Frederick A. Draper
The numerous and daring robberies repeatedly chronicled in the daily papers, where stores, banks and post offices have been entered, the safe blown open and contents taken, would seem sufficient in the way of warning to influence those having valuable property to protect it, especially when this can be done with
Fig. 3 comparatively little expanse and trouble. That is, it can be protected to the extent that an alarm can begiven of an attempted robbery, either on one premises or at a point at a distance, such as a fire or lighting station, or other place where men are on duty both dnight. The fact that a safe or other depository is protected in this way is generally sufficient to prevent any attempt at robbery, and an " ounce of prevention is worth a pound of cure."
The device by which an alarm is given in the event of interference from any cause is here described. It comprises two electrical circuits of differing strengths, working through relays located at the point where an alarm is to be given. Any decrease in the stronger ing np or increase in the weaker one will cause either r the other of the relays to make contact and close a local circuit and ring a bell until a drop has been replaced or the battery run out. Another arrangement of two circuits is with one circuit closed and the Other open, but the latter is not so efficient as the former. With the first system any break in the stronger circuit or " ground" between the two will op-«rate on the relay, therefore the wires cannot be cut, crossed, or circuits diverted from the point protected "without operating the relay. In fact, with this system a safe is not necessary, except as a protection against fire, or to retard robbers until those hearing the alarm can arrive and learn the cause.
Assuming that it is a safe which is to be protected, it is necessary to build a box of peculiar construction large enough to entirely surround the safe without touching it. In making this box, the principal object to be looked after is that no " grounds " are made by tacks, nails or screws between the wiring of the two circuits in the walls of the box. They are composed of two layers of wire screening, insulated from each other and held by a wooden frame. The screening can be obtained of any hardware-dealer.
Each side of the box is made in a separate section, all to be joined about the safe as shown in Fig. 1. The framing of a side is shown in Fig. 2, each frame being made of three thicknesses: a center one 3/8 in. thick, and two outside ones 1/4 in. thick, all about 2 1/2 to 3 in. wide, according to the size of the box; no great strength, only stiffness, being required. Only general dimensions will be given, as the requirements will differ so greatly with each case. It is advisable to make drawings of each part with dimensions before beginning construction so that contacts may be correctly located.
The sides, front and back all rest upon the floor section, with outer edges flush. The front and back lap the sides, the side on which are the hinges of the safe door, coming only to the front line of the safe to permit of swinging the door wide open, that side of the front section having a projecting piece to fill in. The top, bottom and sides project beyond the safe far enough to clear projecting, parts like the knob, by at least 1/2 to | in. The top and bottom also lap the front, which is removed to one side during the hours when the safe is open.
The construction of one section, a side, will be de scribed. As before mentioned, the frame work is made in three thicknesses. A plain, rectangular frame is made with mitred corners and center brace of stock 1/2 in. thick, as shown in Fig. 2: all joints should be glued. This frame is then covered on each side with the screening, the kind ordinarily used for screen doors will answer. Where joints are made on the front edge with contact plates it will be necessary to remove the paint covering the wire with a blow torch or potash. The covers of screening should be large enough to extend, where connected to the contact plates, and with corresponding projections on the bottom sections, a trifle less than 1/4 in. beyond the sides of the wooden frame. At the corners it is cut away for about 2 in. each way to a little less than the frame.
After tacking the screening to the frame both sides are covered with heavy paper, which is laid on with a strong paste or thin liquid glue. The layers of paper are to insulate the wire coverings from each other.
The screening is put on with galvanized iron tacks, on a line marked with gauge or pencil and located to clear the tacks to be put in on the other side by about 3/4 in. long, so that by no possibility can a point come through and " ground " one side with the other. The frame is now turned over, the open spaces are partly filled with straw board, excelsior or other non-conducting substance, so that in the event of moving things about and the box becoming indented, the two surfaces would not come in contact to form a " ground. " This done, the screening for this side is then tacked on. A test for "grounds" can be made with a common bell and battery, one lead placed on one wire and one on the other. If the bell rings, or the hammer even vibrates, there is a "ground" which must be located and removed before proceeding further. This trouble is not likely to occur if care is used in driving the tacks.
The double screen frames are now covered on the outside with cartridge paper or other attractive covering and after nailing the sections together the outside frames are added. These are made of 1/4 in. stock with mitred corners or halved joints as preferred and for the sides are the same size as the inner frame. They are fastened to the part already made with 1/2 in. wood screws, countersinking the heads. Use plenty of screws of small gauge, covering with putty when all is completed. The projecting edges of wire are then soldered to the contact plates, to be described later, four or more being placed on the front edge, as shown in Figs. 4 and 5. The edges, other than the front, are simply soldered together, using care that no stray wires or drops of solder cause a "ground." The joints between sections are carefully nailed.
The contact plates are shown in Fig. 6 and will have to be made up, as they are not commonly on sale of the size and shape needed. Obtain some strip brass £ in. wide and about 1/8 in. thick, cut into 32 pieces 2 in. long. Drill holes in the center of one-half of them of tapping size for 1/4 in. rod, and cut threads with tap. Also drill holes in the center of the other half a trifle over 1/4 in. diameter, say 9-32 in. Holes are also drilled at each end of all the pieces and countersunk to receive the screws for fastening. Get some 1/4 in. brass rod from which 16 studs are made to fit into the 1/4 in. threaded holes. These studs should clear the plate about § in., the best way to make them being to cut the thread with a 1/4 in. die, put on the plate and head over on the under side as with riveting, and then cut off the rod with a hack saw, finishing with a file.
The top, front and back sections are constructed in the same way, with wire projecting at several places with which to connect one section with another, except the front one, on which are put the contact plates as described. These are located on the inner side, four to each edge, two being connected to the inner screening and two to the outer screening, alternating them. The plates with the studs are put on the front section. The screening is soldered to the bottom of the plates before they are screwed down. These plates are put on the framework, or the sheathing with which the inner side of thefront section is covered. On
Fig. 5 the hinge side of this section, a board wide enough to reach the side section is nailed, the contact plates being put on the edge of this board and the wire screening connections carried down to the under side of the plates.
The front of the top section will also require that the front two inches be a plain board, hinged to the rest of the top, so that it may be lifted off from the top of the front section, otherwise the studs would prevent the front from being put in place. To electrically connect this hinged piece and the rest of the top, use four ordinary burglar alarm door springs which are set into the hinged piece and make contact with flat pieces on the edge of the top. These door springs are connected with the contact plates by insulated wire, all joints being soldered.
The floor section will have to be shaped to fit between the feet of the safe, and the two covers of wire can be dispensed with. Make a solid frame of matched boards with 2 in. cleats at the ends, the boards at front and back to be loose until placed in position. Brass angles are then to be located at the edges to make contact with the other sections resting thereon, and these angles connected electrically as shown in Fig. 7, which also shows the leads to the line wire, grounds and day loop.
Another way of finishing the outside of the box, after all wiring has been done and connections made and tested, would be to sheath it all over with thin matched sheathing, which would be more durable than the wall papering mentioned, but much care must betaken to avoid short circuits.
The electrical connections, instruments and operation will be described in the next number.