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Fidget-spinners? Just a passing fad, compared with a curiosity that has captured people’s attention for nearly 400 years: Prince Rupert’s drops. These small, tadpole-shaped glass beads are capable of withstanding almost any crushing force on the head. The slightest hit to the slender tail, however, and the entire thing, including the previously indestructible head, will shatter into smithereens.

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The drops’ mystery begins with their origins. Even the name is misleading; the drops were definitely not invented by Prince Rupert, a German nephew of King Charles I of England who had a distinguished career as a solider, statesman, duke, and scientist. There is no concrete record of their first invention, but the earliest confirmed record of their existence is from Mecklenburg, Germany, home to many fine glass blowers, sometime before 1625. There is speculation that the drops are far older, as hardened glass was such a valuable material that it was frequently kept secret from rivals or competitors.

There are obscure records for the next few decades, but in the 1650s examination of the drops’ weird properties began in earnest. Evidence suggests that they were known and studied in France. Then, in 1661 the drops were presented to the august chambers of the Royal Society (Prince Rupert’s contribution was apparently to be the first to bring them to England). Society members performed experiments in order to better understand the phenomenon. Among the noted experimenters was Robert Hooke, best known for coining an obscure scientific term: “cell.”

Hooke didn’t grasp all the details, and never did understand the tail, but he figured out the gist of the drops’ strength. The drops are made by dripping molten glass into water. The outer layers, in direct contact with the water, cool rapidly, compressing into a teardrop shell that basically compresses on itself, giving it great strength. The shell is balanced against the slower cooling interior, which has high tensile strength and helps maintain the teardrop shape. To confirm this, Hooke sanded away the outside of the drops.

However, glass is brittle, and even microscopic cracks branch and spread, dispersing and causing more cracks. The thin tails of the drops are also a harder shell compressing on a tensile core, but without much core and without the force distribution provided by the heads’ spherical shape. The tail is thus easily cracked, and tail cracks rapidly propagate into the interior of the head. Essentially, the tail shatters the head from the inside out.

The phenomenon continued to interest scientists both in and out of the Royal Society for the next three and a half centuries, up into the present day. Modern analysis has modeled the behavior of atoms in the drops, and calculated the exact strength of the head, which rivals steel.


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Notes and Records of the Royal Society of London, Vol. 41, No. 1 (Oct., 1986), pp.1-26
Royal Society