US3771153A

US3771153A - Defeat resistant burglar alarm contact

Info

Publication number: US3771153A
Application number: US00232871A
Authority: US
Inventors: T Byrne
Original assignee: DISTRICT TELEGRAPH CO
Current assignee: DISTRICT TELEGRAPH CO
Priority date: 1972-03-08
Filing date: 1972-03-08
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06
Also published as: CA998134A; GB1427565A

Abstract

The present invention comprises a defeat resistant burglar alarm contact of miniature size for application on small safes having rotating doors as well as for the protection of other types of doors and closure elements. Special features have been provided to resist various forms of burglarious attack upon the protected closure element.

Description

ted States Patent Byrne Nov. 6, 1973 [54] DEFEAT RESISTANT BURGLAR ALARM 3,444,493 5/1969 Nieuwhof 335/206 CONTACT 2,912,540 11/1959 Sawicki 340/274 UX 3,510,813 5/1970 Bozzone 335/206 X Inventofl Thomas J- y New York, NY 3,544,934 12/1970 Poliakoff 335/206 Assigneez American Disrict Telegraph 2,877,361 3/1959 Chase 340/274 X Company, Jersey City, NJ. Primary Examiner-John W. Caldwell 2] Filed! 1972 Assistant Examiner-William M. Wannisky 2'1 Appl. No.: 232,871 Att0rney-Ronald F. Ball 52 us. c1 340/274, 340/276, 335/219, [57] ABSTRACT 200/6153 The present invention comprises a defeat resistant bur- [51] Int. Cl. 60811 13/08 g alarm Contact of m n atur s z f r application on [58] Field of Search 340 274, 276; small Safes having rotating doors as Well as for the P 335/206, 205, 219; 200/6158; 315/34 tection of other types of doors and closure elements. Special features have been provided to resist various 56 R i Ci L forms of burglarious attack upon the protected closure UNITED STATES PATENTS element 3,161,742 12/1964 Bagno 335/206 X 9 Claims, 6 Drawing Figures DEFEAT RESISTANT BURGLAR ALARM CONTACT BACKGROUND OF THE INVENTION Burglar alarm contacts have been used with notable success for many years by the electrical protection industry to signal the unauthorized opening of doors, windows and other types of closure elements designed to protect property against theft.

The earliest types of contacts were simple mechanical devices capable of opening and/or closing electrical circuits. Shortcomings lay in their relatively large size and more particularly in the troubles resulting from corrosion of current carrying metal surfaces which dften caused burglar alarm systems to become inoperative at the most inconvenient times. The fairly recent advent of miniature switches hermetically sealed in glass envelopes and operated by the approach or withdrawal of a magnet has largely overcome the size and corrosion problems.

A particular problem remained, however, in the case of closure elements of small size as exemplified by safes having round openings with a diameter on the order of six inches. Physical size limitations make it impossible to install most of the mechanical types of contact on such safes. Another serious problem arises when attempts are made to utilize the small size magnetic type of contact for'such installations in that the magnetic contact in its simple form might be compromised by a knowledgeable burglar who may then proceed to loot the safe with no fear of a signal being transmitted by the burglar alarm system.

Past efforts to improve the defeat resistance of the magnetic type of burglar alarm contact are exemplified by United States Pat. No. 2,877,361 issued to L.H. Chase on Mar. 10, 1959, US. Pat. No. 2,912,540 issued to A.T. Sawicki on Nov. 10, 1959 and US. Pat. No. 3,187,129 issued to 1.131. McBrian on June 1, 1965. These patents all require the use of a second or biasing magnet which is employed to establish a balanced magnetic field and experience has shown that such arrangements are difficult to maintain in proper adjustment under service conditions. Still other forms of magnetically responsive switches for use in burglar alarm systems are shown in US. Pat. No. 3,621,417 issued to J.F. Halaby on Nov. l6, 1971.

Accordingly, the principal object of the present invention has been the provision of a defeat resistant burglar alarm contact. Experience has shown that it is foolhardy to refer to any protection device or system as being completely defeatproof, but the preferred contact construction of the present invention has been subjected to repeated. defeat attempts by highlytrained engineers thoroughly familiar with burglar alarm systems and has successfully resisted all such attempts.

Another object has been the provision of a burglar alarm contact of such reduced overall dimensions as to permit installation upon the round doors of small safes.

A still further object has been the provision of a burglar alarm contact that is simple in principle and that does not require difficult field adjustments.

Other and further objects, features and advantages of the invention will be apparent from the following description of the invention.

BRIEF DESCRIPTION OF THE INVENTION The invention comprises the combination of a contact block and an actuating block. The contact block is mounted on the stationary frame of the door to be protected and contains two magnetically operable transfer switches connected in series with a conventional burglar alarm system. One of theswitches is partially shielded by a plate of a high permeability metal to provide defeat resistance properties. The actuating block is mounted on the door and comprises a magnet whose withdrawal as the door is opened operates the contact block to provide an electrical signal in the burglar alarm system. In accordance with a particular feature of the invention, the field of the magnet is partially shunted by the steel door of the safe to provide resistance to a particular form of attack.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in greater detail with reference to the appended drawings in which:

FIG. 1 is a front view showing the manner of installation of the invention upon a safe having a round openmg;

FIG. 2 is a fragmentary vertical sectional view taken generally along the line 22 of FIG. 1;

FIG. 3 is a plan view in section of the actuating block assembly taken along the line 33 of FIG. 2;

FIG. 4 is a plan view in section of the contact block assembly taken generally along the line 44 of FIG. 2;

FIG. 5' is a plan view looking upward, with parts broken away, of the contact block assembly, taken along the line 5-5 of FIG. 2; and a FIG. 6 is a schematic electrical diagram showing the inclusion of thepresent invention in a burglar alarm system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Those skilled in the art will recognize that the burglar alarm contact of the present invention may be applied with equal facility to a wide variety of closure elements and that the following description thereof in connection with a roundopening safe is by way of illustration only and is not intended as a limitation upon the use of the invention.

Referring now to FIG. 1, 10 indicates a round opening safe such as the Model OOX manufactured by the Mosler Safe Company. The safe is 11 inches high, 11 inches wide and has a door opening diameter of 6 1% inches. When the proper combination has been dialed on knob 11, handle 12 and the rotatable cylinder portion 13 of the door 14 are rotated about 30 degrees and the entire door 14'swung forward on hinges 15 to reveal a round opening to the interior of the safe.

Actuating block 16 is mounted on the exposed or outside of rotating portion 13 of the safe door 14 and contact block 17 is mounted on the outside of door 14 by conventional screw fasteners as will be described in more detail hereinafter. Cable 18 connects the contact block 17 to a conventional burglar alarm system.

The contact and actuating blocks are identical in appearance (save for the cable connection) and are approximately inch deep, 2 inches wide and l 14; inches high. Blocks l6 and 17 are not sensitive to orientation in space and may be mounted at any point around the periphery of rotating portion 13 of the door as may be most convenient in the particular installation. It is only necessary to allow operating clearance for the handle 12 and to mount actuating block 16 entirely within the outer diameter of the rotating portion 13 of the door so that block 16 will clear block 17 when portion 13 is rotated.

As best shown in FIGS. 2 and 3, actuating block 16 comprises a cover 19, a magnet 20, a bracket 21 and a rectangularly shaped bar 22. Bracket 21, which may be conveniently made of light gauge aluminum stock, is secured to the rotating portion 13 of the safe by means of two screws 23. Since it is common practice to case harden safes to protect them against burglarious attack, it may not be possible to drill the holes for the screws 23. In such event, a rectangular piece of soft steel (not shown) which may be drilled and tapped is welded to rotating portion 13 to serve as a mount for the-actuating block 16.

Bracket 21 has attached thereto by means of screws 24 a rectangularly shaped bar 22. Bar 22 may be of aluminum stock and has a bore 25 drilled therein for supporting cylindrical bar magnet which is sealed therein by means of an epoxy cement or similar adhesive. Bar magnet 20 may be made from a permanent magnet alloy such as Alnico V which has a composition of aluminum 8 percent, nickel 14 percent, cobalt 24 percent, copper 3 percent, and the balance iron. The cover 19, which may be conveniently drawn from thin gauge aluminum stock, is secured to the bar 22 by two flat headed screws 26.

As best shown in FIGS. 2, 4 and 5, contact block 17 comprises a cover 27, a contact assembly generally indicated as 28, a magnetic shield 29 and a plastic electrical insulating shield-30. The contact assembly 28 consists of a panel 31 for supporting the electrical components, a base 32 and a mounting block 33. Panel 31 and block 33 are conveniently made from a phenolic composition or other nonconducting material. Base 32 may be made of light gauge aluminum stock. The panel 31 is fastened to block 33 by two screws 34 and the base 32 is fastened to block 33 by'two flat head screws 35. Assembly 28 is secured to door 14 by two screws 36. If the safe has been hardened and a soft steel plate has been welded thereto to mount the actuating block 16, as previously described, a similar plate of the same thickness should be employed to mount contact block In the embodiment shown the contact block 17 is mounted on safe door 14 so that the contact block is adjacent the actuating block 16 mounted on the rotating portion 13 of the door when the rotating portion of the door is in its normal closed and locked position, as shown in FIG. 1. It will be understood by those skilled in the art that the positions of the contact block 16 and the actuating block 17 may be reversed. It will be further understood that where the burglar alarm contact is used for other forms of openings, such as building doors, windows and the like, either the contact block or the actuating block is mounted on a fixed part of the door opening and the remaining actuating or contact block, respectively, is mounted on a movable portion of the opening so as to be adjacent the other portion of the block when the opening is in a closed position.

Two magnetically operated transfer switches SW1 and SW2 of the glass enclosed type are mounted bysix conventional terminal posts 37 on panel 31 and connected by means of a four wire cable 18 to the associated external burglar alarm circuit, as will be explained in conjunction with FIG. 6.

Contact block cover 27 is secured to mounting block 33 with two flat head screws 38 and is generally similar to actuating block cover 19 except for the cable attachment 18 and the magnetic shield 29 secured to cover 27 in such position as to partially shield switch SW2 from magnet 20.

Shield 29 may be made from a thin sheet of metal having a high magnetic permeability, such as a nickel-copper-chromium-iron alloy sold under the trademark ,Mumetal of the Allegheny Ludlum Corporation. This particular alloy has a typical composition of 77.2 percent nickel, 4.8 percent copper, 1.5 percent chromium, and the balance iron. Other materials may be used provided they have the desired characteristic of high magnetic permeability. Shield 29 may be secured to cover 27 with an epoxy cement or other suitable adhesive.

Insulating shield 30, which may be conveniently made of a plastic having good electrical insulating properties such as polyethylene terephthalate sold under the trademark Mylar polyester film by E.l. du Pont de Nemours & Co., is inserted within cover 27. Its purpose is to prevent accidental shorting of the terminals 37 by the metallic cover 27. Such shorting would be likely to occur if the cover were dented or otherwise deformed. The device will function normally without the plastic insulator as long as the cover is in its proper position. As will be appreciated by those skilled in the art, it is important to guard against accidental shorting because a false alarm could be produced thereby, resulting in unnecessary dispatching of the police and other undesirable activity. In normal practice a plastic insulator of the type described will be provided when the metallic cover 27 has little rigidity or when there is a possibility of shorting terminals 37 when removing the cover 27 while the current is on.

The burglar alarm contact of the invention may be connected to a conventional burglar alarm system of the central station type as shown in a simplified-form by FIG. 6. A central station 39, which is manned at all times by operators trained .to take appropriate action in response to incoming signals, is connected to a protected premise 40 by a transmission line 41. The line 41, which is ordinarily a telephone line leased for the purpose, may be up to approximately 20 miles in length and may pass through several telephone exchanges (not shown).

The system" is powered by battery 42 at the central station 39 which may supply a voltage in the range of from 52 to 78 volts. The negative terminal is connected to ground at 43 and the positive terminal is connected to the transmission line 41 through a break drop 44 and a ground drop 45.

The break drop 44 is a current sensitive device which responds to a drop in the line current below a predetermined value while the ground drop 45 is a similar device which responds to an increase in line current above another predetermined value. Such devices are well known in the art in the form of relays and as solid state electronic devices. In either form, operation of the devices will result in audible and visual signals to attract the attention of the operator.

Each of the spaced, series connected magnetically actuable switch elements SW1 and SW2 has a front (48, 54 respectively) and back (53, 50 respectively) electrical contact and an armature (47, 49 respectively) movable between the front and back contacts.

Each of the armatures is biased so that in the absence of a magnetic field the armature is in engagement with the back contact. Subjecting either switch to the etfect of a magnetic field will cause the armature to move from the back contact to the front contact.

Switch SW1 and magnetic shield 29 within contact block 17 are located relative to each other and to magnet 20 within actuating block 16 when the rotatable portion 13 of door 14 is in closed position (as shown in FIG. 1) so that switch SW1 is responsive to the magnetic field of magnet 20. Switch SW2, on the other hand, is located relative to magnetic shield 29 so as to be shielded from the magnetic field of magnet 20 by the magnetic shield when the rotatable portion 13 of door 14 is in closed position.

At the protected premises, the current flows from line 41 through a control set 46 of conventional design used to turn the protection system OFF and ON, through armature 47 and front contact 48 of switch SW1 which is held in the operative or front contact position by magnet 20. Switch SW2, however, is in the unoperative or back contact position by'reason of the magnetic shield 29. Thus the current path is via armature 49 and back contact 50 of switch SW2 to the other protection devices indicated as 51 to ground 52 at the protected premises 40. Back contact 53 of the switch- SW1 and front contact 54 of switch SW2 are connected together and to ground 52 by a conductor 55 which forms a shunt path.

Those skilled in the art will recognize that the, term other protection devices is intended to include other alarm contacts used to protect entrance doors and windows, foil patterns installed for the protection of glass windows and doors plus such other intrusion detection devices as floor traps, photoelectric beams and ultrasonic and microwave motion detectors as are normally used for premises protection.

It will also be understood that the alarm contact of the invention need not be. installed between the control set and the other protection devices as shown in FIG. 6. As is well known in the art, it is only necessary that there be enough resistance in the circuit so that the operation of the switches SW1 andlor SW2 will shunt sufficient resistance out of the current path to allow the current flow to increase sufficiently I to operate the ground drop '45. Furthermore, while a ground return has been shown between the protected premises and the central station, a line return could as well be emthan the central station type, such as those connected directly to a police headquarters or other signal receiving location and in local systems arranged to sound a bell or horn outside the protected premises upon the occurrence of an alarm.

OPERATION OF THE SYSTEM FIG. 6 shows the burglar alann system in the protection ON condition withcurrent flowing from battery 42 at central station 39 via transmission line 41 to protected premises 40 and thence through control set 46, contact block 17 and the other protection devices 51 to return to the central station 39 via

ground connections

52, 43.

If an attempt is now made to obtain access to the interior of safe 10 via rotation of door portion l3, the actuating block 16 will be removed from its normal location opposite contact block 17. Switch SW1 is thus freed from the field of magnet 20. Because, aspreviously explained, armature 47 of SW1 is normally biased to contact the back contact 53, armature 47 transfers from front contact 48 to back contact 53. The flow of current in the circuit is first interrupted as armature 47 leaves contact 48 which is detected and annunciated by the break drop 44 at central station 39. When armature 47 makes with back contact 53, a portion of the circuit is shunted via conductor 55 and the resulting increase in current flow is detected and annunciated by the ground drop 45 at central station 39.

Those skilled in the art will recognize that, while the magnet 20 was removed from the presence of switch SW1 by a rotational movement in the foregoing illustration, the direction of removal is not significant to the operation of switch SW1. Accordingly, the alarm contact of the invention may be employed on a wide variety of closure elements which open in various manners.

Should an attempt be made to render the protection system inoperative by severing the cable 18, a break signal will be registered by the break drop 44 as the continuity of the circuit would thus be interrupted. Furthermore, in accordance with one of the features of the invention, an intruder cannot gain access to the interior of either actuating block 16 or contact block 17 when the safe is in the closed condition because the U16 to 1/4 inch spacing between the blocks is not sufficient to permit access to the

screws

26, 38 which secure the block covers 19, 27, respectively, to bar 22 and block 33, respectively. I

A well known form of attack which has been employed successfully against alarm contacts of the magnetic type is to place an external magnet against the contact block in such manner that the field of the external magnet will hold the switch in the normal position while the actuating block is removed. The present invention is resistant against such attack by reason of the switch SW2. The introduction of an external magnet will cause switch- SW2 to operate and armature 49 thereof will transfer from back contact 50 to front contact 54. As was the case when switch SW1 was operated, the circuit will first be opened when armature 49 leaves contact 50 and then grounded when armature 49 makes with contact 54 thereby producing both break and ground signals at centralstation 39.

In accordance with a particularly important feature of the invention, resistance is provided against the type of attack in which the actuating block and the contact block are taped together and removed from the protected safe as a unit. In the illustrated embodiment of the present invention the actuating block is mounted on the movable portion 13 of the safe opening, which portion 13 is of magnetic material, preferably steel. The magnetic field of magnet 20 is partially shunted by the magnetic movable portion 13 of the door. The block mounting screws 23, 36 are not accessible being inside the block covers 19 and 27, respectively. However, if it were possible in some way to detach

blocks

16 and 17 as a unit from the safe, an alarm would still be transmitted because the field of magnet 20 is partially shunted by the steel frame of the safe when the alarm contact is properly mounted. When the magnetic shunt is removed by the withdrawal of the alarm contact from the safe, the effective field 'of magnet 20 will increase sufficiently to cause the armature 49 of the switch SW2 to move from back contact 50 to front contact 54 thus originating break and ground signals at the central station as previously described.

A further resistance to attack against the integrity of the system is afforded because the magnetic shield 29 is secured to the cover 27 as previously described. Removal of the cover 27 will result in simultaneous withdrawal of the magnetic shield 29, thus subjecting switch SW2 to the magnetic field of magnet 20. Armature 49 of switch SW2 will transfer from back contact 50 to front contact 54 producing both break and ground signals at the central station.

While the invention has been described in connection with specific embodiments thereof and in a specific use, various modifications thereof will occur tothose skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

l. A burglar alarm contact for door openings and the like comprising a contact block and an actuating block, one of which is mounted on a fixed part of the door opening and the other of which is mounted on a movable portion of said opening so as to be adjacent said one block when said opening is in a closed position, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said opening is in said closed position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said opening is in said closed position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said movable portion of said door opening is moved out of closed'position, each of said switch elements having a'front and back electrical contact and an armature movable between said front and back contacts, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element contacts the back contact thereof or when said armature of said second switch element contacts the front contact thereof.

2. A burglar alarm contact for door openings and the like comprising a contact block and an actuating block, one of which is mounted on one part of the door opening and the other of which is mounted on a second part of the door opening, said parts of said door opening being movable relative to each other between first and second positions, said contact and actuating blocks being mounted so as to be adjacent each other when said parts of said door opening are in said first position, a magnet,- meansmounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said parts of said door opening are in said first position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said parts of said door opening are in said first position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said parts of said door opening are moved out of said first position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element contacts the back contact thereof or when said armature of said second switch element contacts the front contact thereof.

3. A burglar alarm contadt for door openings and the like comprising a contact block and an actuating block, one of which is mounted on a fixed part of the door opening and the other of which is mounted on a movable portion of said opening so as to be adjacent said one block when said opening is in a closed position, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said opening is in said closed position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said opening is in said closed position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said movable portion of said door opening is moved out of closed position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, the armature of said first switch element engaging said front contact when said opening is in said closed position and the armature of said second switch element engaging said back contact when said opening is in said closed position, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal ien either said armature of said first switch element is removed from contact with said front contact thereof or when said armature of said second switch element is removed from contact with said back contact thereof.

4. A burglar alarm contact according to claim 3 wherein said magnetic shield comprises a thin sheet of metal having a high magnetic permeability.

5. A burglar alarm contact according to claim 4 wherein said magnetic shield comprises a nickel-copper-chromium-iron alloy.

6. A burglar alarm contact for metallic door openings and the like comprising a contact block mounted on a fixed part of the door opening and an actuating block mounted on a movable portion of said opening so as to be adjacent said contact element when said opening is in a closed position, said movable portion of said opening being of magnetic material, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said opening is in said closed position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said opening is in said closed position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said movable portion of said door opening is moved out of closed position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, the armature of said first switch element engaging said front contact when said opening is in said closed position and engaging said back contact when the effect of the magnetic field from said magnet is removed from said first switch element, the armature of said second. switch element engaging said back contact thereof when said opening is in said closed position and engaging said front contact thereof when influenced by a magnetic field other than from said magnet, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element moves from the front to the back contact thereof or when said armature of said second switch element moves from the back to the front contact thereof, the field of said magnet being partially shunted by the magnetic movable portion of said opening whereby removal of said magnetic shunt will increase the effective field of said magnet sufficiently to cause said armature of said second switch element to move from said back to said front contact thereof.

7. A burglar alarm contact for rotatable metallic door openings comprising a contact block mounted on one part of said door opening and an actuating block mounted on a second magnetic part of said door opening, said parts of said door opening being movable relative to each other between first and second positions, said contact and actuating blocks being mounted so as to be adjacent each other when said parts of said door opening are in said first position, a magnet, means field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said parts of said door opening are in said first position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said parts of said door opening are moved out of said first position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, the armature of said first switch element engaging said front contact when said parts of said door opening are in said first position and engaging said back contact when the effect of the magnetic field from said magnet is removed from said first switch element, the armature of said second switch element engaging said back contact thereof when said parts of said door opening are in said first position and engaging said front contact thereof when influenced by a magnetic field other than from said magnet, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element moves from the front to the back contact thereof or when said armature of said second switch element moves from the back to the front contact thereof, the field of said magnet being partially shunted by the magnetic movable portion of said opening whereby removal of said magnetic shunt will increase the effective field of said magnet sufficiently to cause said armature of said second switch element to move from said back to said front contact thereof.

8. A burglar alarm contact according to claim 7 wherein said magnetic shield comprises a thin sheet of metal having a high magnetic permeability.

9. A burglar alarm contact according to claim 8 wherein said magnetic shield comprises a nickel-copper-chromium-iron alloy.

Claims

1. A burglar alarm contact for door openings and the like comprising a contact block and an actuating block, one of which is mounted on a fixed part of the door opening and the other of which is mounted on a movable portion of said opening so as to be adjacent said one block when said opening is in a closed position, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said opening is in said closed position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said opening is in said closed position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said movable portion of said door opening is moved out of closed position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element contacts the back contact thereof or when said armature of said second switch element contacts the front contact thereof.

2. A burglar alarm contact for door openings and the like comprising a contact block and an actuating block, one of which is mounted on one part of the door opening and the other of which is mounted on a second part of the door opening, said parts of said door opening being movable relative to each other between first and second positions, said contact and actuating blocks being mounted so as to be adjacent each other when said parts of said door opening are in said first position, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said parts of said door opening are in said first position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said parts of said door opening are in said first position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said parts of said door opening are moved out of said first position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element contacts the back contact thereof or when said armature of said second switch element contacts the front contact thereof.

3. A burglar alarm contact for door openings and the like comprising a contact block and an actuating block, one of which is mounted on a fixed part of the door opening and the other of which is mounted on a movable portion of said opening so as to be adjacent said one block when said opening is in a closed position, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said opening is in said closed position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said opening is in said closed position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said movable portion of said door opening is moved out of closed position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, the armature of said first switch element engaging said front contact when said opening is in said closed position and the armature of said second switch element engaging said back contact when said opening is in said closed position, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element is removed from contact with said front contact thereof or when said armature of said second switch element is removed from contact with said back contact thereof.

6. A burglar alarm contact for metallic door openings And the like comprising a contact block mounted on a fixed part of the door opening and an actuating block mounted on a movable portion of said opening so as to be adjacent said contact element when said opening is in a closed position, said movable portion of said opening being of magnetic material, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said opening is in said closed position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said opening is in said closed position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said movable portion of said door opening is moved out of closed position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, the armature of said first switch element engaging said front contact when said opening is in said closed position and engaging said back contact when the effect of the magnetic field from said magnet is removed from said first switch element, the armature of said second switch element engaging said back contact thereof when said opening is in said closed position and engaging said front contact thereof when influenced by a magnetic field other than from said magnet, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element moves from the front to the back contact thereof or when said armature of said second switch element moves from the back to the front contact thereof, the field of said magnet being partially shunted by the magnetic movable portion of said opening whereby removal of said magnetic shunt will increase the effective field of said magnet sufficiently to cause said armature of said second switch element to move from said back to said front contact thereof.

7. A burglar alarm contact for rotatable metallic door openings comprising a contact block mounted on one part of said door opening and an actuating block mounted on a second magnetic part of said door opening, said parts of said door opening being movable relative to each other between first and second positions, said contact and actuating blocks being mounted so as to be adjacent each other when said parts of said door opening are in said first position, a magnet, means mounting said magnet within said actuating block, said contact block comprising a magnetic shield, spaced series connected first and second magnetically actuable switch elements arranged to be connected to an external alarm circuit, said first of said switch elements being located relative to said magnet and said magnetic shield when said parts of said door opening are in said first position so as to be responsive to the magnetic field of said magnet, said second of said switch elements being located so as to be shielded from the magnetic field of said magnet by said magnetic shield when said parts of said door opening are in said first position, said second of said switch elements being unresponsive to the movement of said magnet relative to said magnetic shield when said parts of said door opening are moved out of said first position, each of said switch elements having a front and back electrical contact and an armature movable between said front and back contacts, the armature of said first switch element engaging said front contact when said parts of said door opening are in said first position and engaging said back contact when the effect of the magnetic field from said magnet is removed from said firsT switch element, the armature of said second switch element engaging said back contact thereof when said parts of said door opening are in said first position and engaging said front contact thereof when influenced by a magnetic field other than from said magnet, and alarm circuit means coupled to said switch elements and arranged to transmit an alarm signal when either said armature of said first switch element moves from the front to the back contact thereof or when said armature of said second switch element moves from the back to the front contact thereof, the field of said magnet being partially shunted by the magnetic movable portion of said opening whereby removal of said magnetic shunt will increase the effective field of said magnet sufficiently to cause said armature of said second switch element to move from said back to said front contact thereof.