What does the term "transformer" refer to?

The term "transformer" specifically refers to a device designed to transfer electrical energy between two or more circuits through electromagnetic induction. This fundamental operation allows transformers to step up (increase) or step down (decrease) alternating current (AC) voltage levels.

In practical applications, transformers are essential in power distribution systems. For instance, when electricity is generated in power plants, it's initially at a lower voltage. To transmit this electricity over long distances efficiently, the voltage needs to be stepped up to a higher level. Once it reaches the destination, the voltage is then stepped down to usable levels for homes and businesses. This ability to adjust voltage levels without changing the frequency of the current is what makes transformers vital components in the electrical grid.

The other options describe devices with different functionalities that do not align with the specific role of a transformer. For example, devices that store electrical energy are capacitors or batteries, while rectifiers perform the conversion from AC to DC. Measuring electrical current typically involves the use of ammeters rather than transformers. Therefore, the function of stepping up or stepping down AC voltage accurately defines what a transformer is and does.