the contacts in the starter is sufficient to ionise the
gas there. This allows a current to flow in the metal
strips and through the gas (and also through the
filaments of the main tube, which heats them and
facilitates the subsequent ionisation). The heat
generated by the current flow through the gas causes
the bimetallic strip to bend towards the other. When
the contacts finally touch, two things occur: firstly
the gas in the starter de-ionises, and so the bimetallic
strip begins to cool. Secondly, as the impedance of
the circuit falls, the current through the ballast inductor
and the filaments of the main tube increases.
A few tenths of a second later, the bimetallic strip
has cooled sufficiently to bend back slightly,
reopening the gap. The sudden increase in
impedance and consequent sharp reduction in
inductor current causes a large overvoltage across
the inductor. Given the correct conditions (see
section 2.2 below for a discussion of these) this
overvoltage is large enough to cause ionisation of
the gas in the main tube. At this point the impedance
of the fluorescent tube falls to a minimum, and the
voltage drop across it falls to a level below that
required to ionise the gas in the starter contact gap.
The contacts thus remain open until the lamp is next
turned on.