You remember calculus, right? In a time before mechanized computing was performed by computers, complex (or sometimes just clever) machines were used to automate calculations. One example that has always impressed and fascinated me is the wheel-and-disk integrator, a simple machine capable of solving the calculus equations you labored over in high school without breaking a sweat. While this concept was used most impressively in Vannevar Bush's differential analyzer, an analog computer built in 1931, the chances are good that you've seen one in a more mundane application around your house: the power meter. Click on the photo gallery to see inside one and how it works, and follow the jump for more in-depth electro-geekery.
The mechanical style of electric meter makes use of the wheel-and-disk integrator  concept to calculate the amount of energy used by a home or business. Recall from your basic electricity concepts Ohm's Law, which tells us that P = VI; that is, the instantaneous Power (P) passing through a circuit element (such as the entire electric load of a house) at any given moment is equal to the Voltage (V) potential across that element multiplied by the Current (I) passing through that element . Now, were we to represent that quantity of Power in some way and then to take the sum of each instantaneous power value over a period of time, we would have the amount of power used during that period of time. This is known as Energy, and energy consumption is what is measured by a power meter.
Here is how the power meter does it: