Clock pendulum materials — select the alloy used for thermal length stability Which material is widely used for making the pendulums of precision clocks due to its extremely low thermal expansion?

Introduction / Context:
Pendulum length determines timekeeping accuracy in mechanical clocks. Temperature-induced length changes alter the period, causing time drift. Thus, materials with minimal thermal expansion are preferred for pendulum rods or compensating devices.

Given Data / Assumptions:

• Room-temperature operation with moderate daily temperature variation.
• Availability of specialty low-expansion Fe-Ni alloys.

Concept / Approach:
Invar, a nickel-steel alloy (approximately 36% Ni), exhibits a near-zero coefficient of linear expansion near ambient temperature, making it ideal for precision pendulums, measuring tapes, and instrument frames. The umbrella term nickel steel captures this class of alloys in many exam syllabi. Ordinary stainless, high-speed, or heat-resisting steels expand significantly more with temperature, degrading timekeeping unless mechanical compensation (gridirons or bimetallic devices) is used.

Step-by-Step Solution:

Verification / Alternative check:
Historical chronometers and precision wall clocks employed Invar pendulums; metrology standards likewise adopted nickel-steel alloys for dimensionally stable components.

Why Other Options Are Wrong:

• Stainless/heat-resisting/tool steels: higher coefficients of expansion; not ideal for uncompensated pendulums.
• High-speed steel: selected for hot hardness in cutting tools, not thermal stability of length.

Common Pitfalls:
Equating corrosion resistance or high-temperature strength with low expansion; confusing Invar with Kovar (a controlled expansion alloy tailored to glass sealing).

Final Answer: