For many years people have always wanted to know what was going on in the mind of other people, though there is no true way to answer this. What if you knew how to tap into the mind of someone and provide yourself with the security of knowing you're not being lied to? You'd take it, and the government and corporations do too though whether or not they're very accurate is up to you.
Lie detectors (also called polygraphs) may be used to tell whether or not someone is lying, but it is not all that complicated of a machine. It's simply a medical tool used to examine changes in the human body. These changes include heart rate, blood pressure, respiratory rate and sweat. What it does is measure these events and the examiner (also called a forensic physiologist or an FP) reads them and tries to differentiate the changes that occurred in comparison to what the levels of the subject are normally.
There are two kinds of polygraph tests, the ones you normally see in movies with a needle marking lines on a scrolling paper is called an analog polygraph test, though it reads the measurements the same as a newer digital polygraph test the only real difference is how it prints the information. A digital polygraph is what is used today and is read by a computer for the examiner to read. The main thing to remember is that both types of tests work overall the same.
The respiratory rate is one of the things your body increases when under stress, your respiratory rate is simply put, how fast you breathe. What the polygraph test does is the examiner puts an air filled tube around your waist and abdomen and when you breathe the air that is inside of the tube gets pushed in or out and the polygraph machine measures it.
Another thing a polygraph machine measures is galvanic skin resistance. Galvanic skin resistance is called electrodermal activity and simply put is just a measure of the sweat on your fingers. The fingertips sweat more than most parts of your body so this is the area used for this test. Fingerplates are attached to the subject's fingers and what this does is measure how well the skin can conduct electricity. Since skin with more moisture conducts electricity better, when you sweat more the polygraph machine can pick it up.