Fig. 54.
Fig. 55.
§ 57. In all forms of burglar or thief alarms, the ordinary system of having the circuit broken, until contact is made by the intruder involuntarily making contact at some point, presents one great disadvantage; and that is, that if "notre ami l'ennemi," viz., the thief or burglar, be anything of an electrician (and alas! to what base uses may not even science be perverted) he will begin by cutting all suspicious-looking wires before he
attempts to set about any serious work. This disadvantage may be entirely overcome by the adoption of a simple modification, known as the "closed circuit system" of bell ringing. For this the bells, etc., are continuously in contact with the batteries, but owing to the peculiar connections, do not ring unless the circuit is broken. To render the working of such a system clear to my readers, I quote the description given in the English Mechanic, by one of our leading electricians:—
Writing on the subject of Closed Circuit Bell-ringing, Mr. Perren Maycock says:—"This is principally adopted for alarm purposes. Its superiority over the open circuit system lies in the fact that notice is given on opening (breaking) the circuit, which is the reverse to the usual practice. In the ordinary method it becomes necessary to have a contact maker, differing in form for various purposes and situations, which, along with the leading wires, must be artfully concealed. All this entails great expense; besides which one can never be sure that the contacts and wires are in proper order without actually trying each one. On the other hand, with the "closed circuit" system, one has merely to place the wire in any convenient position, it being better seen than hidden. The very fact that alarm is given on breaking the contact renders the method applicable in circumstances and under conditions which would render the "open" method difficult and expensive, if not impossible. One can always be certain that everything is in order. The modern burglar, electrically educated