What device is designed to convert alternating current into direct current?

The device specifically designed to convert alternating current (AC) into direct current (DC) is the rectifier. This process is crucial in various applications, especially in electronics, where many devices require a steady DC source to operate effectively.

A rectifier uses components such as diodes to allow current to flow in only one direction, effectively converting the AC input, which periodically changes direction, into a unidirectional flow of current. This transformation is essential in power supplies for electronic gadgets, battery chargers, and other equipment that necessitates a stable power source.

In contrast, a transformer is used primarily to change the voltage level of AC signals but does not convert AC to DC. A generator produces AC or DC electricity but does not involve conversion between the two. An amplifier boosts the strength of an electrical signal but does not perform any transformation of AC to DC. Therefore, the rectifier is indeed the correct answer as it fulfills the specific function of converting AC to DC effectively.