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On the Construction of Locks and Keys by John Chubb
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CONSTRUCTION OP LOCKS AND KEYS. 27
The double acting tumbler would only release the bolt, by being lifted to the exact height required for releasing it, and no higher ; for if the tumbler was lifted any higher, it caught the bolt anew, and (by what was called " overlift") detained it as securely, as if the tumbler had not been lifted high enough. In attempting to lift this tumbler with a picklock, there was nothing to indicate when the tumbler was lifted to its exact height, and with two such tumblers requiring to be lifted independently, each one to its own proper height, but no higher, it was difficult to conceive how picklocks could be available.
Chubb's lock was a very improved modification of Barren's, containing six double-acting tumblers combined together, and also possessing the important adjunct of the "detector." In no instance had one of Chubb's locks been opened by picklocks, and, indeed, with a combination of six tumblers, it became exceedingly difficult to make a false key sufficiently accurate, to open a lock, because each step of the key required to be just sufficient to lift the tumbler, to which that step belonged; if the step was too long, the tumbler would be overlifted, and would thereby detain the bolt, or if the step was too short, it would not lift the tumbler high enough to release the bolt; no indication could be obtained by the trial of a false key in the lock, as to which of the steps was too long, or too short. The lock would be secured against unlocking, by any one, or more of the six tumblers, being either overlifted, or not lifted high enough; but it could not be ascertained which tumbler detained the bolt, or which step of the false key was incorrect. In such a state of uncertainty, all attempts to rectify the inaccuracy of the false key, must be directed by mere guess, and alterations were as likely to be made in the steps which were nearly correct, as in those which were wrong.
It was formerly thought, that a skilful workman, furnished with impressions taken from the true key, in wax, or soap, could make a false key to open any lock; and in common locks with the most elaborate wards, but with only one common tumbler, also in Bra-mah's locks, there was much truth in the notion; but for a lock with six double-acting tumblers combined, a false key made ever so carefully, according to impressions, would not be likely to open the lock, for want of exactitude in the lengths of the several steps; and if the key could not be made exact from the impressions, there would be no chance of rectifying it by trial, in the lock, on account of the total uncertainty as to which part required alteration.
Chubb's detector being combined with the six double-acting tumblers, added very greatly to the security of the lock ; for in the course of making trials with a picklock, or false key, if any one of
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On the Construction of Locks and Keys by John Chubb