QUESTION CORNER

Working of a thermostat

QUESTION: How does a thermostat work?

P.V.Srinivasan, Chennai

ANSWER 1: Thermostat is a device that automatically regulates the temperature of a system by maintaining it constantly or varying it over a specific range.

Most thermostats depend on the expansion of a substance corresponding to an increase in the temperature. In water type thermostat numerous flat disk shaped shells called sylphons are either filled with gas or partially filled with liquid that boils at a particular temperature. On heating the internal pressure increases, expanding the flexible sylphons and resulting in a mechanical motion that activates the control valves. Lowering the temperature causes contraction and opposite motion.

Another widely used thermostat depends on the differential elongation upon heating between two strips made of different metals and bonded together. If one end of the bimetallic strip is fixed, an increase in temperature will elangate one metal more than the other causing the strip to curve. This motion is used for switching off the current for heating.

Other devices such as thermocouples and thermistors can also be used as temperature sensing element in thermostats. In the thermistor, the resistance varies with the temperature and hence the current will also vary. This variation in the current is amplified to operate the relays to switch off the current. When the temperature decreases, the relays are back to the normal position and the heating starts.

Gopalakrishnan, New Delhi

ANSWER 2: A thermostat is commonly used in electrical appliances namely in an automatic electric iron, refrigerators to control temperature.

It mainly consists of a bimetallic strip of brass A and iron B fixed at one end and free at the other end. The free end will be in contact with a screw P in a electric circuit.

When heating is continued and the desired temperature is obtained, the metallic strip bends due to uneven expansion of the metals and loses contact with the screw. Thus the circuit is broken as and when the required temperature is obtained. When the temperature decreases, the contact is re-established. Thus the temperature is maintained constantly.

A bi-metallic strip consists of 2 similar rods of brass B and iron, I are rivetted side by side as a compound rod called bimetallic strip. When it is heated more brass rod expands more than iron, then it bends with brass on conve side. Similarly, when it is cooled below room temperature then brass contracts more than iron and bends with iron on convex side. It is mainly used in thermostat, fire alarms,'' etc.

A. Bhujanga Kumar, Visakhapatnam

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