It is believed that a mint worker named Samuel Brown made the coin, managing to change the die in order to create a coin bearing a bogus date.

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After each call, Gates is shown checking the coin slot for loose change.

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Controlling the rate at which coins were created, he said, meant there would never be a surge or shortfall in the number of bitcoins in circulation, either one of which could rapidly change the value of each coin.

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But Prof Chrystal says that in general the smallest denomination coin is the most likely to disappear as habits change, but not for a while yet.

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All agreed that health and climate change could be two sides of the same coin.

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