The Chubb Detector lock: security with a memory
In the summer of 1817, someone walked into Portsmouth Dockyard with a false key and walked out with whatever they pleased. The locks protecting one of Britain’s most important naval installations had not been picked so much as politely circumvented. The British government, stung, announced a competition: one hundred guineas to any inventor who could produce a lock that no false key could open.
Jeremiah Chubb was twenty-four years old and running a ships’ ironmonger’s business with his brother Charles on Daniel Street in Portsea, a few streets from the dockyard. He had trained as a blacksmith, which gave him an instinct for tolerances. On February 3, 1818, he filed his patent and claimed the prize.
The mechanism built on the four-lever tumbler design — the double-acting principle Robert Barron had patented in 1778 — but added something no previous design included. Inside the lock body, Chubb fitted a sensitive detector spring attached to its own small lever. Each of the four main levers had to be lifted to a precise height for the bolt to slide free. If a picker overlifted any lever — pushed it past the correct position, as picking instruments invariably do — the spring tripped and drove the detector lever into a locking slot, seizing the mechanism entirely. The correct key, turned in reverse, would reset it; anything else would not. The lock could not be defeated quietly.
He named it the Detector. It was a precise choice. The lock did not merely resist attack; it registered it. A frozen lock told the returning owner, without ambiguity, that someone had tried.
The government remained skeptical, so a test was arranged. A convict aboard one of the prison hulks moored in Portsmouth Harbour — the decommissioned warships the authorities were still using as overflow jails in the years after Waterloo — was offered a deal: pick the Chubb Detector, receive a full pardon and the hundred-guinea reward. He was reportedly capable of opening any lock set before him. He spent two or three months on the Chubb. He admitted defeat, and declared it the most secure lock he had ever handled.
The prize money funded a proper workshop. By June 1818, the brothers had opened a factory on Temple Street in Wolverhampton, already the centre of British lock manufacturing. Early customers included the Duke of Wellington and the Bank of England. By 1847, Chubb had extended the design to six levers, multiplying the legitimate key combinations into the millions and narrowing the gap through which any picker might work.
The six-lever lock eventually met its match — Alfred Charles Hobbs, an American salesman with considerable skill, opened it at the Great Exhibition of 1851 — but by then, Chubb’s real contribution had already taken hold.
Before 1818, security meant resistance. Chubb added memory. The two ideas have not separated since.
Sources
- Chubb detector lock — Wikipedia — mechanism, the convict test, six-lever extension, Hobbs at the 1851 Exhibition.
- A Brief History of Chubb 1818–1990s — Chubb Archive — patent date (February 3, 1818), government reward of 100 guineas, company founding in Wolverhampton.
- The Chubb Detector Lock — History of Safes — six-lever design, early customers, Bank of England adoption.