviour’.
The three main physiological areas the polygraph monitors are respiratory rate, which is measured by affixing two pneumographs (rubber tubes filled with air) to the test’s subject’s chest and abdomen; blood pressure/heart rate, which is measured by the traditional manner of fastening a cuff around the subject’s upper arm; and galvanic skin resistance, which detects how sweaty you are, measured by attaching fingerplates called galvanometers to the subject’s fingertips.
Traditionally the information garnered from these instruments was translated and displayed on an analogue polygraph system, which consisted of a scrolling sheet of paper and a series of pen-filled mechanical arms, each attached to a set of bellows that in turn were attached to the individual instruments.
So, for example, when a subject’s chest muscles expanded due to heavy or fast breathing, the bellows would inflate and deflate, controlling the movement of the arm and the marks it left on the sheet of paper. Over the past 20 years, however, digital polygraph machines have become the machine of choice, utilizing computer software to decode the instruments’ results.