Jeff Bonja, age 11, of Princeton, N.J., for his question:

WHAT ARE DIODES?

A British scientist named John Ambrose Fleming in 1904 built the first electronic vacuum tube that could be used commercially. It was a two electrode, or diode, tube that could direct radio signals.

The tube proved much more effective than earlier magnetic or crystal detectors and gradually replaced them almost entirely.

In time, diode tubes also became widely used to rectify alternating currents supplied by commercial power plants.

Rectification allows electric current to flow in one direction only. A rectifier performs this function. Most equipment today uses semiconductor diodes to perform rectification.