A polygraph is a machine, or a test using said machine, which measures the changes in a person's body, such as pulse, breathing rate, and skin temperature.It is used as a lie detection test.It is based on the principle that when lying, one will become nervous and have an increased pulse and need for air. polygraph is a machine, or a test using said machine, which measures the changes in a person's body, such as pulse, breathing rate, and skin temperature.