FREDERICK A. DRAPER
As mentioned in the previous chapter, the electrical arrangement consists of two closed circuits of sufficiently different strength so that short circuits or breaks, whether intentional or other wise, will operate either one or the other or both relays. This is accomplished by inserting resistance coils in the circuit. These are shown in the illustration of circuits as R1, which may be 50 ohms, and R2, which may be 150 ohms resistance.
To further illustrate the subject, assume that arrangements are made with a near-by fire station for placing the alarm instruments therein, and from which the response to an alarm will be made. Such an arrangement is a good one, as even in small towns two or three men are always present, except when called away by a fire, and at such times so many people are passing on the main streets that a robbery requiring the use of explosives would hardly be attempted. The reason for having the alarm station located where there are always several men present is to prevent robberies such as one occurring in a large Massachusetts town near Boston. In the case mentioned, a band of seven or eight robbers captured singly the two night police officers, locked them up in the town jail, and then blew open the safe of a bank and disappeared with the contents, amounting to several hundreds of dollars.
An alarm station with only one attendant, unless securely protected, could be similarly "captured " and the whole system made useless. Where only one attendant is possible, however, he should be located in a room where he cannot be reached without being alarmed in time to be ready for emergencies and have an opportunity to give a signal over a telephone line which cannot be cut, or otherwise bring ample assistance to the premises being guarded. He should also make a thorough test of all parts of the alarm system early every evening, and again on that part necessary to signal others before retiring.
At the alarm station are located the batteries, relays, resistance coils, signal bell and continuous ringing alarm bell. At the protected premises there is a signal bell and strap key for sending and receiving signals at the opening and closing hours. These also permit of signals during the daytime, if desired, by arranging the circuits so they may be closed by a switch.
The installing of the alarm station will first be described. A double balance relay, Fig. 6, is necessary. The circuits pass through both sets of coils. One side is adjusted at the highest tension so that any reduction of current will release the armature. The other side is adjusted just beyond where the strength of the current will pull down the armature, so that any increase in current will move it.
From the open sides of the relay, wires are run to an automatic drop, Fig. 7. which operates whenever either armature of the relay is thrown, and causes a bell to ring continuously until the drop is replaced or the battery runs down. The continuous ringing feature is necessary to assure that the alarm will be noticed and receive attention. A short ring might escape notice because of other noises, or the temporary absence of attendants.
The line circuits through the relay are also connected to a ground which may be a near-by iron or water drain pipe, or a direct wire. If a water or drain pipe is used, it must be examined to learn that no telephone or other wires are grounded thereon, as other circuits on the ground might cause the relay to operate and thus cause false alarms. A direct wire ground should have the earth end buried several feet in moist soil, and is best arranged by turning that end into a spiral of several turns, or soldering to a copper plate 10 or 12 in. square.
The batteries for the line are of the ordinary gravity type, two to four cells being necessary. They are connected in series and located between the ground and the relays with zinc pole on the relay side, although a reverse arrangement may work best in some places.
A signal bell, Fig. 8, and strap key, Fig. 9, are also connected into the line by a shunt around relay, which is thrown out during the open hours by the switch fitted to same. If daytime signals are not needed, these may be omitted and the "O. K. " signals given through relay and drop, which would also serve as tests for the line on one side of the relay. The wiring arrangement of the alarm station for the arrangement described would be shown as in Fig. 10.
At the premises to be guarded, a signal bell and strap key are also installed in the same circuit as that at the alarm station, provided answering " O. K. " signals are desired. If such signals are not thought necessary, the signal bell here is omitted, the key being necessary, however, to advise the alarm station when opening and closing hours arrive. The circuits at this end must also be connected to ground as previously described. The bell and key require a circuit in shunt with the safe cover. If the line wires are run outside of buildings, lightning arresters should be used where the lines enter buildings. The type used for telegraph lines is suitable.
After the work of fitting up the frame, installing instruments, etc., is completed and the battery reaches its full strength, the relay is adjusted and the circuits tested. If these directions have been carefully followed, the system should be found to work satisfactorily, and if the alarm station has vigilant attendants any attempt at robbery will give sufficient warning to prevent its being successful.