Every household uses electricity meters, so electricity meters are one of the most popular items, and the kilo watt-hour meter is the most popular item.

Also, the voltage coil of the watt-hour meter is small, and AC voltage is applied. Since the current is slightly behind the voltage that generates it, it starts to flow when the voltage tends to the highest value. When the voltage drop approaches zero, since the current still lags, it gradually tends to the maximum. The lines of magnetic force are also generated as the current changes.

The "current coil" has a thicker wire and fewer turns. Because it is connected in series with the load (that is, the appliance that uses electricity), the load current flowing in it is also called the use current. When the load is electric heaters and incandescent lamps and other electrical appliances that use the thermal effect of current to work, the load current does not lag behind the power supply voltage. Moreover, the lines of magnetic force at this time are also generated simultaneously with the change of the load current.

The voltage coil and current coil of the three phase electricity meter are also the same.