A vacuum tube consists of electrodes in a vacuum in a (usually tubular) insulating heat-resistant envelope. Many tubes have glass envelopes, though some types such as power tubes may have ceramic or metal envelopes. The electrodes are attached to leads which pass through the envelope via an airtight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement. The simplest vacuum tubes resemble Incandescence|incandescent Incandescent light bulb|light bulbs in that they have a electrical filament|filament glass-to-metal seal|sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal plate inside the envelope, if the plate (also called the anode) is positively charged relative to the filament (or hot cathode|cathode). The result is a flow of electrons from filament to plate. This cannot work in the reverse direction because the plate is not heated and does not emit electrons. This very simple example described can thus be seen to operate as a diode: a device that conducts current only in one direction. The vacuum tube diode conducts conventional current from plate (anode) to the filament (cathode); this is the opposite direction to the flow of electrons (called electron current). Vacuum tubes operate primarily on the function of the heat gradient difference between the hot cathode and the cold anode. Because of this operating requirement, vacuum tubes are inherently power-inefficient; enclosing the tube within a heat-retaining envelope of insulation would allow the entire tube to reach the same temperature, resulting in electron emission from the anode that would counter the normal one-way current flow. Because the tube requires a vacuum to operate, convection cooling of the anode is typically not possible. Instead anode cooling occurs primarily through black body|black-body radiation and conduction of heat to the outer glass envelope via the anode mounting frame. Cold cathode tubes do exist but are used primarily in lighting systems, where unidirectional power regulation is not the functional purpose of the tube. The vacuum tube is a ''voltage-controlled device'', with the relationship between the input and output circuits determined by a transconductance function. The solid-state device most closely analogous to the vacuum tube is the junction gate field-effect transistor|JFET, although the vacuum tube typically operates at far higher voltage (and power) levels than the JFET.