If your goal is to save on electricity, you must first measure the ac current flowing through your appliances. The formula for power is the product of the ac line voltage and the measured current. Presented here is a demonstration of how to build an ac power meter using a current transformer (see the figure).
The current transformer is similar to an ordinary voltage transformer, except that a primary wire is generally absent around the core. The primary coil is constructed by winding either one of the wires connecting an ac plug and an outlet around the core (two turns, in this case).
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