Filtered 15 V de is applied to a series circuit consisting of thermistor R2 and parallel combination of resistors Rl and R3. Transistor Ql acts as a switch whose state is determined by the setting of potentiometer R3, which is first set so just enough current flows into the base to switch on when the thermistor is in contact with air. When the resistance of the thermistor decreases, the voltage at the base of Ql rises. When the base current reaches the preset level, the transistor conducts and passes current through the reed relay coil, closing the reed relay contacts.
Current at the base of transistor Ql is determined by the environment into which the termistor is inserted.