crystal set

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An early radio receiver using a crystal detector.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An early radio receiver, having no electrical power supply except the radio waves themselves; it used a crystal of lead or cadmium sulphide and a wire (the cat's whisker) touching the crystal's surface.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an early radio receiver using a crystal detector