Application: The Watt-Hour Meter A useful device based on the interaction of induced eddy currents in a conductor is
the common watt-hour meter found in many homes. The meter is built of an aluminum disk on a spindle. The disk is placed
between the poles of a magnetic yoke, as shown in Figure 10.26. Three coils are wound on the yoke. The upper, center coil is
called a voltage coil since the current in the coil and, therefore, the field it generates depends on the line voltage. The two
lower coils are connected in series with the load. These coils generate a field that is proportional to the current in the load.
Both the current and voltage coils generate eddy currents in the conducting disk. However, either one, by itself, produces no
torque in the disk. When both are present, the interaction of the current and voltage fields produces a torque, proportional to
the product of current and voltage. The speed of the disk is therefore proportional to power. In addition, the meter employs
permanent magnets as retarders or braking devices whose braking force is proportional to the speed of the disk. This is
required to avoid acceleration of the disk and, therefore, inaccurate measurements. The shaft of the disk is geared and turns
indicator dials that register the energy consumption. Although there are more modern instruments that use direct measurement
of current and voltage (as well as phase angle) and display power consumption digitally, the common watt-hour meter
is still used extensively because it is an accurate, reliable, and inexpensive instrument. The rotating disk in watt-hour meters
is normally set such that it can be seen through a window in front of the instrument, giving a simple indication of its
operation.