Locks and Keys

Locks and Keys

Locks can be either mechanical or electronic, the latter being a modern variation for which a specific numeric code is required to release the locking mechanism. Much more common is a mechanical lock, opened by purely physical means. Locks do not have an independent existence; they must lock something or someone in or out, and they must have a key. The key is based on principles that go back to ancient times, using one of the most rudimentary types of machine known to humankind: the inclined plane.

Historical background. In the history of physics and technology, there are three simple machines: the lever, the inclined plane, and the hydraulic press. The last of these only came into existence during the 1600s, but the first two date to a time before the dawn of civilization. The simplest form of inclined plane is a ramp, which makes it possible to move an object across a vertical distance with a smaller amount of exertion than would be required to lift it straight upward. Other modifications of the inclined plane are wedges, knives, axes, screws, corkscrews, and a key and lock mechanism.

The earliest locks date back to ancient Egypt, and even the more modern variations on lock design that developed in the wake of industrialization still harken back to the design used in the pharaohs' palaces. For example, American locksmith and inventor Linus Yale, Jr., whose name remains an important one in the lock and key industry, based his cylinder lock in the 1860s on the Egyptian design. The latter consisted of a wooden housing containing wooden pegs of varying length, fitted into holes bored into the top of a wooden bolt. Only when a long wooden key with pegs of specific lengths was inserted into the bolt could it be opened.

Basic workings of a lock. Modern locks and keys are made of steel rather than wood, but otherwise the design is not remarkably different from that used to lock doors thousands of years ago. Inside a modern mechanical lock is a row of pins, usually five in number. Each pin has its own cylinder, and when the lock is locked, they hold together two pieces of metal rather as the "teeth" of a belt hold together two sections of a piece of leather. The pins are of varying length, meaning that in order to open the lock, it is necessary to raise them all together so that the bottoms are in alignment.

The solution to this problem is, quite literally, a key, whose serrated edge is actually a row of inclined planes fitted to the configuration of pins inside the lock. The notches on the key are made to push the pins upward just the right amount for each pin, so as to force them all into their respective cylinders and separate the two blocks from one another. The shape of the notches is such that the key can be withdrawn from the lock after use, at which point springs push the pins back downward into their original place.

Mechanical and electronic variations. A variation on this model is Yale's cylinder lock. In this design, the pins are lined up along a larger metal cylinder, which they hold in place inside a cylindrical housing. Inserting the proper key raises the pins and frees the cylinder so that, when it is turned, it rotates and draws back a cam that holds a bolt in place. The bolt is spring-loaded, such that when the key is withdrawn, the spring pushes the bolt back into place, turning the cylinder back to its original position and making it possible to withdraw the key.

There are other variations on the mechanical lock, most notably the old-fashioned lever lock, but the basic principle is the same. By contrast, an electronic lock requires the use of a keypad and a numeric code. The user enters a code, which the machine interprets as a series of binary (on-off) electric pulses. These pulses are bits in a number sequence, which are read by a computer chip. Assuming the sequence matches the one encoded on the chip, the latter sends out an electric signal that opens a mechanical bolt holding the lock in place.



Macaulay, David, with Neil Ardley. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Phillips, Bill. The Complete Book of Locks and Locksmithing. New York: McGraw-Hill, 1995.

Roper, C. A. The Complete Book of Locks and Locksmithing. Blue Ridge Summit, PA: Tab Books, 1983.

Sloane, Eugene A. The Complete Book of Locks, Keys, Burglar and Smoke Alarms, and Other Security Devices. New York: Morrow, 1977.


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