The Polygraph Instrument

Table of Contents

The polygraph machine or “lie detector” is an instrument or device that produces recordings of physiological phenomena that could be used to diagnose the truth or deception using a reliable technique. There are no specific physiological reaction linked to lying. The instrument can only record the physiological phenomena/changes that occur when the subject is deceptive. It cannot detect lies or deception. The examiner simply uses the instrument to attempt to detect deception.

Two Kinds of Polygraph Instrument

Most polygraph machines measure four to six physiological reactions that are recorded by three different medical instruments that are combined into one machine.

Long strips of paper move slowly beneath pens that record the various physiological responses on older polygraph machines. This is called the conventional or analog type of polygraph instrument.
Transducers are used in newer equipment to convert information into digital signals that can be stored on computers and analyzed using complex mathematical algorithms in the CPS or computerized polygraph system.

Major Components of the Polygraph Instrument

The polygraph instrument is composed of four major parts: the pneumograph, the galvanograph, the cardiosphygmograph, and the kymograph. These components work simultaneously and help in the administration of the examination.

pneumograph detects the thoracic and abdominal breathing patterns

galvanograph measures the subject’s skin resistance to a very small amount of electricity

cardiosphygmograph measures the changes in the subject’s blood pressure and pulse rate

kymograph chart recording unit of the polygraph instrument

Note, however, that the digital polygraph no longer has the kymograph. Instead, it has the computerized polygraph.

Additionally, the digital polygraph also has the activity sensor seat pad to monitor the activity on the lower body of the subject.