Electric Companies use meters calibrated to measure 1 kilo-watt-hour and also measure maximum demand, which is the peak kilo-watt drawn during the billing period. Using the collective information from total usage and maximum demand, a bill is generated. The most common electromechanical induction meters operate through electromagnetic induction by counting the revolutions of a disc which is made to rotate at a speed proportional to the power passing through the meter.
With the advent of digital electronics and the Internet, electricity companies generate readings automatically and post bills on their website, where customers login and settle their dues. In such situations, where visits from the company are rare, unscrupulous customers sometimes use methods of tampering such as attaching magnets to the outside of the meter to saturate the magnetic fields and stop the disc from spinning. To tamper proof these meters, Reed Sensors are placed near the front cover of the meter, in the vicinity of the rotating disc. These sensors are specially selected, having a low AT band so that it detects the weakest of external magnetic fields. This ensures that the tampering is detected well before the external magnet has any effect on the rotating disc, and triggering an alarm to the electricity company.