Lock

It was as a result of this invention that the modern English lever was brought into general use. Different combinations, which are almost unlimited, are obtained by raising or lowering the position of the horizontal slot, or gate, through which passes the talon on the bolt. The correct key will raise the lever (shown in its lower position in fig. 6) so that the talon (c) on the moving bolt will pass through the gate (b). An American named Hobbs invented the safety lever.

Linus Yale's Cylinder Lock 1848.—The other outstanding invention is that of Linus Yale, an American, who, in the year 1848 conceived the idea of adapting the Egyptian lock to modern requirements. To do this he brought about a revolutionary change by separating the key mechanism from the lock itself, thereby making it possible for a very small key to be used, as it did not have to pass through the door. The cylinder (see fig. 7) or the part in which the key operates, consists of an outer barrel which is fixed to the door and a cylindrical plug which is rotated by the key and has a tongue on its back end which projects into the lock. The upper pins, five in number, in the fixed part of the cylinder, fall down into corresponding holes in the plug, which contains five similar pins which are raised to the level of the circumference of the plug by the correct key, the top pins being kept in the required position by means of phosphor bronze springs, vide fig. 7. If a key is inserted which does not raise the pins in the plug to the required height, the plug cannot be turned. A further degree of security is provided by the irregular shape of the key. The notches of the average key may be cut in eight depths, so that, as there are five notches, the number of different keys possible is eight to the power of five, i.e., 32,768. The notches are cut automatically in a milling machine to the required depth, and the pins in the plug are made afterwards to correspond.

Still a further number of key changes is obtained by milling grooves lengthwise on the sides of the keys to correspond with similar grooves in the keyway in the plug. By varying the shape and location of these grooves quite a large range of key changes are made possible.

The illustration (fig. 8) is of the most common form of cylinder lock, i.e., the cylinder rim night latch, the term meaning a spring bolt cylinder lock operated by a key from outside and a knob from inside. This lock is almost universally accepted as a front or street door lock. The invention was therefore of great impor

tance. It is the cheapest lock offering such a degree of security and having so many differing keys. The invention of Joseph Bramah, in 1784, of a lock also depending for its security on moving parts, has to a large extent fallen into disuse, but Bramah's name was an important one in the trade when he placed his patent on the market.

Keyless Locks.

There are several kinds of keyless locks, the two principal being those which are operated by an arrangement of letters or numbers, and special safe locks which are made to open only at a given time. In both cases the opening position in the disc or levers is achieved with out the aid of a key. It is usual in the case of time locks to make use of three watch movements, in order to reduce to a minimum the possibility of failure which would be likely to occur if only one were used. The movements are wound up until the time at which the safe is to be opened is indicated on the clock faces inside the lock. When the lock is set in this manner it automatically goes off guard at the appointed hour, allowing the safe then to be opened.

The growth of safe deposits from the latter half of the 19th century has necessitated the design of special locks, one of the conditions required being that each locker should be opened only by the renter and the guardian together. These are termed duplex key action locks, a number being operated through a single keyhole and others through two. Another variety of safe deposit lock has changeable keys, i.e., all the locks are identical in every respect, but all keys are different. Any lock may be set to any key by the simple action of locking out the bolt with the desired key, which then becomes the only key which will unlock until the lock is set to another key.

Construction of Modern Locks.—Most door locks are used to keep the door in a shut posi tion besides securing it. Indeed this is one of the principal func tions of a lock, and for this pur pose it is provided with a bevelled spring bolt which is operated by a knob or lever handle, the dead bolt being thrown out or with drawn by the key. The method of moving this spring bolt is called the "action" of the lock, and upon the action the easy and sweet closing of the door depends. When it is considered the different sizes, weights and types of doors that are in use, it will be understood that many different kinds of actions must be de signed so that the lock may correctly perform its function.