Triode were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament and a plate (anode). Triodes came about in 1906 when American engineer Lee De Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a grid, between the filament and plate to control current. Von Lieben's partially-evacuated three-element tube, patented
Electrical ballast Early tube-based color TV sets used a ballast triode, such as the PD500, as a parallel shunt stabilizer for the CRT acceleration voltage, to keep the CRT's deflection factor constant. Electrical ballast An electrical ballast is a device placed in line with the load to limit the amount of current in an electrical circuit. It may be a fixed or variable resistor. A familiar and widely used example is the inductive ballast used in fluorescent lamps to limit the current through the tube, which would otherwise rise to a destructive level due to the negative differential resistance of the tube's