A polygraph measures and records several physiological indices such as blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

, pulse

Pulse

In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...

, respiration

Respiration (physiology)

'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

, and skin conductivity

Galvanic skin response

Skin conductance, also known as galvanic skin response , electrodermal response , psychogalvanic reflex , skin conductance response or skin conductance level , is a method of measuring the electrical conductance of the skin, which varies with its moisture level...

 while the subject is asked and answers a series of questions. The belief is that deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers.

The polygraph was invented in 1921 by John Augustus Larson

John Augustus Larson

John Augustus Larson was a police officer of the Berkeley Police Department, California, United States, and famous for his invention of modern polygraph used in forensic investigations...

, a medical student at the University of California at Berkeley and a police officer of the Berkeley Police Department

Berkeley Police Department

The Berkeley Police Department is the municipal police department for the city of Berkeley, California, USA.-History:Shortly after Berkeley was incorporated in 1878, a town marshal and constables were elected to provide law enforcement. In 1909, the town marshal was appointed to be the first...

 in Berkeley, California.