In standards and metrology history, the international prototype metre bar kept at the International Bureau of Weights and Measures (BIPM), Sèvres (near Paris), was composed of a specific platinum–iridium alloy. Identify the correct composition.

Introduction / Context:
Before the modern definition of the metre based on the speed of light, international length standards relied on a physical artefact: a carefully manufactured platinum–iridium bar preserved at the International Bureau of Weights and Measures (BIPM) in Sèvres, near Paris. Understanding its alloy composition is part of the history of measurement science and metrology.

Given Data / Assumptions:

• The question asks for the alloy composition of the international prototype metre bar.
• Options list various platinum–iridium percentages.
• The historically adopted composition prioritized hardness, corrosion resistance, and dimensional stability.

Concept / Approach:

Platinum alone is noble and corrosion resistant but relatively soft. Adding iridium increases hardness and wear resistance while preserving dimensional stability. Metrologists selected a specific composition after extensive evaluation to ensure minimal creep or deformation under handling and time.

Step-by-Step Solution:

Verification / Alternative check:

The same 90–10 alloy was also used for the international prototype kilogram (IPK) for similar reasons: stability, hardness, and corrosion resistance, corroborating the choice made for primary artefact standards of that era.

Why Other Options Are Wrong:

• 50–50, 60–40, 70–30, 80–20: These mixes deviate from the historically adopted 90–10 standard and would alter mechanical and thermal properties away from the chosen optimum.

Common Pitfalls:

• Confusing the metre bar composition with generic platinum alloys used in jewellery or laboratory wares.
• Assuming higher iridium automatically means better stability without considering long-term creep and manufacturability.

Final Answer:

90% platinum and 10% iridium