How does the mercury flame switch operate to close contacts?

The mercury flame switch operates based on the principle of thermal expansion of mercury. When the pilot flame is present, it generates heat that warms the mercury contained within the switch. As the temperature of the mercury rises, it expands. This expansion is crucial because it leads to the movement of a float or a lever within the switch mechanism. When the mercury expands sufficiently, it reaches a level that triggers the closure of the electrical contacts, which completes the circuit.

This process is designed to provide a reliable safety feature in gas appliances. The operation is automatic and responds to the presence of the pilot flame, contributing to the overall functionality and safety of the gas system. The other choices do not accurately describe the function of the mercury flame switch in relation to how it operates to close contacts. The mechanism relies on temperature changes affecting the physical state of the mercury rather than electrical forces or manual interventions.