Seems like an electric version of a 'thermocouple' - a heat/expanding aire pressure sensor to guarantee flame presence for a pilot fired gas burner. My dad changed over from oil fired to gas when we moved into our old house, and I remember having to swap them out every few years.
Nowdays, many control modules (Pilot? Why would I need to fly my boiler?) incorporate a "flame proof" (sensor), to ensure that there is, in fact, a burning flame, and not just a big cloud of gas vapors in your furnace/boiler. If the flame proof fails, it d/c's the power to the gas valve (solenoid), and shuts down the flow of gas. Usually, the chain of activity follows like this:
Thermostat calls for heat.
Pre-light gas flow (25% of "regular" flow for 5-10 secs)
Ignition fire (piezo for 5-10 secs)
Flame proof test-
Flame yes? then open valve rest of way.
Flame no? shut valve. Retry after 60-90 secs.
If burning, continue until thermostat is happy.
I assume that your Firematic operates in the same fashion.
I got to know the control module intimately due to a failure of mine one cold winter a few years back (!*#%^$( solid state (*&^#&!!!).
