How exactly do those fancy electronic scales measure body fat?

I have an electronic scale that measures body fat. How does it do this, and how accurate is it?

Public Comments

1. usually they're within 3 percent either way or your correct body fat, they send a current through your body...i'm not sure exactly what it does though.
2. They use bioelectrical impedance. They measure how well your body conducts electricity and assume that higher conductivity means lower body fat percentage. However, they do not account for hydration state or your electrolyte levels. As a result, they are not very accurate. Here is an article about the different ways to measure body composition: http://www.spartafit.com/articles/measuring%20body%20composition.php
3. They send a low powered charge through your body and measure the impedance. This impedance has a definite siganture when passing through the parts that make up the body i.e water, bone, fat, muscle etc...The impedance is converted into a bodyfat percentage based on a known constant for each item making up the body.