lock and key fastening fitted to an entryway, such as a gate or door, or a container, such as a cabinet, drawer or safe, to keep it closed and/or prevent unauthorized access or use. Locks typically consist of a sliding, pivoted, or rotary bolt protected by a fixed or movable object. A lock may be opened by a mechanical, magnetic, electric, electronic, or electromechanical key or by employing a code or sequence of numbers or letters.

Mechanical Locks

There are two basic types of mechanical locks, each with variations. The oldest and simplest is the warded lock, which is essentially a spring-loaded bolt in which a notch has been cut. The key fits into the notch and slides the bolt backward and forward. The lock takes its name from the fixed projections, or wards, inside the lock and around the keyhole. The correct key has notches cut into it that match the wards, which block the wrong key from operating the lock. The ward lock is the easiest to pick and now is used only for cheap padlocks.

The tumbler lock contains one or more pieces of metal (called tumblers, levers, or latches) that fall into a slot in the bolt and prevent it being moved. The proper key has serrations that raise the metal pieces to the correct height above the slot, allowing the bolt to slide. There are three types of tumbler locks, pin-tumbler, disk-tumbler, and lever-tumbler. Pin-tumbler locks are the most common. The tumblers in this type of lock are small pins. The modern door lock is a compact pin-tumbler cylinder lock of the type developed (1860) by the American inventor Linus Yale. Door locks on automobiles and most high-security locks have pin tumblers. Disk- or wafer-tumbler locks, use flat disks, or wafers, instead of pins. When the proper key is inserted, the disks retract, releasing the bolt. Disk-tumbler locks are often used in desks and file cabinets. Lever-tumbler locks employ a series of different-sized levers resting on a bolt pin to prevent the bolt from moving. When the proper key is inserted, all the levers are raised to the same height, enabling the bolt pin to release the bolt. Lever-tumbler locks are often used in briefcases, safe-deposit boxes, and lockers.

The first of the keyless locks was the combination lock, developed at the beginning of the 17th cent. In it a number of rings inscribed with letters or numbers are threaded on a spindle. To open the lock the rings must be turned to form a code word or number, which causes the slots inside the rings to align and permits the spindle to be drawn out. A variant of the combination lock employs a movable dial with a series of numbers around it in place of the rings. The dial must be turned clockwise and counterclockwise in the proper sequence of numbers to align disk tumblers and open the lock. Once used only for padlocks, combination locks began to be used in safes and strong-room doors during the last half of the 19th cent. The time lock, first used successfully c.1875, has a clock mechanism that is set to permit opening only a certain time.

Electric and Magnetic Locks

Recent lock developments include the magnetic-key lock, in which the pins are actuated by small magnets on the key, which has no serrations. When the key is inserted into the lock, these magnets repel magnetized spring-loaded pins, raising them in the same way that the serrations on a tumbler-type key would. The card-key lock is actuated by a series of magnetic charges; the card-key is popular where security is vital, because a new series may be electronically defined for each new user, without having to change the lock itself. Similarly, electronic card access systems are used in many hotels and office buildings. A special "key" system uses a paperboard or plastic card, on which a code is recorded as a series of holes or bumps, or a microchip or a magnetic strip on which a code is stored. A card reader at the lock location reads the code and sends the information to a computer, which sends a signal to release the bolt if the code is correct. Electronic combination locks similarly use a computer to compare a combination stored in memory with one entered on a keypad; access is permitted if the combinations match. In a biometric entry system the numeric keypad is replaced by a scanner, which captures an individual's fingerprint, palmprint, signature, or other personal characteristic and compares it with that in the computer's memory. Biometric entry systems are most often used in high-security areas, such as nuclear power plants.

In an electromagnetic lock a metal plate is attached to the door and an electromagnet is attached to the doorframe opposite the plate. When the current flows, the electromagnet attracts the plate, holding the door closed, When the flow of current is stopped, the door unlocks. A variation places the plate and electromagnet so that the door is held open when current flows, enabling the door to be closed automatically when the current stops.

Keyless entry systems, which are common in motor vehicles, rely on a keychain fob that contains a remote-control unit consisting of an integrated circuit and a radio transmitter. The fob sends a low-powered radio signal to a receiver in the motor vehicle, and, if the received code is the correct one, the receiver in the vehicle relays the signal to a microprocessor, which opens the lock. The acceptance of such entry systems has led to devices that allow additional functions within the vehicle to be activated remotely.

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.

█ FURTHER READING:

BOOKS:

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.

SEE ALSO

Black Ops
Covert Operations
Lock-Picking

lock-and-key theory A theory to explain the mechanism of enzymatic reactions, in which it is proposed that the enzyme and substrate(s) bind temporarily to form an enzyme–substrate complex. The binding site on the enzyme is known as the ‘active site’ and is structurally complementary to the substrate(s). Thus, the enzyme and substrate(s) are said to fit together as do a lock and a key.

lock-and-key theory A theory to explain the mechanism of enzymatic reactions, in which it is proposed that the enzyme and substrate(s) bind temporarily to form an enzyme–substrate complex. The binding site on the enzyme is known as the ‘active site’ and is structurally complementary to the substrate(s). Thus the enzyme and substrate(s) are said to fit together as do a lock and a key.

locks and keys A system of memory protection in which segments of memory are assigned identification numbers (the locks) and authorized users are provided the numbers (the keys) by the operating system. This provision is done by a privileged process in some location, such as a program status word, not accessible to the user.