Thermostatic room temperature controller is mounted on radiators,which is combine with valve and thermostatic head.
valve opens automatic or becomes large gradually when the room temperature is lower than setting value, the radiator will heat up till the room temperature up to setting value. The liquid-filled thermal bulb will expand that leads to valve turn down or turn off, then the radiator decrease heat speed or stop heating. So that the room temperature at constant temperature purpose.

Forging brass, nickel plated
Liquid-filled thermostat
Marking “*”7°C for frost protection setting
Marking”0” for valve closed
Marking”1...5” for 7…28°C
The arrow shows in valve body for flow direction