How do bathroom scales measure body fat percent? My bathroom scale has 2 metal strips each for a foot and the producer claims that it can measure body fat percent. There is a warning in the manual, that if I have heart problems (I guess pacemaker), then I must not use the scale. I tested it, and the results appeared to be correct. I was wondering how is it possible to measure body fat content with I guess conductivity?
 A: You will get a pretty good answer by reading this paper. In short: fat, water, and muscle each have different electrical properties as a function of frequency. By measuring the (very small, on the order of micro amps) current that flows between your legs when frequency of the driving voltage is changed, you can create a model of the body as parallel and series resistors and capacitors (looking at both the phase shift between voltage and current, and the magnitude of the signal). It is then possible to solve for the amount of water, fat, and muscle.
As an example (Figure 7 from the above paper) here is the relationship between real and imaginary impedance of a body as a function of frequency:

The circular shape of the curve connecting the points corresponding to different frequencies is characteristic for a mixture of fat and muscle. The radius of the circle and the location of its center can both be used to help estimate the percentage of body fat. 
Typically when these devices are built into a bathroom scale they have at least four electrodes - two on each foot. This allows for something called a "Kelvin" connection - you apply the voltage (driving current) on one set of electrodes and measure the voltage with the other one. By measuring the voltage with a high impedance electrode (no current flowing at all) this allows you to eliminate the effect (variability) of the skin resistance - which is a strong function of how sweaty your foot is, for example.
"Better" instruments have multiple electrodes - you might hold electrodes in both arms as well as standing on electrodes with your feet. Such configurations allow current to flow across the torso as well as up and down the body and give a more detailed picture of the distribution of fat and muscle (down to giving you the muscle mass in each arm, for example).
Google "Bioelectrical Impedance Analysis" for much more information.
