Compound interest turned Franklin's tiny bequest into millions over 200 years

When Benjamin Franklin died in 1790, his will hid a 200-year time bomb of arithmetic. In a codicil he left 1,000 pounds sterling each to the cities of Boston and Philadelphia - roughly $4,000 apiece - with peculiar strings attached. The money could not simply be spent. It had to be loaned out at interest to young married tradesmen starting their careers, the repayments feeding the fund and the fund feeding new loans.

Franklin, ever the systems thinker, had run the numbers. “If this plan is executed, and succeeds as projected without interruption for one hundred years,” he wrote, “the sum will then be one hundred and thirty-one thousand pounds.” At the first century mark the cities could draw down most of the pile for public works; the remainder would compound for a second hundred years before final distribution.

A thousand pounds, left alone, becomes a fortune if you simply refuse to touch it.

The experiment was messy - loans defaulted, managers bickered, the funds wobbled - yet the core bet held. By the time Franklin’s two-century clock ran out in 1990, Boston’s fund was worth about $4.5 million and Philadelphia’s roughly $2 million.

The lesson Franklin engineered into law was the one he preached in life: money makes money, and the money that money makes makes money. Given enough patience, even a modest sum becomes a live demonstration of compounding.