a vacuum tube, also called a valve in british english, is an electronic device used in many older model radios, television sets, and amplifiers to control electric current flow. the cathode is heated, as in a light bulb, so it will emit electrons. this is called thermionic emission. the anode is the part that accepts the emitted electrons. the device may have other parts. vacuum tubes must be hot to work. most are made of glass, thus are fragile and can break. vacuum tubes were used in the first computers like the eniac, which were large and need much work to continue operating.