<P> Heat escapes the device by black body radiation from the anode (plate) as infrared radiation, and by convection of air over the tube envelope . Convection is not possible in most tubes since the anode is surrounded by vacuum . </P> <P> Tubes which generate relatively little heat, such as the 1.4 - volt filament directly heated tubes designed for use in battery - powered equipment, often have shiny metal anodes . 1T4, 1R5 and 1A7 are examples . Gas - filled tubes such as thyratrons may also use a shiny metal anode, since the gas present inside the tube allows for heat convection from the anode to the glass enclosure . </P> <P> The anode is often treated to make its surface emit more infrared energy . High - power amplifier tubes are designed with external anodes which can be cooled by convection, forced air or circulating water . The water - cooled 80 kg, 1.25 MW 8974 is among the largest commercial tubes available today . </P> <P> In a water - cooled tube, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator to prevent high voltage leakage through the cooling water to the radiator system . Water as usually supplied has ions which conduct electricity; deionized water, a good insulator, is required . Such systems usually have a built - in water - conductance monitor which will shut down the high - tension supply if the conductance becomes too high . </P>

What kind of gas is in a tv tube