Early lever tumbler lock could easily be picked by testing the springs until the right one is activated in order to release the bolt and open the lock. In 1818 Jeremiah Chubb patented a lock that used a lever tumbler system with an added security mechanism that kept one lever in the up position if the incorrect key was used, meaning that the bolt could no longer be released. The lock would remain in this position until it was reset using a special key.
How does the mechanism work?
This lock was so revolutionary because the security system meant that once the lock had been tampered with the thief could not continue to pick it. Inside the lock the main part of the mechanism featured the traditional lever tumbler lock - the original design had 4 levers but this later increased to 6. It worked on the same principle of the lever having to be the correct height in order for the bolt to be released, the only difference was a pin lying across the levers which would trip the security mechanism if any of the levers were lifted fractionally too high. If tripped an independent lever would catch the lever that had been lifted too high and keep it in the upright position, making the lock impenetrable.
The Extra Key
The Chubb Lock was designed so that when it jammed in the up position the only way to return the lock its original state was to insert a special key that reset the mechanism. This meant that if someone picked the lock and triggered the security mechanism they would have to then work out how to reset the lock before they could begin the process again, and this would require an enormous amount of skill!
This security feature meant that the homeowner was alerted to an attempted break-in when they next went to open the lock. In 1824 the lock was improved further by the Chubb brothers and the need for the extra key was eliminated. The lock could be reset using the true key but it would need to be re-locked and then unlocked, thus alerting the owner that someone had tampered with it.
This lock, and the legend that King George IV accidentally sat on a Chubb Lock with the key inserted, cemented Chubb at the forefront of the lock making business, where they still remain. It also set a benchmark for lock security from which industry leaders have developed the locks we use today.