Locks are appliances by which doors, lids of desks, trunks, etc., are temporarily fastened in such a manner that only the possessor of a key of peculiar form can loosen or unlock the fastening by the slipping of a bolt; and this key is also necessary to repeat the operation of closing or locking.
The French Lock (fig. 37), which is one of the simplest forms of lock now extensively used, is well suited for the doors of dwelling-houses. The bolt stands in a quadrangular box or rim. Its head—or double head (as in the Figure)—after passing through the cramp of the rim, enters the corresponding opening or openings of the catch fastened on the door-jamb or in folding doors on the leaf not g-enerally moved. The end positions of the bolt are fixed by a spring lever, the tumbler, which must always be lifted by the wards of the key before the bolt can slide. The effective turning of the key results, first, in lifting the tumbler by the bolt, and, second, in moving the bolt, both in locking and in unlocking. The return of the tumbler to its resting-place is effected by a spring.
To increase the difficulty of imitations of the key, wards (fig. 38) com posed of sheet metal are set inside the rim, which prevent the key from being turned if its wards have not opening-s shaped to correspond exactly with the form of the profile of the guard. Changing the form of the guard furnishes a simple method of varying the locks and keys of a house or of a story in a flat or hotel, the arrangement being otherwise similar. At the right of the top of Fig-ure 37 is the key of a French lock with complicated wards; under this is a blank key, by which, if it is covered with wax, the profile form of the ward may be taken from the outside if it is desirable or necessary to make a skeleton key or "pick," by which an effective snbsti tute for the reg,ular key is obtained. False keys in two forms are shown on the right of the lower part of Figure 37.
Padlocks, the head of the bolt is not to enter a box staple, but the end of the hasp forming part of the lock box, which is to be hung in a staple or ear fastened to the door-jamb, the appliance assumes the form of the padlock (fig. 39). Other modifications in form arise when the bolt is so changed in shape and position as to afford security from unauthorized intrusion to desk-s (fig. 4o), trunks (fig. 41), etc.
Combination fact that all locks with a single tumbler are comparatively easy to open with keys other than those expressly made for them has led to the invention of the combination lock. In it the move ment of the bolt can occur only after a nmnber of movable tumblers have all been brought into certain positions; so that the inisplaciu.- of even a single one of these tmnblers will prevent the bolt from inovino In the letter lock (pi. 7, fig. 42) tumblers take the shape of rotating rings, and the letters for making the combination stand on the outside. In Chubb's lock (jig. 43) the tumblers are arranged as levers that rotate about a pin common to all the tumblers in the lock box, and they have perforations of a peculiar form into which a pin or dowel on the bolt pene trates. Each of these levers is brought into the position requisite for free ing the bolt by a special part of the wards of the k-ey. It therefore assumes an irregularly notched form on the edge of the main ward, and the differ ences in lengths and grouping of the required notches or divisions of the ward afford facilities for easily obtaining a notable variety in locks of this kind which are alike in general features. In Price's lock- Cfig. 44) the tumblers surround the keyhole, so that the part accessible from the outside is made as small as possible, for the purpose of preventing the destruction of the lock by gunpowder. This, as well as Newell's permutation lock (fig. 46a), may be regarded as a variation of the Chubb lock, in which the divisions of the key-ward corresponding to the different tumblers (fig. 46b) can be separated, and thus interchanged (fig. 46c). By ingeniously divid ing each tumbler into two parts, it becomes possible to make such changes inside the lock by a change of the key and closing the lock by its aid that the former arrangement of the key becomes useless for opening the lock. This peculiarity becomes valuable if there are grounds for suspecting that the key has been duplicated or that attempts are about to be made to manu facture a false key. Locks of this kind have been used in New York on safes in which money or valuable securities were deposited.