How does a lie detector work?
A lie detector is a kind of machine that records changes in such things as a person’s breathing and heart and heartbeat while he is being questioned. Sometimes it is hard to know when a person tells a lie. But most people feel guilty when they lie.
When a person lies, certain body reactions may take place which he cannot control or may not even be aware of. The may be a change in blood pressure or breathing, a faster heartbeat, or other emotional signs.
A lie detector is a kind of machine that detects and records these changes in the body.
A person who takes a lie-detector test sits in a chair with an arm cuff to measure blood pressure and pulse. A respiration tube around the chest records the rate of breathing. Still other devices may be attached to detect other body changes.
The body’s reactions to questions are recorded on a moving chart. If the chart shows changes in breathing, heartbeat, or other signs while the person is being questioned, chances are he is not telling the truth. He usually will not show these signs when he answers questions truthfully. – Dick Rogers