Generally when you lie, your system will chance, you're breathing or heart rate may increase and you may start sweating etc. It basically just tests for an strange activity in your body.
However, some experts believe that it's not 100% proof that someone is lying.
A polygraph is used to help detect if someone is lying.
Polygraph operator.
polygraph
Yes. Polygraphs are designed to detect heightened levels of anxiety, they cannot detect whether someone is telling the truth or lying. Most people when they lie become anxious/nervous, however if someone suffers from anxiety or is particularly nervous the polygraph will also detect this. If the examiner refers to the polygraph as a "lie detector" then the chances are they have little understanding as to the function of the machine and would likely mistake someone with anxiety as a chronic liar.
Because polygraphs measure stress. The presumption is that if you are stressed, you are lying. In reality, there are many causes of stress and many reasons other than lying that one can fail a polygraph. The polygraph has been known for many years to be junk science, and the fact that it is still legal to use it is one of the shameful aspects of law enforcement and security. BTW: the polygraph does not say you are lying. The "technician" who interprets it is the one who decides that, and he or she is not even a trained psychologist.
A lie detector picks up changes in your pulse and heart rate. Generally when you tell a lie, your pulse will increase because you don't want to tell the person what you are hiding and consequently become nervous. It has been confirmed that a lie detector can not be trusted fully to determine whether you are lying or not.
advantage: you come to know who is lying disadvantage: the principle is based on the human skin resistance, is the person who is subjected to the test is lying confidently then it will be difficult for the lie detector circuit to catch a lie because in this case you wont be sweating or feeling nervous
There isn't a defiant way, but polygraph tests come close to detecting lies because they measure the bodies physiologic reactions to questions. Some people also indicate certain facial expressions, body movements, or start to release hormones in the adrenal glands, but it isn't a for sure way to tell if someone is lying.
It is possible that people may be aware when someone is lying, as there are often signs or cues that can indicate deception.
There are several signs that can indicate if someone is lying, such as avoiding eye contact, fidgeting, inconsistencies in their story, and changes in their tone of voice. Additionally, physiological responses like sweating or increased heart rate can also be indicators of deception. It is important to consider these cues in context and not rely on any single indicator to determine if someone is lying.
A polygraph (often called a lie detector) is actually a biomedical instrument. This device looks at and charts or logs a number of physiological characteristics (blood pressure, pulse, respiration, breathing rhythm, body temperature and the conductivity of skin) and the changes in these features as questions are asked of the subject and he answers.The idea is to ask questions that establish a "baseline" physiology. If and when the subject provides false answers, changes in some "baseline" physiological factor will appear. These can provide some distinctive contrasts that could indicate that the individual is lying. The physiological changes monitored are controlled by the sympathetic nervous system, and as the questions are asked and answered, physiological responses are tracked. A link can be found below to check facts and learn more.PolygraphA polygraph is a machine in which the multiple ("poly") signals from the sensors are recorded on a single strip of moving paper ("graph").How does it work?About 4 to 6 sensors are attached to body parts of a person during polygraph test and the sensors record data from these locations.(see the link below)
A polygraph instrument measures three basic bodily functions but may also measure additional (optional) features. At a minimum, the polygraph must measure (1) relative blood pressure by placing a blood pressure cuff on the upper arm, (2) electrodermal response by placing metal plates on the fingers or self-stick pads on the palm of the hands, and (3) breathing patterns by placing two soft rubber tubes across the upper and lower abdomen. Additional features may also be measured, but these are the basics. - Michael Martin (President, Global Polygraph Network)