Their job is to convert high-voltage alternating current from the mains into low-voltage direct current for mobile phones, laptops, iPods and other electronic gadgets.

ECONOMIST: How a boring gadget saved $2 billion-worth of electricity

Large-scale electricity transmission usually uses alternating current, in which both current and voltage oscillate.

ECONOMIST: Superconductors are starting to become useful

High-voltage DC can carry power over long distances at lower cost than alternating current, but the transformers still cost too much to make the idea widespread.

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