The device for detecting a lie is one of the tools in a criminal investigation and is used by the police in many countries.
However, the data obtained using this device is not accepted everywhere as direct evidence in the courts, because this method of discovery of “truth” the science has not yet accepted.
Testing by means of a lie detection is based on the fact that on the body of the person lying, certain reactions occur:  facial flushing, increased heart rate (pulse), swallowing saliva and other manifestations of excitement.
The lie detector device records such changes in the body. The success of these devices, depend on the sensitivity of its components, and also from the expert who is performing the test.
In order to have most relevant data, during the tests a preference is given to those reactions that the person being tested is unable to control, and even is not aware of, such as blood pressure, pulse and respiration.
Neither of these reactions is not in itself, sufficient proof that the test person is lying, but taken together, they are presumably still fairly reliable sign.
On the body of the person being tested are attached various parts of the device, in order to obtain a record (paper or digital) of his reactions while he is answering the questions.

The examiner must first be thoroughly familiar with the case. Only in this way he will be able to ask specific questions. The examiner then asks a lot of questions that have nothing to do with the case, in order to discover the “normal” reaction of the test person.
Thereafter, the examiner asks questions of both types, like those that have no connection with the case, and those that have. In doing so, three or more “reactions” of the test person are recorded, and later during the analysis of these records, unusual emotional activities can be detected.
To avoid the possibility of accidental response, the response should occur at least at two different records on the same reaction.