Question

To keep the correct time, modern watches are fitted with a balance wheel made of
A. Steel
B. Platinum
C. Invar
D. Tungsten

Answer 1

Hint:Mechanical watches use a balance wheel in the form of a time-keeping device. Consequently, it is required to be made of a durable material that helps in keeping the correct time in all sorts of atmospheric conditions.

Complete step by step answer
A mechanical watch works with the help of a mainspring which must be wound either periodically by hand or via a self-winding mechanism on it. Its force is transmitted through a series of gears to power the balance wheel. This balance wheel is a weighted wheel which oscillates back and forth at a constant rate. It is used as a time-keeping device just like simple pendulums in a large pendulum clock.
The balance wheel returns towards its centre position by a spiral torsion spring, known as the balance spring or hairspring. It transforms the rotating motion of the watch gear train into impulses that are delivered to the balance wheel. Each swing of the wheel allows the gear train to advance a set amount, moving the hands forward.
However, this wheel is susceptible to damages made by the effect of temperature change which may adversely affect the time measurement. This is why the balance wheel is made of materials with low thermal expansion coefficient like Invar.

Hence, the correct answer is option C: Invar.

Note: The thermal expansion coefficient is a measure for the change in size in size of an object with temperature. It is inversely proportional to the temperature change, and hence a low coefficient of thermal expansion implies a large change in temperature but a comparatively smaller change in size, which is the property useful in making clocks.