Editing Triode (section)

=== Low power triodes ===
Low power triodes have a concentric construction ''(see drawing right)'', with the grid and anode as circular or oval cylinders surrounding the cathode. The [[cathode]] is a narrow metal tube down the center.  Inside the cathode is a [[electrical filament|filament]] called the "heater" consisting of a narrow strip of high resistance [[tungsten]] wire, which heats the cathode red-hot (800 - 1000&nbsp;°C).  This type is called an "[[Hot cathode|indirectly heated cathode]]".  The cathode is coated with a mixture of [[alkaline earth]] oxides such as calcium and [[thorium oxide]] which reduces its [[work function]] so it produces more electrons.  The grid is constructed of a helix or screen of thin wires surrounding the cathode.  The anode is a cylinder or rectangular box of sheet metal surrounding the grid.  It is blackened to radiate heat and is often equipped with heat-radiating fins. The electrons travel in a radial direction, from cathode through the grid to the anode. The elements are held in position by [[mica]] or ceramic [[Insulator (electricity)|insulators]] and are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins. A "[[getter]]", a small amount of shiny [[barium]] metal evaporated onto the inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time.