The importance of locks is to protect against intruders and thieves. Over the years several different types of lock have been invented. Each new development of the lock was to improve the previous one.

The first big attempt to improve the lock was made by Robert Barron. He made a double-acting tumbler lock. A tumbler is a lever that falls into a slot in the bolt. It prevents it from being moved until it is raised by the key’s ridges to exactly the right height out of the slot. Then the key slides the bolt. The Barron lock had two tumblers and the key had to raise each tumbler by a different amount before the bolts could be unlocked.

In 1818 Jeremiah Chubb of England improved on the Barron lock by making a detector. He made a retaining spring that caught and held any tumbler which was raised too high by someone who was trying to pick the lock. This prevented the bolt from being withdrawn and also showed that the lock had been messed with.

Joseph Bramah of England made a different type of lock in 1784. His lock used a very small, light key that gave more security than any other. The Bramah key is a small, metal tube that has long narrow slots cut in its end. When the key is pushed into the lock, it pushes down some of the slides to the depth controlled by the slots. Only when all the slides are pushed down to the right level can the key be turned and could slide the bolt.

In the mid-19^th Century, the demand for locks grew quickly after the Industrial Revolution. In this period new locks were being invented often and quickly. Robert Newell of the firm Day and Newell of New York City made the Parautoptic lock. Its special feature was that it not only had two sets of lever tumblers, but it also used a plate that revolved with the key and prevented the inside from being inspected. The key also had bits that could be interchanged so that the key could be changed.

In 1878 American Linus Yale made a pin tumbler lock which used a small flat key with a jagged edge. The pins in the cylinder are raised to the right heights by the jagged edge which allows the cylinder to turn. It has probably the most familiar lock and key in the world. It is used for outside doors of buildings and automobile doors.

To stop bank robberies, in 1873, James Sergeant of New York developed a lock using a clock that allowed the safe to be opened only at a certain time. This idea was based on an idea previously used in Scotland.

The keyless combination lock came from the "letter-lock" used in England at the beginning of the 17^th Century. It had rings with either letters or numbers in them and were threaded on a spindle. The rings can be turned to form words or number combinations. Then the spindle can be taken out because slots inside the rings all fall in line. These locks were used for padlocks and trick boxes until the last half of the 19^th Century when they were used for safes and strong-room doors. There are an unlimited number of combinations and are very easy to manufacture. A simple combination lock with four rings and 100 numbers on the dial present 100,000,000 possible combinations.

The basic types remain the Bramah, lever, Yale, and combination locks. Many variations have been made. Sometimes these variations combine features of each.

The Swiss Kaba lock, for example, uses the Yale principle but its key has flat sides marked with deep indents. Four sets of pin-tumblers are pressed into the indents.

The Finnish Abloy lock is a compact combination lock. The rings are moved to the right positions by a single turn of a small key, instead of separately by hand.

Magnetic forces can be used in locks based on the Yale lock. The key has no cuts. It has several small magnets. When the key is inserted into the lock, these magnets repel magnetized spring-loaded pins. This raises them in the same way that the cuts on Yale lock key raises them mechanically. When these pins are raised to the right height, the cylinder is free to rotate the barrel.

The importance of locks as a protection against professional thieves went down after World War II because explosives started being used. Even though most safe locks and strong-room locks became unpickable, criminals started to blow them off with explosives instead. One way to prevent this from happening is by using a second series of bolts, not connected to the lock mechanism. They are automatically inserted by springs when an explosion happens.

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