In which of the following device is the expansion of solid not required? I. Simple pendulum II. Fire alarms III. Thermostat IV. Bimetallic thermometer

Step 1: Analyze the question We need to find the device(s) that do NOT rely on the expansion of a solid to work.

Step 2: Check each device

• I. Simple pendulum: It is a bob on a string swinging under gravity. Its period depends on length and gravity (T = 2π√(l/g)). It does not need solid expansion to operate. ✔ (expansion not required)
• II. Fire alarm: Many fire alarms use a bimetallic strip. Heat makes it bend and close a circuit, so solid expansion is used. ✘
• III. Thermostat: It uses a bimetallic strip that bends when heated and breaks or makes the circuit to control temperature, so solid expansion is used. ✘
• IV. Bimetallic thermometer: It works on the differential expansion of two different metals. ✘

Step 3: Conclusion Only the simple pendulum works without needing the expansion of a solid.

Answer: C (I only)

Tip: Whenever you see "bimetallic strip", think "differential expansion of solids". Fire alarms, thermostats, and bimetallic thermometers, as well as flashers and some circuit breakers, all use it. The pendulum is about gravity and length, so it is the odd one out.