If you have ever looked up at power lines, you’ve surely seen these barrel-shaped devices attached to utility poles. The devices are called transformers. To understand their purpose, you should know that all metal transmission wires have some amount of resistance to the passage of electric current, which means that a fraction of electric power is converted to heat. The amount of energy lost to the surroundings is proportional to the square of the electric current. The electric power transmitted over these lines can be expressed in watts and is equal to the voltage times the amperage of the current. A current of 100 amps and a voltage of 50,000 volts transmits the same amount of power as 5 amps and 1,000,000 volts. Since the energy loss is related to the amperage squared, using the 5-amp current instead of the 100-amp current would make a tremendous difference in terms of efficiency. It is clearly wise to increase a current’s voltage and reduce its strength before sending it over long transmission lines. When the current reaches houses and other buildings, however, the appliances at those destinations cannot use a current of 5 amps with a voltage of 1,000,000 volts. Therefore, the most efficient system for the transmission of electric power is to “step-up” transform the current to a higher voltage before long-distance transmission and then “step-down” transform the current to a lower voltage to ready it for use. These transformations are carried out by the transformers on the power lines. In this chapter, you will learn more about transformers and other applications of electromagnetic induction.