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When I attach a Digital Multimeter (DMM) to read the resistance between my right and left hands, the resistance starts off high and reduces over time. It appears almost like "RC-type" behavior. Can someone explain to me what it is that I am seeing on the DMM in some detail?

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It has nothing to do with any capacitance. It's all about skin resistance. The resistivity inside your body is much lower than that of the skin. As a result, your measurement is really showing you the sum of two resistances thru the skin.

The main reason skin has higher resistance than the body internally is because the skin is dry. However, the skin can get moister by sweating and external blocking of evaporation. When you grip the probes, evaporation is blocked in the immediate vicinity, so moisture builds up. Your body may also produce a bit more sweat when you grip hard.

Try licking your fingers before touching the probes and you will see the resistance reduced substantially. You can also reduce the resistance by gripping harder. That increases the contact area and also makes better contact.

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  • $\begingroup$ great answer! How would one go about measuring human resistance accurately then? $\endgroup$ – Jesus Nov 21 '13 at 18:49
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    $\begingroup$ @Jesus - the problem is that the human body is grossly nonuniform, so the term "human resistance" is not defined. There's the infamous case of "body mass index" meters which claim to measure fat content via resistance but whose output varies significantly depending on stomach and bladder contents. $\endgroup$ – Carl Witthoft Nov 21 '13 at 19:16
  • $\begingroup$ @Jesus, As Carl said, "human resistance" will vary widely over a number of factors. Variation in skin resistance is actually one of the parameters measured by a lie detector (polygraph). $\endgroup$ – Olin Lathrop Nov 21 '13 at 20:35
  • $\begingroup$ @Jesus The slow response you see could be related to the skin conductance level (SCL). SCL is the slow varying skin conductance value, changing over the course of minutes. See my answer. However your probes are too far apart for consistent skin readings and you could be just seeing some residual error voltages in the meter. $\endgroup$ – user6972 Nov 21 '13 at 23:36
  • $\begingroup$ Whoever downvoted this, it would be useful to know what exactly you think is incorrect. I have re-read what I said and still think it's right. $\endgroup$ – Olin Lathrop Nov 22 '13 at 14:34
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Just to add to the answer a DMM is not considered a good way to measure such a high impedance reading like skin conductivity. A typical Galvanic Response Sensor (GSR) has the ability to adjust filters/rates and is optimized for high impedance measurements to reduce the signal to noise ratio. DMM's will often pick up environmental noise on very high impedance circuits (including probe motion across changing surface contact impedance).

Also the DMM probe shape is not ideal for a good skin contact which makes reading fluctuate wildly and is not repeatable. GSR probes usually consist of two bands or large pads for good contact area and are applied to fingers, palms or soles of feet. These areas are considered to be the most reliable for measurement.

The combined changes between galvanic skin resistance and galvanic skin potential make up the galvanic skin response. It is not simply just "skin resistance" but a mix of resistance and galvanic reactions. A typical GSR will measure both the skin conductance level (SCL) and skin conductance response (SCR) as they vary with sweat gland (eccrine) activity due to stress, arousal or emotional excitement.

There is a lot of quackery too. The background on E-Meter used in Scientology makes for fun reading. The Mark VII Super Quantum E-meter will run you about $5000. (Yes, Super Quantum!)

But there has been clinical trials using a standard GSR. This type of study is termed Electrodermal System Response (ESR) or EDA (activity). They have been used as a tool in lie detectors and in diagnosis of abnormal psychotic patients (C17 is normal and P5 has a psychotic disorder).

compare

Experiments continue with trying to measure skin responses. This project discusses how they tried to separate and quantify these biofeedback responses.

When measuring skin conductance, there is the skin conductance level (SCL) and skin conductance response (SCR). SCL is the slow varying skin conductance value, changing over the course of minutes. SCR is the fast varying skin conductance value, changing over the course of seconds. The SCR that rides on top of the SCL reflects a person's mental response to various stimulus. SCR has several statistics that are useful for psychologists. First is latency, which is the amount of time between the stimulus and the rise of the wave. Next is the rise time, how long it takes for the skin conductance to shoot up to it's peak. Amplitude is the height of the SCR. Half recovery time is the amount of time it takes for the wave to fall back to half it's amplitude.

To see how your DMM compares to a EDA meter try this device.

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