US3197594A - Thermo-sensitive switch plug - Google Patents

Description

D. J. SCHMITT THERMO-SENSITIVE SWITCH PLUG July 27, 1965 2 Sheets-Sheet 1 Filed Dec. 28, 1962 III:

s9 INVENTOR. DONALD J. SCHMITT BY M 14/23 FIG. 3

ATTORNE YS July 27, 1965 D. J. SCHMITT 3,197,594

THERMO-SENSITIVE SWITCH PLUG Filed: Dec 28,. 1962 2 Sheets-Sheet 2 ll FIG. 6

FIFG: 5. 2O

MIC FIG. 8

INVENTOR. DONALD J. SCHMITT ATTORNEYS United States Patent ce O-Disc, incorporated, Manstieid, (ihio, a corporation of Ghio nine Dec. as, 1962, Ser. Ne. 247,9ss 4- Claims. ea. sea-teas This invention relates to an electrical fitting of the type employed in attaching an electric cord to a conventional outlet or source of power embodying an electric circuit breaker or switch therein and more particularly to a connector plug having a thermo-sensitive switch as an integral part thereof which is responsive to excessive heat produced by current overloads for interrupting the flow of current through the fitting.

Heretofore, it has been the practice to provide protective devices for electric motors used with various types of tools and appliances to avoid the effects of overheating resulting from current overloads. However, this practice was substantially confined to motors of a size in which the protective device could be easily or economically incorporated in the motor or in close proximity to the windings thereof. Thus, such protective devices were used primarily with medium and larger sized motors. Considerable trouble has been experienced heretofore with motors used in connection with small hand power tools and similar appliances. The improper use or abuse of such tools and appliances has contributed greatly to the burning out of the motors and damage to essential operating elements of the devices. The relatively small size of the motors used in connection with such devices precluded the use of the customary protective devices. It was, therefore, desirable that suitable protective means be developed and provided for motors used with power tools and other appliances where current overloads might be experienced.

It is among the objects of the present invention to provide a new and improved means for overcoming the dirfi culties heretofore experienced in protecting motors employed in operating small hand tools and similar appliances.

Another object of the present invention is to provide a device for protecting electrical motors employed in operating small hand tools and similar appliances which causes interruption of the circuit only after a sustained overload of sufficient duration effective to damage the motor windings and avoids nuisance tripping of the circuit control means in response to overloads of short duration resulting from harmless current surges.

Another object of the present invention is to provide an electrical plug embodying a current overload responsive switch, which includes a thermal element having two positions of concavity, which element is free to pass automatically from its first position of concavity to its second position upon a rise in temperature and is effective to open the switch contacts in its second position and which may be returned to its first position only by manually operated reset means.

Another object of the invention is to provide cal plug embodying a thermostatic switch having manually operable reset means therefor, wherein the switch mechanism is trip-free of the manual resetting means.

A still further object of the invention is to provide an electrical fitting having a thermo-sensitive switch which because of its simplicity, economy of construction and reliability and efliciency in operation is particularly well adapted as a protective device in controlling electric circuits wherein the current is employed for operating purposes in small hand power tools and appliances.

These and other objects and advantageous features of an electri- 3,197,594 Patented July 27, 1965 the invention not at this time more particularly pointed out will become apparent as the nature of the invention is better understood from the following detailed description taken in conjunction with the accompanying drawings wherein like reference characters denote corresponding parts and wherein:

FIGURE 1 is a plan view of the electrical fitting of the present invention showing the operating button of the manual reset means projecting through the housing of the fitting,

FIGURE 2 is a longitudinal sectional View taken on the line 22 of FIGURE 1 illustrating the elements of the device,

FIGURE 3 is a transverse sectional view taken on the line 33 of FIGURE 2 showing the contacts of the thermostatic switch in closed position,

FIGURE 4 is a transverse sectional View taken on the line 3-3 of FIGURE 2 showing the contacts of the thermostatic switch in open position,

FIGURE 5 is a transverse sectional view taken on the line 3-3 of FIGURE 2 and illustrating the manual reset mechanism in depressed position, such depression having restored the thermal element to its initial position of concavity from the position shown in FIGURE 4,

FIGURE 6 is a longitudinal sectional view of the device taken on the line 6-6 of FIGURE 2 and illustrating the mounting of the thermal disc and contacts and terminal prongs within the device,

FEGURE 7 is a longitudinal sectional view taken on the line 7-7 of FIGURE 2 showing the mounting of the heater element and the electrical connections thereto within the interior of the housing of the device, and

FIGURE 8 is an end elevational view taken on the line 8-8 of FIGURE 1.

Referring more particularly to the drawings there is illustrated a device embodying the present invention which takes the form of an electrical plug for insertion into a conventional wall receptacle. A molded housing for the plug, broadly indicated by the numeral 10 contains the switching assembly for controlling the circuit through the plug and is preferably formed of a suitable electrical insulating material which is heat resistant to a fair degree such as a phenolic condensation product or the like. EX- tending from one end of the housing 10 are two prongs 11 and 12 and grounding pin 13 arranged to engage within a standard wall outlet, and from the opposite end a connecting cord 14 having three insulated conductors 15, 16 and 17, as shown in FIGURE 1.

The plug housing it) is of substantially rectangular shape and comprises two mounting parts, one of which may be termed a hollow base member 18 and the other a hollow cover member 119. In assembled relation the base 13 and cover 19 define a housing chamber 26 having opposed longitudinal side walls and end walls. One end wall is formed with a cylindrical opening 21 therethrough to receive the connecting cord 14 and the other end wall is arranged with three rectangular slots 22, 23 and 24 extending therethrough for receiving the prongs 11 and 12 and grounding pin 13, respectively, projecting outwardly therefrom.

The cover member 19 is secured to the base member 18 by machine bolts 25 and 26 extending through opposed aligned circular openings 27 and 23 in the housing members l8 and 39 and which engage nuts (not shown) imbedded in the outer surface of the base member 18. The cover member 1% has opposed side walls 29 and 30 (FIG- URE 6) preferably molded to provide a pleasing outer apperance. The side Walls 2h and 3d and end Walls 31 and 32 define a rectangular cavity 34, within the cover for receiving and positioning the elements of the switch.

Located in the top surface 33 of the cover member 19 (FIGURE 1) is a circular recess 35 for receiving a manually actuated reset button as, to be described hereinafter. Surrounding the recess 35 is an upstanding cylindrical wall 37 extending from the top surface of the cover 19 as shown in FIGURES 1 and 2 and serving to protect the reset button 36 from being accidentally depressed. Extending from the base of the recess 35 and concentric therewith is a circular recess 33 of smaller diameter which terminates in a bore 39 extending through the housing into the cavity 34 of the cover member as best shown in FIG- URE 2. Surrounding the recess 3? and projecting upwardly into the recess 35 is a circular wall member til.

Projecting from the inner wall surface of the recess 35 and located within the rectangular cavity 34 of the cover member 19 is a circular wall 41 defining a cylindrical chamber 42 concentric with the circular recess 35 in the top surface of the cover 19 and adapted to receive the switching assembly hereinafter described. Integrally formed with the base of the cylindrical chamber 4-2 are diametrically extending upstanding wall members 43 which form opposed guadrants 44 and 45, the former having openings 46 and 47 (FIGURE extending therethrough and the latter an upstanding integrally formed post 48 (FIGURE 2). The opening 4-6 receives the shank portion of a rivet 49 which has formed on the outer end thereof a fixed contact button 541?. Similarly, the opening 47 receives the rivet 51 having formed on its outer end the fixed contact button 52. The circular wall 41 of the chamber 42 is provided with two rectangular openings 53 and 54 at approximately 90 apart for receiving current carrying members also forming parts of the switch assembly. The switch chamber 42 is closed at its lower end by a removable cup-shaped closure member 55 which fits snugly within the wall 41 of the chamber 42. Formed integrally with the wall 41 and extending outwardly therefrom is a wall member 56 which terminates in a rectangular supporting post 57 upon which the inner end of the grounding pin 13 rests. A circular bore 57a is formed in the top thereof to receive the shank of the terminal screw 58 thereaded through the end of the grounding pin 13 for attaching the ground lead thereto.

The base member 18 has longitudinal wall-s 59 and dtl and end walls 61 and 62 forming a rectangular cavity 63 as shown in FIGURE 7. Located at the line cord end of the cavity 63 is a rectangular recess d4 (FIGURE 2) for receiving the end of a ground lead 17 which is attached to the grounding pin 13 by a terminal screw 58. Extending from the recess 64 to the end wall 61 of the base member 18 there is formed a rectangular chamber 65 for receiving the grounding pin 13.

One insulated conductor 15 from the line cord 14 is attached by a terminal screw 66 to the angulated end of the terminal prong 11 within the housing 18. As shown in FIGURE 6 the terminal prong 11 is positioned within the cover member 19 and extends from its angulated inner end along the inner surface of the side wall 29 and is bent at right angles thereto along the inner surface of end wall 32 and then reversely bent and projected outwardly through the slot 23. The prong 11 is retained in position by a raised shoulder portion formed on the end wall 61 of the base member 18 which engages within the open end of the slot 23.

The other insulated conductor 16 of the line cord 14 is secured by a terminal screw 67 to an L shaped conductor arm 68 which is mounted on the cover member 19 by means of a suitable rivet 69. The terminal screw 67 extends through a threaded boss 70 in one leg of the L shaped arm 68 and the shank end thereof may be received in a cylindrical recess 71 formed in the cover member. The other leg of the L shaped conductor arm 68 extends through the opening 53 in the circular wall 41 and terminates in the switch chamber 42. Secured to the leg of the conductor arm within the switch chamber 42 is one end of a heating element 72 formed of a suitable electric resistance material which becomes heated to a desired temperature when a current overload is experienced in the circuit. The heater element may be bent into any desired shape and is of a length sufficient to provide the desired resistance. I have found that the element 72 should be formed of a material of the proper gauge and composition to provide the desired related overload time and temperature characteristic fer the desired results. Usually an overload condition must exist for a period of time before enough heat will be generated to cause damage to the windings in an electrical motor. Therefore the heater element is selected which will generate sufiicient heat to be effective on the switch elements to break the circuit before this time has elapsed. On the other hand, nuisance tripping caused by instantaneous current surges is to be avoided. The other end of the heating element 72 is secured to the leg 73 of a sheet metal strip 74, which leg projects into the switch chamber 42. The heating element 72 is situated within the chamber 55 as defined by the cup-shaped closure member 55. The sheet metal strip '74 is fastened to the closure member 55 of the switch chamber 42 by tabs 75 which are wrapped around the rim 76 of the closure member. The metal strip 74 engages the upstanding leg 77 of an angulated conductor member 78 and provides circuit continuity therewith. The leg 77 of the conductor member '78 is positioned against the wall of the cylindrical chamber 4-2 as shown in FIGURE 5 and the other leg extends inwardly from the wall and is secured in position by the rivet 49 below the contact button 5t) formed on the end thereof.

The rivet 51 carrying the contact button 52 anchors in position the laterally extending end 79 of the prong 12 which passes through the opening 54 in the circular wall 41. The prong 12 engages the side wall 3t) of the cover member 19 to the end wall 32 where it bends at right angles thereto along the inner surface of end wall 32 and is reversely bent and projected through the slot 22. Similar to prong 11, prong 12 is retained in position within the slot 22 by a raised shoulder portion formed on the end wall 61 which engages within the open. end of the slot 22.

Mounted within the cylindrical chamber 42 is a switching assembly arranged to provide a thermal actuated breaking of the circuit when a predetermined temperature is reached by reason of a current overload causing heating of the element 72 to generate an excessive amount of heat. Extending through the circular opening 39 at the center of the recess 38 is an internally threaded tubular shaft 8t having a circular flange which bears against the margin of the opening 39 on the inner surface of the cover member 19. The shaft fit) is slidably mounted in the opening to be capable of occasional movement inwardly into the cylindrical switch chamber 42 within selected limits. The shaft 89 threadedly receives an adjustable mounting screw 81 which extends beyond the flanged end thereof and has a shouldered end forming mounting post 81.

The post 81 serves to provide a center mount for a snap acting thermo-sensitive, bi-metallic disc 32 having two positions of concavity and which is so positioned in the cylindrical switch chamber 42 as to snap from one position of concavity to the second position in response to a rise in temperature. The disc %2 is of a proper size, composition and configuration to provide the desired thermal responsive characteristics. This will depend to a great extent upon the particular heater element used as the relative reaction of these two members will determine the time delay between the commencement of the overload condition and breaking of the circuit. Such a delay is necessary to prevent nuisance tripping caused by instantaneous current surges in the circuit. A circular opening in the center of disc $2 receives the post 81 and the end thereof is riveted over to fasten the disc thereto. The disc 32 is formed with an opening 32a which receives the end of post 48, the latter serving to guide the disc in its movement within the switch chamber.

ttached to the bottom face of the disc 82 in diametric relation are mobile contact elements 83 and M, which are arranged to engage the stationary contacts 5t? and 52;, respectively, when the disc is in its switch closed position. Under current overload condition. however, the disc becomes heated to a predetermined temperature by the heating element '72 and snaps to its second position of concavity thus breaking the circuit as shown in FIGURE 4. The reset button 36 is positioned in the circular recess 35 in the cover member 1% as best shown in FIG- URES 1 and 2 and has a circular opening centered therein which receives the upper end of the tubular shaft 8%. Bearing against the lower surface of the reset button 36 is a helical spring 85 which is positioned around the shaft and rests against the bottom wall of the circular recess 38. The button 36 is secured to the tubular shaft 3%) by a retaining ring 86 which is pressed on the end of the shaft. A reset button cover plate 87 fits on the top of the button and conceals the shaft and retaining ring. The button is thus spring loaded in an upward position but may be manually depressed to push the bi-metallic disc through the center into its switch closed position of concavity.

To attach the plug to the end of a line cord, the unit may be disassembled by loosening the screws 25 and 26 and removing the cover. The conductors l5? and 16 from the line cord are then suitably looped at their ends and placed around the loosened terminal screws 66 and 67 with the ground lead 17 attached to screw 58 so as to be in engagement with the grounding pin member 15. The terminal screws may then be tightened to secure the conductors in position. The cover member 19 and base member 18 are reassembled and the device is ready for use.

In the operation of the device, in the event of a current overload, the element 72 would provide resistance to current flow sufiicient to generate a rise in temperature which would heat the cylindrical chamber 42 to the critical temperature of the bi-metallic element. This series of events would not occur instantaneously but would require definite periods of time so that a temporary overload would not cause the circuit to be broken. After a suitable time delay, the heat radiated by the element causes the bi-metallic disc 82 to snap to its switch open position as shown in FIGURE 4. Once the bimetallic disc has snapped to this open switch position, it will remain so even though the temperature of the element returns to normal since the forces in the element are insuflicient to snap it over center under normal temperature conditions.

Once the element has reached normal temperature, however, the bi-metallic disc may be manually snapped back into its switch closed position by pressing the reset button 36. This causes the disc to be forced downward at it center as shown in FIGURE 5. In so doing, the outer edge of the disc is brought into engagement with the wall of the closure member 55' and further pressure on the central portion of the disc moves it through center, and the disc is caused to snap back to its initial position of concavity. When the button 36 is released, the mobile contact buttons 83 and 84 are engaged with the fixed contacts Stl and 52, respectively, and the switch is closed.

It will be noted, however, that if the element '72 has not cooled down sufficiently when the reset button 36 is released the bi-metallic disc will immediately return to its switch-open position of concavity. Release of the button 36 will therefore not close the switch. This feature prevents closing the circuit when an overloaded condition continues to exist in the line.

The choice of bi-metal used for the disc 82 and the temperature set applied thereto are critical factors in providing a device having the desired current overload time and temperature responsive characteristics. Also, the choice of wire gauge, length and composition for the heater element 72 is important so that the delay in the operation of the switch element is effective to preclude nuisance tripping of the circuit under instantaneous overloads or line surges and so that the device is effective only when the line is subjected to an overload of suflicient duration to threaten to cause damage to the motor windings of the electrical appliance or tool with which the plug is used.

It is to be understood that in accordance with the patent statutes, variations and modifications of the specific device herein shown and described may be made without departing from the spirit of the invention.

Having described my invention, I clam:

it. An electric plug for attaching a line cord from a small hand power tool or other electrical appliance to an electric power outlet comprising: a rectangular base member a cover member therefor fitting together to form an elongated plug housing of electric insulating material defining a cavity, a pair of prongs of conductive metal mounted in said housing on opposite longitudinal sides of said cavity having portions projecting outwardly generally parallel to the opposite longitudinal sides of said housing, said housing having slots at one end to receive said prongs and a line-cord-receiving opening at its opposite end, a pair of laterally spaced line-cord-receiving terminals mounted within said housing adjacent said linecord-receiving opening, wall portions within said cavity defining a circular chamber having a lateral opening and a longitudinal opening connecting said chamber with said cavity, one of said terminals being electrically connected to one of said prongs, means for carrying an electric current through the other of said prongs to the other of said terminals including switch means connected within said chamber and having a stationary contact and a mobile contact and means for opening said switch means including a bi-metallic disc operably connected to said mobile contact to move the same into or out of contact with said stationary contact and an electric heating element mounted in said chamber adjacent said disc and receiving an electric current from one of said terminals, said disc having two stable positions of opposite concavity and holding the contacts together to close the circuit through said current carrying means when in the first of said stable positions and moving the contacts out of contact when heated suiriciently to snap to the other stable position, said disc being mounted with its central axis generally parallel to the longitudinal side walls of the plug and generally perpendicular to the base and cover members of said plug housing, and means for electrically connecting said switch means with said other prong through said lateral opening and with said other terminal through said longitudinal opening and a spring-loaded manual reset button engaging the central portion of said disc and projecting out of the cover member of said housing for moving said disc to said first stable position.

2. An electric plug for attaching a line cord from the motor of a small electric appliance to an electric power outlet comprising: a rectangular base member and a cover member therefor fitting together to form a thin elongated plug housing of electric insulating material defining an internal cavity and narrow side walls, a pair of spaced prongs of conductive metal mounted in said housing and having end portions projecting outwardly from one end of the housing generally parallel to said side walls, said housing having a line-cord-receiving opening at the opposite end of the housing, a pair of laterally spaced linecord-receiving terminals mounted within said housing adjacent said line-cord-receiving opening and electrically connected to said prongs, wall portions within said cavity defining a chamber having a lateral opening and a longitudinal opening connecting said chamber with said cavity switch means in said chamber for making and breaking the electrical circuit through the plug comprising a pair of fixed contacts, a pair of movable contacts, and a snap acting bi-metallic disc supporting said movable contacts at its opposite sides and conducting current between said movable contacts when the switch means is closed, an electric heating element mounted in said housing adjacent said disc to receive current from one of said terminals, said disc having two stable positions of opposite concavity and being mounted in said chamber between said side walls generally parallel to the base and cover members of said housing, said disc holding the movable contacts against said fixed contacts to close the said electrical circuit when in the first of said stable positions and moving the contacts out of engagement to break said electrical circuit when heated sufl'lciently to cause it to snap to the other stable position, means for electrically connecting said switch means with one of said prongs through said lateral opening and with one of said terminals through said longitudinal opening, means comprising a manual reset button connected to the central portion of said disc and projecting through the cover member of said plug housing for moving said disc to said first stable position in a direction to close said switch means, and means biasing said reset button and said disc in a direction to close said switch means.

3. An electric plug for attaching a line cord from the motor of a small power tool or other electrical appliance to a conventional electric power outlet comprising: a thin generally rectangular base member therefor and a thin generally rectangular cover member fitting together to form an elongated plug housing of electric insulating material with a length corresponding to the length of said members, said housing defining a cavity with two prongreceiving openings at one end and a line-cord receiving opening at the opposite end, a pair of prongs of conductive metal mounted in fixed positions in said housing on opposite sides of said cavity and projecting outwardly through said prong-receiving openings generally parallel to the sides of said housing, a pair of laterally spaced line-cord receiving terminals mounted in said cavity adjacent said line-cord receiving opening, wall portions within said cavity defining a chamber having a lateral opening and a longitudinal opening connecting said chamber with said cavity, one of said terminals being electrically connected to one of said prongs, means for carrying an electric current through the other of said prongs to the other of said terminals including switch means mounted within said chamber and having stationary contacts and movable contacts and means for opening said switch means comprising a generally circular bi-metallic disc connected to said movable contacts to move the same into or out of contact with said stationary contacts and an electric heating element mounted in said chamber adjacent said disc to receive an electric current from one of said terminals, said disc having two stable positions of opposite concavity and holding the movable contacts against said stationary contacts to close the circuit through said current carrying means when in the first of said stable positions and moving the contacts out of engagement to break said circuit through said current carrying means when heated sufliciently to cause it to snap to the other stable position, said disc being mounted in said chamber generally parallel to the cover member, and means for electrically connecting said switch means with other of said prongs through said lateral opening and with other of said terminals through said longitudinal opening, a fixed stop mounted in said cavity on one side of said disc for engaging the marginal portion of said disc, and a manual reset button mounted on said cover member and connected to the central portion of said bimetallic disc on the side thereof opposite said stop for pressing said disc against said stop in a direction to open said switch means to return the disc to said first stable position, said reset button projecting through the said cover member.

An electric plug for attaching a line cord from the motor of a small power tool or other electrical appliance to a conventional electric power outlet comprising: a thin generally rectangular base member and a thin generally rectangular cover member therefor fitting together to form an elongated plug housing of electric insulating material with a length corresponding to the length of said members, said housing defining a cavity with two prong-receiving openings at one end and a line-cord receiving opening at the opposite end, a pair of prongs of conductive metal mounted in fixed positions in said housing on opposite sides of said cavity and projecting outwardly through said prong-receiving openings generally parallel to the sides of said elongated plug housing, a pair of laterally spaced line-cord receiving terminals mounted in said cavity adjacent said line-cord receiving opening, wall portions within said cavity defining a chamber having a lateral opening and a longitudinal opening connecting said chamber with said cavity, one of said terminals being electrically connected to one of said prongs, current carrying means for connecting the other of said terminals in series with the other of said prongs comprising switch means mounted in said chamber between said cover memher and said base member and having a pair or" stationary contacts, a pair of mobile contact buttons, and a snapacting bi-rnetallic disc supporting said mobile contact buttons at diametrically opposed portions of the disc, an electric heating element mounted in said chamber adjaent said disc to receive an electric current from one of said terminals, said disc having two stable positions of opposite concavity and holding the mobile contact buttons against said stationary contacts to close the circuit through said current carrying means when in the first of said stable positions and moving the contacts out of engagement to break said circuit through said current carrying means when heated sufficiently to cause it to snap to the other stable position, said disc being mounted in said chamber generally parallel to the base member and cover member, means for electrically connecting said switch means with said other of said prongs through said lateral opening and with said other of said terminals through said longitudinal opening, a fixed stop mounted in said chamber on one side of said disc for engaging the marginal portion of said disc, and a manual reset button connected to the central portion of said bi-metallic disc on the side thereof opposite said stop for pressing said disc against said stop in a direction to open the switch means to return the disc to said first stable position, said reset button projecting through the said cover member.

References Cited by the Examiner UNITED STATES PATENTS 2,199,307 5/40 Bolesky 200-122 X 2,427,390 9/47 Dear 200-136 2,783,329 2/57 Jackson 200 2,839,638 6/58 Epstein 200-113 2,911,503 11/59 Garbcrs 200122 BERNARD A. GILHEANY, Primar 1 Examiner.

ROBERT K. SCHAEPER, Examiner.

Claims (1)

1. AN ELECTRIC PLUG FOR ATTACHING A LINE CORD FROM A SMALL HAND TOOL OR OTHER ELECTRICAL APPLIANCE TO AN ELECTRIC POWER OUTLET COMPRISING: A RECTANGULAR BASE MEMBER AND A COVER MEMBER THEREFOR FITTING TOGETHER TO FORM AN ELONGATED PLUG HOUSING OF ELECTRIC INSULATING MATERIAL DEFINING A CAVITY, A PAIR OF PRONGS OF CONDUCTIVE METAL MOUNTED IN SAID HOUSING ON OPPOSITE LONGITUDINAL SIDES OF SAID CAVITY HAVING PORTIONS PROJECTING OUTWARDLY GENERALLY PARALLEL TO THE OPPOSITE LONGITUDINAL SIDES OF SAID HOUSING, SAID HOUSING HAVING SLOTS AT ONE END TO RECEIVE SAID PRONGS AND A LINE-CORD-RECEIVING OPENING AT ITS OPPOSITE END, A PAIR OF LATERALLY SPACED LINE-CORD-RECEIVING TERMINALS MOUNTED WITHIN SAID HOUSING ADJACENT SAID LINECORD-RECEIVING OPENING, WALL PORTIONS WITHIN SAID CAVITY DEFINING A CIRCULAR CHAMBER HAVING A LATERAL OPENING AND A LONGITUDINAL OPENING CONNECTING SAID CHAMBER WITH SAID CAVITY, ONE OF SAID TERMINALS BEING ELECTRICALLY CONNECTED TO ONE OF SAID PRONGS, MEANS FOR CARRYING AN ELECTRIC CURRENT THROUGH THE OTHER OF SAID PRONGS TO THE OTHER OF SAID TERMINALS INCLUDING SWITCH MEANS CONNECTED WITHIN SAID CHAMBER AND HAVING A STATIONARY CONTACT AND A MOBILE CONTACT AND MEANS FOR OPENING SAID SWITCH MEANS

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