There are two filaments, one at each end of the tube.
Light is emitted only when the gas inside the tube is ionized.
For that very high voltage is needed.
But once the gas is ionized, the voltage can be reduced to a great extent for the normal operation of the tube light.
The purpose of the choke is to provide a very high voltage initially
between the filaments (across the two ends of the tube light).
Again once the gas in the tube is ionized the choke provides a low voltage.
A choke is a coil of wire. When the current through it made to on and
off very frequently, every time the current is zero, a back voltage is
produced between the ends of the coil. The value of the back voltage is
high if the frequency of the on and off operation is made frequently.
This is achieved by the use of the starter. The starter switches on and
off the current through the choke frequently. A very high back voltage
is formed between the ends of the choke and hence between the two ends
of the tube.
This high voltage ionizes the gas.
The starter will continue to function till the time the gas is ionized.
Once the gas is ionized, there being an easy path for the current to
flow between the two filaments, there will be no current through the
starter. The starter stops functioning.
Since the starts stops functioning, no more high voltage is across the choke.
And the voltage across the choke (and between the filaments) is very much reduced.
This is the function of a choke.
