On switching ON the circuit , the two electrodes of the lamp are heated up so as to emit electrons as in case of electron gun. The emitted electrons will bear in-elastic collision i.e. loss of energy will take place. These emitted electron will strike the atoms of gas present in tube. After collision , the bounded atoms will release electron and photon as they have absorbed energy from electrons. The photons will emit wavelength of ultra violet region and therefore they will not be visible. The electron emitted from gas molecule will enhance conduction and therefore conductivity increases as current increases. That’s why these lamps are called as negative differential resistance circuit. The photon produced will fall on the inside coating of lamp and transfers its energy and a visible spectrum light is emitted.
Since it is negative differential resistance circuit therefore if it will be placed across constant power supply it will be self destructive and hence a current limiter is installed. In normal household lamps this limiter is simply an inductor which has magnetic disks isolated from each other. These disks will limit the AC supply. These limiters are known as ballast. In some lamps these ballast have in build step-up transformer to provide additional energy to ballast.
As to make lamp conducting the gas inside tube must be charged. For this type of charging starters are provided in parallel with tube. These starters have gasses which expand on application of current. When switch is ON , then gas inside starter will expand and this expansion will push metallic leads to connect with lamp and choke. This causes arcing and lamp starts glowing. After certain time , the expanded gas cools down bringing the leads to their actual position and hence circuits open. This process will repeat itself until there will be sufficient amount of charged atoms. After this the conduction begins as ballast will be providing sufficient voltage.Capacitor should be used to compensate power factor due to choke coil.