Transformers increase or decrease the voltage of electricity, making it usable in our homes, businesses and even individual appliances.
Regardless of shape and size, all transformers have two sides, a high-voltage side and a low-voltage side. In normal operation, electricity flows into the transformer on the high-voltage side, where it goes into a coil of wire wound around an iron core. As the electricity flows through this coil, it creates a magnetic field that induces a voltage in the opposite coil.
Each coil has a different number of turns that regulate how the voltage is changed. The coil on the high side will have more turns than the coil on the low side. As a result, the voltage induced on the low side is smaller. The same principle is at work in every type of transformer, from the massive units inside utility substations, to the gray cylinders hanging on utility poles, to the charger for your cell phone. At each step, the voltage is reduced to make it suitable to use.
It is important to remember that transformers work in both directions. Electricity flowing in on the low side will be stepped up to the voltage of the high side. This is why home generators must be installed properly to avoid feeding electricity back to the power grid during outages. A generator feeding 220 volts into a residential transformer will produce whatever voltage the transformer is rated for on the other side, creating a deadly risk for line crews working to restore power.