What Is Polygraph?- Hogan Polygraph & Investigations

What is Polygraph

Polygraph is the use of scientific instruments known as the polygraph instruments or lie detector to monitor the pulse rate, sweat gland activity (skin conductivity), blood pressure and respiration of a subject or examinee when the subject is being asked series of questions by a competent polygraph examiner. The aim is to detect the examinee’s truthfulness or deception with regards to a particular issue in an ongoing investigation. Or simply saying; polygraph, popularly referred to as a lie detector test, is a device or procedure that measures and records several physiological indicators such as blood pressure, pulse, respiration, and skin conductivity while a person is asked and answers a series of questions.

Electro-dermal activity (EDA) plates

The EDA monitors sweat gland activity (skin conductivity). Whenever we lie in our response to a question, the amount of sweat produced by the sweat gland increases but this can not be seen by mere eyes. The EDA plates are attached to the index finger and the ring finger of the subject during a polygraph test and the function of the plates is to monitor the amount of sweat produced by the sweat gland while the subject reacts to questions. This activity in the sweat gland is sent through the instrument box to the computer in form a moving chart for analysis.

Cardio cuff

The cardio cuff looks like the blood pressure cuff but performs a different function. It measures the flow of blood during the test while a stimulus (question) is being introduced. The cardio cuff is usually fixed on the upper arm of the subject during a polygraph test.


The Pneumographs are two rubber tubes with metal ends. They are attached to the chest and the abdominal areas of the examinee. They measure the respiratory activity of the subject during the test. Data collected through the pneomographs are also transferred through the pneumograph (lie detector) box to the computer and they are represented by two moving thin lines with curves indicating subject’s reaction to stimulus (questions).

Seat sensor

This rectangle pad usually placed on the polygraph seat for the subject. Sear sensor measures movement from the lower body of the subject during the test. Deceitful subjects tend to move their body around and not stable in order to cover their reactions during a polygraph test.

The polygraph instrument box

The instrument box is what connects all other instruments to the polygraph computer. All reactions are sent through it to the computer for analysis after the polygraph examination.

Complete polygraph instrument

Pictorial representation of the complete polygraph instrument can be seen right