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Angeldrawers, a long, long time ago, a radio was called a wireless. An essential working-part of the radio was called a vacuum-tube. It was a tube, with a partial vacuum in it, that contained metal filaments . When you turned the radio (wireless) on, these filaments had to warm up and actually glow a reddish colour before you could hear any sound. The tubes came in various sizes and shapes, and there was more than one in the set. If the wireless didn't work, you looked at the tubes to see if they were glowing: if one (or more) were not then you would take them to a shop and put them in a tube-tester. Usually, you would end up buying replacements.