From a technical point of view, measuring the resistance of something is very simple. You put a voltage over the subject you want to measure the resistance of, measure the current flowing through it and apply Ohm’s law (R = U / I - resistance is voltage divided by current) to get the resistance. For our purposes it’s even easier: we don’t actually care about the real resistance values, just the changes to it over time. Since the voltage over a certain subject is dependant on its resistance (with an input voltage that remains the same), all we need to measure is voltage: when the resistance of our subject (the hand palm) goes down (sweaty palm), the voltage over it will go down too. So all we’d have to do is use a voltage meter and record the measurements over the time spanning the interrogation of our ’suspect’.
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This is the first in a series of posts in which I’ll document my quest to find an answer to the question that has for centuries been keeping millions of people awake at night (or maybe not quite): is it possible to build a lie detector on a budget and make it work to some degree of reliability?

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