Home transformer cut open to reveal the bobbins inside. Museu Hidroelèctric de Capdella.
The device consists of an iron nucleus which is wrapped in coils (wires around the bobbins). The primary bobbin is the one that receives the input voltage, while the secondary one provides the outlet for the transformed voltage.
When the voltage was changed from 125 V to 220 V, many homes installed voltage tension transformers like this one. The device allowed housholders to continue using the home appliances and light bulbs they already had, as it transformed the alternating current that entered the house to the voltage which powered the existing appliances.
