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The balance wheel of a watch is made of:
(A) Brass
(B) Invar
(C) Steel
(D) Platinum

Last updated date: 14th Jun 2024
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Hint: Watch contains a balance wheel. The material is supposed to be mostly unaffected by external conditions. Esp. the elasticity should remain the same so that it maintains the uniformity required to measure time. We know that all pure metals are significantly affected by temperature.

Complete step by step solution:
It is an alloy made from copper and zinc. More content of zinc in brass makes it in such a state which is not in working conditions.
Invar is an alloy made using nickel and iron. It’s coefficient of linear expansion is lower. So, a little amount of change in length as the temperature is changed. Therefore, time taken by the watch will be more accurate and that too for a wide range of temperatures.
It is an alloy which is made using iron and carbon. Other types of material can also be used in it. It is low in cost and has tensile strength. It is used to make ships, infrastructure, machines, electrical appliances etc.
It has physical properties like malleability, ductility, dense, does not react, transition metal. It is the rarest element. It is expensive but very precious metal; it is least reactive. It is used in electrical contacts, electrodes, thermometers etc.

Therefore, the correct solution is B that is Invar.

Although brass is a good choice, it is difficult to manufacture a brass balance wheel, because it is not in a working mode. Rest all options are bad because they don’t have such a low coefficient of linear expansion.
If we use platinum then it would be very expensive and least available. Therefore, we are left with only one correct option and that is invar.