On the other side of the transformer you find two diodes wrapped in rubber insulation.

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Turning the AC current into DC current

On the other side of the transformer you find two diodes wrapped in rubber insulation. The diodes act as a rectifier, turning the AC current into DC current.

Most transformer cubes that you find around the house produce a low-voltage DC current (3 to 12 volts, and less than an amp of current). DC current is necessary because rechargeable batteries store DC current, because most electronics require low-voltage DC current and because small DC motors run directly from batteries and are the least expensive motors available.

The big question remains, however - why can't somebody standardize all of these little cubes?!