Editing Vacuum tube (section)

===Space charge===
When a cathode is heated and reaches an operating temperature around {{convert|1050|K}}, free electrons are driven from its surface. These free electrons form a cloud in the empty space between the cathode and the anode, known as the [[space charge]]. This space charge cloud supplies the electrons that create the current flow from the cathode to the anode. As electrons are drawn to the anode during the operation of the circuit, new electrons will boil off the cathode to replenish the space charge.<ref>''Designing Tube Preamps for Guitar and Bass, 2nd ed.'', Merlin Blencowe, Wem Publishing (2012), {{ISBN|978-0-9561545-2-1}}</ref> The space charge is an example of an [[electric field]].