How It Works: the Body-Fat Scale

You've used them before: body-fat scales. But do you know how they work?

Step 1: When you step on the scale's electrodes, a 500 microampere current shoots up your leg. Human skin can sense currents only stronger than 1 milliamp, which is twice as powerful as this charge.
Step 2: As the current moves throughout your lower body, it encounters resistance in tissue. Water is a much better conductor than fat, so muscle--which is about 73 percent water--provides easier passage.
Step 3: The scale detects the total time it takes for the current to compete its path. The faster it travels, the more muscle and less fat you have. With this data, it computes your body-fat percentage.

Personal experience leads me to advise that you empty your bladder before using the scale to estimate body fat percentage.