Rudimentary Treatise on the Construction of Locks

by A. C. Hobbs; George Dodd

The following are the salient points of the discussion that followed the reading of his paper:—

The Chairman remarked that the paper just read gave a very excellent and clear description of the detailed working of the new lock, and he thought this construction of lock was a most valuable one, as affording real security against all fraudulent attempts. He inquired whether there would be any possibility of tampering with the lock by examining it upon the inside of a safe door, whenever the door might happen to be left unlocked.

Mr. Fenby replied that there was no means of tampering with the lock from the inside of the door, as the two keyholes for working the lock were only in the front face of the door, and the lock was all closed up on the inside of the door, excepting the hole through which the bit was allowed to drop out; but this would be useless for the purpose of tampering with the lock, as the bit dropped down a tube leading to the bottom of the door, through which no examination of the lock could be successfully made.

The Chairman inquired whether there was any provision against the bit being accidentally locked up inside the safe, in which case it appeared the lock could not be opened again.

Mr. Fenby replied that the owner of the safe must of course be careful after unlocking the safe to take the bit out before locking it again, otherwise there would be no means of opening the lock afterwards with that key. As a precaution, however, against any such accident, each lock was provided with three bits, all duplicates, one of which would be kept in the pocket for use, while the two others would be preserved in a place of safety for the chance of any such contingency. Moreover, in most of the safes fitted with these locks, the tube through which the bit dropped had been made of such a length as to carry out the bit on opening the door, dropping it into a small tin tray outside the safe; and by this means the accidental locking in of the right bit was rendered impossible. One of the advantages of the new lock was that the stem of the key was not required to be kept constantly in the possession of the owner, but it might be left in the lock, as the bit alone was the valuable part of the key; and as the bits were of such small size and convenient shape, a number of them might readily be kept in the pocket by a person having charge of a number of safes, without the inconvenience attending a large bunch of ordinary keys. In the case of an attempt being made to open the lock with a counterfeit bit, the advantages of retaining the counterfeit inside the safe were not merely that the person attempting the lock was deprived of his instrument, while the proprietor immediately discovered the attempt upon the next occasion of opening the safe; but the retention of the counterfeit itself afforded the means of judging, by a comparison with the true bit, whether the attempt had been made altogether in the dark as to the actual construction of the lock, or whether it was likely that some clue regarding the true bit had been obtained by means of a wax impression or otherwise. In the latter case the owner of the safe might think it desirable to have the lock taken off, and the arrangement of the levers altered, and a new bit made so as to baffle any further attempts.

Mr. W. S. Longridge observed that the inconvenience that had been alluded to with the new lock, of accidentally locking up the bit inside the safe, was no greater than occurred with an ordinary safe lock if ever the key was accidentally lost; in either case, unless the precaution was taken of keeping a duplicate in reserve, it would of course be necessary to have the safe broken open.

The Chairman inquired how the ideas had been arrived at of separating the bit from the key, and of preventing all access to the works through the keyhole, and also of retaining the bit inside the door after any attempt at unlocking.

Mr. Fenby replied that his attention had in the first instance been attracted to the subject of the picking of locks as a mechanical problem, and he had found that there had hitherto been no principle in lockmaking which could effectually baffle persevering attempts at picking. For although there were certain complicated constructions of locks, having many points of excellence, they had all yielded in time to the picking instrument in clever hands; and it must be remembered that any individual lock when once constructed remained stationary as regarded subsequent improvement, whereas the art of picking that lock was continually progressing towards success, with all previous constructions of locks, and it was clear therefore that the lock must ultimately be defeated. He had been further stimulated in the investigation of this subject by the occurrence of the great gold robbery referred to at page 188; and the circumstance which had struck him most forcibly in connection with that robbery had been that locks of the best make hitherto known had admitted of seven successive trials being made upon them without detection, each trial furnishing the information for further perfecting the counterfeit key, until the locks were at length opened.

These considerations had led him to the conclusion that two points were established and were required to be kept in view for the construction of any lock that should be really secure against fraudulent attempts. The first point was that wherever a man could get instruments into the lock he could ultimately solve any problem laid before him by the maker of the lock, as the lock when once made could be tried any number of times if an instrument could be got into it at all. Hence he had concluded that it was requisite for all access to the interior to be cut off, so as to preclude all possibility of getting a pick-lock in; and this was accordingly accomplished by adopting the plan of separating the bit from the stem of the key. The second point established was that it was necessary to prevent the possibility of making a succession of trials with the same counterfeit key; and it had then struck him that, if the bit of the key were arranged to drop inside the safe in unlocking, there would be no means of going on gradually improving and touching up the counterfeit from the results of previous trials, as the false bit would be irrecoverably lost in the very first attempt, without furnishing any clue whatever as a guide for alteration in a subsequent trial. The first lock that he had invented for meeting the requirements thus pointed out had been made with a solid block having a tunnel through it, but involving the same principle of retaining the bit of the key and keeping the levers inaccessible from the outside. Subsequently, however, he had abandoned that construction and produced the new lock shown in the drawings, having the revolving barrel with radial slot.

The Chairman proposed a vote of thanks to Mr. Fenby for his paper, which was passed.