In the triode, electrons are released into the tube from the metal cathode by heating it, a process called thermionic emission. The cathode is heated red hot by a separate current flowing through a thin metal filament. In high power triodes, the filament itself is the cathode, while in most the filament heats a separate cathode electrode. Virtually all the air is removed from the tube, so the electrons can move freely. A positive DC voltage, from 20V to thousands of volts in power tubes, is applied to the anode. The negative electrons are attracted to the positively charged anode, and flow through the spaces between the grid wires to it, creating a flow of electrons through the tube from cathode to anode.