A fluorescent lamp is a glass tube containing some mercury vapour, with a phosphorescent coating on the interior surface of the tube. The mercury vapour is made to glow by using a high voltage across its electrodes that sets off an electric arc discharge in the tube. The discharge can also be described as conducting plasma
The resulting flow of current through the mercury vapour is stable and well-controlled by external ballast, or loading device, which consists of a high-inductance choke-coil that is connected in series with the tube. The continuous discharge glow is invisible to the human eye but it causes the phosphorescent coating on the interior surface of the tube to emit visible light