In order to shoot the bolt the height of the key steps must be so proportioned to the depth of their tumblers that all the gates in the tumblers are simultaneously raised to the right level for the stud to pass through them, as in Fig. 218. Here you will observe that the tumbler D on the extreme right (lifted by step 2 of the key) has a stud, D S, projecting from it over the other tumblers. This is called the detector tumbler. If a false key or picking tool is inserted it is certain to raise one of the tumblers too far. The detector is then over-lifted by the stud D S, and a spring catch falls into a notch at the rear. It is now impossible to pick the lock, as the detector can be released only by the right key shooting the bolt a little further in the locking direction, when a projection on the rear of the bolt lifts the catch and allows the tumbler to fall. The detector also shows that the lock has been tampered with, since even the right key cannot move the bolt until the overlocking has been performed.
Fig. 218.—A Chubb key raising all the tumblers to the correct height.
Each tumbler step of a large Chubb key can be given one of thirty different heights; the bolt step one of twenty. By merely transposing the order of the steps in a six-step key it is possible to get 720 different combinations. By diminishing or increasing the heights the possible combinations may be raised to the enormous total of 7,776,000!
Fig. 219.—Section of a Yale lock.
THE YALE LOCK,
which comes from America, works on a quite different system. Its most noticeable feature is that it permits the use of a very small key, though the number of combinations possible is still enormous (several millions). In our illustrations (Figs. 219, 220, 221) we show the mechanism controlling the turning of the key. The keyhole is a narrow twisted slot in the face of a cylinder, G (Fig. 219), which revolves inside a larger fixed cylinder, F. As the key is pushed in, the notches in its upper edge raise up the pins A1, B1, C1, D1, E1, until their tops exactly reach the surface of G, which can now be revolved by the key in Fig. 220, and work the bolt through the medium of the arm H. (The bolt itself is not shown.) If a wrong key is inserted, either some of the lower pins will project upwards into the fixed cylinder F ([see Fig. 221]), or some of the pins in F will sink into G. It is then impossible to turn the key.
