Lie detector - Polygraph - How it works and how to buy it
If you want to purchase a lie detector, also known as polygraph you must notice that it is not really easy to find something cheap, but still very accurate. In this article I will point out some basic info on how polygraphs work (with pointing out what is relation between measuring a stress level and lie detection), but also recommend you what polygraph to buy and explain why.
Parameters that different lie detectors use
There are many lie detectors based on voice stress analyzation, but those detectors has very poor detection quality - I didn't find any evidence that these devices work at all - actually there are several scientific papers that says the opposite (Wikipedia has a whole article and many references about that)
Serious lie detector (that are used for example in police stations) are based on following parameters:
Skin resistance:The most important parameter - when you lie you sweat (not like while you exercise - you can not notice that just by feeling and looking your skin, it is about very very small quantity of sweat).
Think on a situation where you are telling some important lie. You feel high amount of stress while you are doing that. This stress induces the sweat. The whole phenomena is known in medicine for years and is called psychogalvanic reflex. You can read a lot on the Internet about this phenomena (best overview, as usually, you can find on Wikipedia). When you sweat your skin resistance is decreasing - recall that people always say that you have to be extra careful with electricity if your hands are wet. Salty water, like sweat, is a good electrical conductor. When your skin generates sweat it becomes better electrical conductor - its resistance is decreasing. So, measuring your skin resistance is a great measure of how much sweat did your skin generate or how stressed are you. This measurement is typically done by placing two electrodes on your fingers - a very small current (don't worry - it's not dangerous, it's really very very small current) is going from one electrode to another - change of your skin resistance is captured as change in the amount of current (Ohm's law: lover resistance implies higher current and vice versa). Signal is taken from the electrodes to an amplifier and filter that should send a nice and clean signal to the display par of the polygraph. So during the measurement you see on the screen what is happening with your skin resistance - if the signal is still, there is no stress, if the signal significantly decreases stress (sweating) it on.
Hearth rate: is the number of heart beats per unit of time. The more you are stressed, your hearth will beat faster. This is something accepted as common knowledge (see for instance this article or Wikipedia again for a nice review with many references). But, let's go into details. :) What means that we are under stress? We feel some pressure, we start to think very intensively, we prepare for/doing some action, very fast... Our body needs more oxygen to produce more energy. So our heart has to pump faster to get the oxygen around our body more quickly. Hearth rate in polygraphs is typically measured by placing a sensor on one finger. The sensor consists of infrared light emitter on one side of the finger, and infrared receiver on the other side of the finger. As heart pumps the blood (let say once per second) the quantity of blood in the finger is changing in the same rhythm. Change of quantity of blood in the finger implies change of transparency of the finger for the infrared light. So the quantity of infrared light that infrared receiver will get is going to change exactly in rhythm equal to the hearth rate. Even though this whole system may sound a little clumsy, it is actually extremely accurate and widely used. On the polygraph screen you typically have a graph trace of hearth rate - hearth beating faster is indication of stress.
Respiration rate: Similar as for the hearth rate - the fact that we need more oxygen during stress means that we have to bread faster. So this is an another useful parameter for stress detection. Even though there are several methods that are used for the respiration rate measurement, in polygraph application a sensor is typically placed around the belly - change in the radios of the belly corresponds to the respiration rate. These sensors are called piezoelectric sensor - made of piezoelectric material (PZT ceramic or some crystal materials) that has a property of generating voltage when is pressed - in this application it is pressed by the belly. Here again you typically have a graph trace of respiration rate on the polygraph screen - higher rate is indication of stress.
Beside these three, in some polygraph applications some other parameters are also used: blood pressure, brain electrical activity, ECG, body motion sensors etc. But the parameters described here are the crucial and usually enough (more parameters you have will typically allow you to detect faster if the person is telling the throuht).
How we come from stress detection to Lie detection
Very important part now. :) Logical question after reading everything above would be: OK, so we can detect stress, but what does it tell us if somebody is lying or not? Moreover, even if I am telling the truth I will still be stressed if the test is important for me - when it comes to crucial question I will definitely be stressed.
To make polygraph testing successful you must make a good concept of the experiment - this is the most important part. If the concept of the experiment is bad, no expensive equipment will help. Let say you want to find if Mr. X killed Ms. Y. You should get some prior knowledge. For example assume that you have a murder weapon - a gun. If Mr. X says that he has nothing to do with the murder and has no idea on the weapons that was used, show him several different guns repeatedly. If the gun used for the murder if familiar to him, every time you show that gun you should see the change of the three parameters form the above, clearly different from the change when you show him other guns.
Take another, more real life example :) If you simply ask "do you cheat me with him/her" you will always get stressful reaction (if you don't he/she probably doesn't like you at all, so he/she doesn't care and thus there is no reaction). You must get some prior knowledge again, for this example, you should have a suspect. Let say Marry is your suspect - you suspect that he is cheating you with Marry. Let say you have some other friends like Anne and Clara. Ask him if he cheats you with each of these ladies - if he doesn't know that you suspect on Marry and you get every time big reaction (bigger than for Anne and Clara) when you ask about Marry than you know your answer.
How to cheat on a lie detector test
This is actually very difficult and if you don't have a lie detector that you can use to practice on at home I wouldn't advise trying to do that. Usually the easiest way to find out if somebody is lying is if he tries to trick the lie detector. The tricks you can find online usually include holding your breath, squishing some of your muscles etc. - this is all clearly visible as abnormal activity on the polygraph screen, and when person that is conducting the test notice that it is more than clear that you want to trick the machine and you wouldn't do that if you were telling the through, so the conclusion is than simple.
What price should you pay for functional polygraph, where to buy a cheap one
First I don't recommend extremely simple systems (like like this) with red and green light and few electronic parts: unfortunately these devices are more like toys and they are not really useful. After a lot and a lot of "Googling" I end up with quality plug and play solution called USB Polygraph that uses the three parameters I described above, costs around 70$ (at least at time when I am writing this), connects to USB and comes with software that seams to have all that you will need. Here is the link.
At the end
I hope you found this article useful, if you are interested in socializing write me down what device did you buy or send some comment, critics, suggestions, quations... - all through the contact page.
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is the number of heart beats per unit of time. The more you are stressed, your hearth will beat faster. This is something accepted as common knowledge (see for instance 
Similar as for the hearth rate - the fact that we need more oxygen during stress means that we have to bread faster. So this is an another useful parameter for stress detection. Even though there are several methods that are used for the respiration rate measurement, in polygraph application a sensor is typically placed around the belly - change in the radios of the belly corresponds to the respiration rate. These sensors are called 
) with red and green light and few electronic parts: unfortunately these devices are more like toys and they are not really useful. After a lot and a lot of "Googling" I end up with quality plug and play solution called 
that uses the three parameters I described above, costs around 70$ (at least at time when I am writing this), connects to USB and comes with software that seams to have all that you will need. Here is the 
