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Q & A: human resistance

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Most recent answer: 10/22/2007
Q:
Human body contains 70% of water so it is good conductor , but if we measure body resistance by electronic-multimeter then the resistance is about 1 mega-ohm. Please explain .
- Amol shivaji koc (age 23)
shahada, maharashtra, INDIA
A:
The water in the body is a pretty good conductor, but only because it contains a lot of salt. Very pure water has very few charged ions, and conducts poorly.

Anyway, so why do we conduct poorly when we are mostly salt water? Our bodies are made of cells. These are little pockets of salty water (and proteins, DNA, etc) surrounded by membranes, which are made of lipids (fats and oils) and proteins. Oils are very poor conductors. Every electrical path has to cross many such cell membranes. Although the membranes have channels in them that allow some conductance, it's still much less than you would have if there were no membranes.

If you put some vegetables in a blender, their conductivity goes way up, because you break cell walls and membranes. I haven't tried this with meat, which has only membranes and no walls, but I bet you'd find the same effect.

Once the electric current passes into the blood stream, however, it may proceed with very low resistance, because blood plasma is mostly saltwater. You may notice quite a lot of variation in the resistance you measure with your multimeter. One of the main determinants of the resistance you measure is how good the electrical contact is between the probes of your multimeter and your skin. The top layer of skin is made up of dead skin cells, oils, and sweat (more saltwater). If your skin is sweaty, it will make better contact with the probes and you will measure a lower resistance. This is one of the measurements made by polygraphs (known as "lie detectors"). Polygraphs do not detect lies, but they can tell when the fingertips of a subject become more sweaty (which presumably happens when he or she lies).

You may also notice a change in the resistance if you squeeze the probe against your skin, improving the electrical contact.

Mike W. (and Tom J.)

(published on 10/22/2007)

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