Mr. Yale's most important invention; brought out in 1860 to 1864, was the key lock univer sally known as the Yale lock, the United States patents for which were issued on 29 Jan. 1861 and 27 June 1865. This combination of a flat key and revolving "plug' has almost entirely superseded the crude and bulky lock, of indiffer ent security and inferior workmanship, which was opened by a round key, and the intro duction of the Yale or cylinder type com pletely revolutionized the art of lock-making in America. • Yale's inventions led to the dial lock of James Sargent, but as the user of both of these, the Yale and the Sargent, could be coerced to open them, and were indeed picked by ex perts, the inventors turned their attention to making the only lock which is absolutely unas sailable—the time-lock. This was first sug gested in 1831 by an Englishman, William Rutherford; in 1857 Holbrook and Fish of the United States devised another, but the first successful time-locks put on the market were the Sargent and Yale time-locks, brought out in 1875, and followed later by the Pillard, the Hornet, the Hall and others.
Warded the majority of this class there is a "back spring" or dog, the feel ing of which, when using the key, resembles that of a tumbler, but which adds nothing to the security.
Lever Tumbler These locks may contain several tumblers, the number of key changes usually being limited to from 12 to 24 and possibly 72, while in a good three-tumbler lock of this style as many as from 200 to 500 key-changes are possible. Round, barrel and flat keys are used.
Cylinder and The former term applies to locks in which the pin tumbler is contained in a cylinder separate from the lock case; the latter is applied to those wherein the pin tumblers are contained in the lock itself.
A series of locks is said to be "master-keyed' when so constructed that each lock can be operated by its own key, which fits no other lock of the series, and also by another key which will operate every lock in the entire series. Warded locks are master keyed by means of a °skeleton° key, with the bit cut away sufficiently to avoid all the wards in all of the locks of the series. Lever-tumbler locks are master-keyed in three ways, either by providing fwo "gatings* on each tumbler, one of which is brought in line with the corre sponding °fence or post on the bolt by the change-key and the other by the master-key; by providing a "lifter," which, when actuated by the master-key, moves the tumblers in the same manner as does the change-key when the latter acts directly on the tumblers; or by pro viding a set of secondary levers, which, when operated by the master-key, move the primary tumblers in the same manner as the change-key which acts on them directly. Pin-tumbler locks
are master-keyed by cutting each pin in two places, or by encircling the plug (which con tains the key-way) with a larger annular plug, thus providing two points at which each tum bler may be set to permit the plug to rotate, and utilizing one set of these points for the change key and the other set for the master-key.
Time of Chronometer In these the mechanism is actuated by clock-work, and is used, in connection with the heavy bolt work of a safe door, to prevent the unlocking of the latter except upon the hour at which the clock is set. There are generally three chronometer movements, each of which will of itself actuate the lock, so that should one or two be disabled or inoperative, the other will open the door at the time indicated. In connection with the time lock an automatic bolt operating device is often used.
Dial or Combination These con sist essentially of a bolting mechanism guarded by a set of changeable tumblers or wheels, and actuated by a spindle passing through the door, provided on the outer end with a graduated dial, by rotating which in a certain manner the tumblers can be set and the lock be operated.
The first locks made in this country naturally were patterned after those made in Europe, but whereas the Euro pean artisan used wrought metal in con struction, the American manufacturers soon turned from this and employed cast metal. This change, together with the introduction of vastly improved machinery, soon reduced the cost of production, so that now locks of the best type can be purchased at a merely nominal cost, and while, for a few years after 1870, the sharp competition among manufac turers tended to reduce the quahty of the production, the mechanical advancement since that time has been marked and at the present time the highest grade of workman ship and mechanical skill is manifest in the articles on the American market. The chief centres of lock-making are in Eastern Pennsyl vania and Connecticut. Consult Fox-PiW Rivers, the Development and Distribution of Primitive Locks and Keys' (London 1883) ; F. E. Kidder, section on Locks in Construction' (part 2, pp. 580-603, New York 1913) ; and H. R. Towne, CLocks and Hard ware> (ib. 1904).