What device allows current to flow in only one direction?

A rectifier is a device that allows current to flow in only one direction. This functionality is crucial in converting alternating current (AC) into direct current (DC), a process that is essential for various applications, including power supplies for electronic devices and battery charging.

Rectifiers utilize components such as diodes, which inherently allow current to pass in just one direction. This property is what enables the conversion process to take place seamlessly, ensuring that the resulting direct current is steady and consistent. This makes rectifiers an integral part of electrical engineering and electronics design, where controlling current flow is often necessary for the proper functioning of circuits.

In the context of the other choices, a transformer is primarily used to change the voltage of alternating current without rectifying it. An inverter, on the other hand, does the opposite of a rectifier by converting direct current back into alternating current. A switch is simply a mechanism to open or close a circuit but does not inherently control the direction of current flow like a rectifier does. Hence, the rectifier is the correct answer for a device that allows current to flow in only one direction.