SPST switch can be knife switch. The line above switch is string or thread that a flame could burn apart. This string is threaded through eye screws, staples, or other guides to hold string close to areas where you want flame detection. Inductors at far left are mechanical spring holding string attached to blade of knife switch holding switch open. Inductor below switch is weaker mechanical spring to pull knife switch closed when upper string burns apart.
Alarm circuit could be simple doorbell or extinguisher system circuit.
The string detector works to protect large areas or threaded in very small spaces in equipment where other detectors would be too large.
