Physics · the experiment
Make a copper round speak
A one-ounce copper round, the study's R3, resting on its two pegs. Make it ring, measure what really vibrates, and recover the stiffness of its metal.
1 · Make the round ring
Beginner
Why is the frequency of a ring not enough, on its own, to recognise silver?
2 · Measure what vibrates
With callipers, the round measures 39.30 mm across and 3.31 mm thick. Weighed suspended in water, it displaces 3.21 cm³: its true volume.
Intermediate
What is its fill factor, true volume divided by the volume of the 39.30 × 3.31 mm cylinder?
Put the thickness that really vibrates into the formula: the calliper thickness times the fill factor.
3 · Keep the calliper reading
A larger thickness makes the computed wave speed smaller, almost in proportion.
Expert
With the calliper thickness, 3.31 mm, which metal would the computed speed have suggested?
4 · Ring some tungsten
Gold and tungsten differ by only 0.3% in density. Their plate wave speeds: 2,240 and 4,810 m/s. At equal size, a coin's frequency is proportional to that speed.
Expert
A tungsten fake with the dimensions of a gold coin would ring…
Round unmasked
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