US3474388A

US3474388A - Terminal connector in electromagnetic assembly

Info

Publication number: US3474388A
Application number: US527292A
Authority: US
Inventors: Wilbur F Jackson; Henry C Braucksiek
Original assignee: Robertshaw Controls Co
Current assignee: Robertshaw Controls Co
Priority date: 1966-01-17
Filing date: 1966-02-14
Publication date: 1969-10-21
Anticipated expiration: 1986-10-21
Also published as: SE336829B; JPS4811536B1; NL6700599A; NL153344B; FR1507902A; ES335999A1; BE692685A; DE1673507A1; GB1181302A; JPS4920003B1

Description

- Oct. 21, 1969 W JACKSON ET AL 3,474,388

TERMINAL CONNECTOR 1N ELECTROMAGNETIC ASSEMBLY Filed Feb. 14, 1966 2 Sheets-Sheet 1 INVENTORS WILBUR F. JACKSON 8! HENRY C. BRAUCKSIEK F|G.4 BY

ATTORNEY Oct. 21, 1969 W F, JACKSON ET AI- TERMINAL CONNECTOR 1N ELECTROMAGNETIC ASSEMBLY Filed Feb. 14, 1966 2 Sheets-Sheet 2 INVENTORS WlLBUR F. JACKSON 8! HENRY C. BRAUCKSIEK BYMAKW" ATTORNEY United States Patent U.S. Cl. 339-92 16 Claims ABSTRACT OF THE DISCLOSURE An electromagnetic assembly has a centrally bored and transversely slotted supporting base with a peripheral groove adjacent the free end thereof; a terminal connector has a center portion disposed in the central bore and end portions disposed in the transverse slot and a plastic cup-shaped retainer has an annular rib disposed in the peripheral groove to retain the elements in assembled relationship. The terminal connector includes a pair of conductors spaced and insulated from each other and locked in the end portions of the connector body with central portions being exposed in opposite ends of the central bore.

The present invention relates to control devices and in particular to a thermostatic control device for preventing dangerously high temperature conditions in a heating appliance such as a hot water heater. This application is a continuation-in-part of application Ser. No. 287,061 filed June 11, 1963 now Patent No. 3,286,923.

In modern heating appliances, such as domestic hot water heaters, it is standard practice to control a supply of fuel to a main burner and to a pilot burner, with the main burner flow being thermostatically controlled and the pilot burner flow being controlled by a thermoelectric safety device that shuts off flow to both burners. The thermoelectric safety device conventionally includes a valve member operatively associated with an armature that is biased to a valve closing position but is retained in a valve opening position when an electromagnet is energized by a thermocouple responding to the flame at the pilot burner; the voltage generated by the thermocouple is great enough to hold the armature in its valve open position but is not great enough to attract the armature, so reset means is utilized to move the biased armature against the electromagnet. As long as the electromagnet is energized, the main burner flow is thermostatically controlled as by a rod and tube type thermostat.

Explosions from dangerously high temperatures being developed in the hot water tank have presented a problem which has been approached by the prior art. For example, U.S. Patent No. 2,781,977 includes a lower cycling thermostat and a thermostatic switch that is attached to the upper exterior of the hot water tank and is connected in series with the thermoelectric circuit. While this arrangement presents some protection against abnormal temperature conditions, it includes certain disadvantages such as cost of installation, being responsive only to the temperature at the top of the hot water tank, and difl'iculty in the electrical connection between the abnormal temperature switch, and the thermoelectric circuit for the safety holding magnet.

It is, therefore, an object of this invention to construct an economical, unitary assembly for the thermoelectric circuit and safety maget of a thermostatic control device having integrated normal and abnormal temperature control features.

Patented Oct. 21, 1969 Another object of this invention is to simplify the construction of an electromagnet assembly for a thermostatic control device so as to be easily connected to a heating appliance without requiring the services of a skilled technician.

The present invention has another object in that the electromagnet assembly for a thermostatic control device includes a built-in junction terminal.

The present invention has another object in that a terminal connector for connecting a safety switch in series with an electromagnet assembly is readily connected to the magnet base thereof.

Another object of the present invention is to slot the magnet base of an electromagnet assembly for the reception of a terminal connector therein.

It is another object of the present invention to retain a terminal connector on the magnet base of an electromagnet assembly so as to be ready for connection to thermocouple means associated With a heating appliance.

This invention has another object in the retaining means for incorporating a terminal connector to the magnet base of an electromagnetic assembly defines a guide for thermocouple means inserted into such magnet base.

Still another object of the present invention is to construct an electric terminal assembly from substantially identically formed half sections.

This invention has another object in that an electric terminal assembly is provided with a plurality of recesses and cavities of different dimensions for securing insulated terminal leads therein. 7

It is another object of this invention to provide an electrical terminal assembly with a mechanical strain relief for the lead wires therein.

A further object of the present invention is to retain a plurality of lead wires in an electrical terminal assembly.

In practicing the present invention, an electromagnetic assembly is constructed with a housing, electromagnetic means in the housing, support means having a portion defining a wall of the housing and a portion defining a cavity, conductor means connected to the electromagnetic means and extending through the support means into such cavity and adapted for connection toenergizing circuit means, and means on said support means cooperating with said cavity and defining terminal connection means whereby said conductor means is adapted for connection to controlling circuit means.

Other objects and advantages of the present invention will become apparent from the following description of a preferred embodiment, taken in connection with the accompanying drawing wherein:

FIG. 1 is a schematic diagram of a fuel burner control system embodying the present invention;

FIG. 2 is a side elevation of a detail of FIG. 1 with a part broken away and a part in section;

FIG. 3 is an exploded perspective view of FIG. 2, on an enlarged scale with a part added;

FIG. 4 is a central cross section of a detail of FIG. 3 on an enlarged scale;

FIG. 5 is a top plan view of the terminal assembly of FIG. 3;

FIG. 6 is an elevation view of half of FIG. 5 taken substantally along the section line 66 of FIG. 5;

FIG. 7 is a top plan view of a detail of FIG. 6;

FIG. 8 is a top plan view of another detail of FIG. 6;

FIG. 9 is an elevation view similar to FIG. 6 with parts removed; and

FIG. 10 is an enlarged cross section taken along the line 1010 of FIG. 9.

While the present invention may be applicable to various types of control devices and various types of heating appliances and may be utilized in connection with electrical heating elements as well as gaseous fuel burning heater elements, it will be described in connection with a fuel burner control system for a hot water heater.

As is illustrated in FIG. 1, the present invention is embodied in a control device including a casing, indicated generally at 10, having an inlet port 12 for receiving fuel from a gas source and communicating with a common internal passage 14 from which a pair of branch passages are controlled. One branch passage defines a pilot flow passage 16 leading to a pilot flow outlet port 18 which communicates with a conduit 20 for supplying fuel to a pilot burner 22; the other branch passage defines a main flow passage 24 that is controlled by a manually operable on-oif valve 26 and a thermostatically operated valve 28, which valves are upstream of a main flow outlet port 30 that communicates with a conduit 32 for supplying fuel to a main burner 34.

The pilot flow passage 16 communicates with the common passage 14 intermediate its opposite ends which define upstream and downstream valve seats. The downstream valve seat 36 is controlled by combined reset and valve means which includes a valve member 38 carried adjacent one end of a reset stem 40. The upper end of the stem 40 extends through a sealing collar 42 on a plunger housing 44, in which a coil spring 46 encircles the stem 40 and is mounted in compression between the collar 42 and a disc 48 on the end of the stem 40. The stem 40 is retained in the housing 44 by means of a hollow push button 50 which receives the stem disc 48 and which includes an annular bottom flange 52 engaging the housing wall surrounding an opening therefor.

The upstream valve seat 54 is controlled by an electromagnetic device, indicated generally at 56, having a safety shutoff valve member 58 movably disposed for cooperation with the valve seat 36. The valve member 58 is fixed to one end of an armature stem 60 which has its other end fixed to an armature 62 located in a fixed magnet housing 64 so that the stem 60 is slidably disposed relative to the housing 64. The housing 64 has a generally cylindrical configuration with a flattened wall portion 66; a coil spring 68 surrounds one end of the armature stem 38 and is mounted in compression between the top end wall of the magnet housing 64 and the rear surface of the valve member 58 which is thus biased to a closed position against the valve seat 54. The valve member 58, stem 60 and armature 62 reciprocate as a unit between released and attracted positions relative to fixed electromagnetic means in the form of a generally U-shaped magnet core 70 and an electric coil 72 wound thereon. One end of electric coil 72 is connected to a ground terminal 74 and the other end is connected to an electrical conductor 76.

As is shown in FIGS. 1 and 2, the bottom end wall of the magnet housing 64 is defined by support means in the form of a magnet base 78 having external threads so that the entire assembly 56 may be threaded into a suitable opening in the casing 10. In order to prevent any fuel leakage from such opening, the threads are provided with a sealing compound and a tapered sealing lip 80 on the periphery of the magnet base 78 has a seal tight relationship with the periphery of such opening.

The conductor 76 extends through the top portion of the base 78 and has a concave surface disposed in the upper part of a cavity formed in the bottom portion of the base 78. A hexagonal periphery 82 on the exterior of the base bottom portion receives a wrench or tool for tightening the assembly 56 in the threaded opening of the casing 10. Between the hexagonal periphery 82 and the lowermost end of the base 78, the exterior of the base bottom portion has an annular groove 84.

The magnet core 70 and conductor 76 are electrically insulated from each other and from the magnet base 78 by any suitable means (not shown) and all three are secured together as an integral unit. In order to connect the conductor 76 to controlling circuit means, the bottom portion of base 78 is slotted at 86 transversely to the axis of the cavity 88. The slot 86 is perpendicular to one of the flats on hexagonal periphery 82 (FIG. 3) and axially extends from the lowermost end of base 78 to the upper part of cavity 88 where conductor 76 is positioned. As is apparent from FIG. 3, that part of the slot 86, which is located upwardly of the base sealing lip 80, tapers upwardly and inwardly from the periphery towards the cavity 88. An electric terminal assembly, indicated generally at 90, includes a body member with diametrically opposed

ends

91 and 92 extending from a cylindrical center part 93; the

ends

91 and 92 are disposed in the slot 86 and the center part 93 is disposed in the cavity 88. Conductor leads 94 and 95, which extend from the terminal end 92 have connecting portions inside the terminal center part 93 as will be described more fully hereinafter.

The terminal assembly is retained in the slot 86 and cavity 88 by an annular retainer 96 having a generally cup-shaped configuration with a central aperture 97 in its bottom wall and an annular, inwardly projecting rib 98 adjacent its peripheral mouth. The retainer 96 is made of plastic material and fits over the lowermost end of base 78 with the annular rib 98 being disposed in the annular groove 84. With this arrangement the electromagnetic assembly 56 is ready for easy connection to a thermocouple cable connector.

The thermocouple cable connector includes an arcuate conductor head with a convex contact surface 100 and a threaded portion 102 which are inserted through the retainer aperture 97 and into the cavity 88 which is similarly threaded throughout its length. Inasmuch as the aperture 97 is only slightly larger in diameter than the cavity 88, the wall of aperture 97 serves as a thread guide during the insertion assembly to prevent cross threading. When the thermocouple cable connector is threaded into place, the arcuate conductor head is disposed in the lower portion of terminal center 93 whose upper portion surrounds the bottom of conductor 76. Thus, the convex surface 100 is forced into contact with the conductor of lead and the concave surface of conductor 76 is forced into contact with the conductor of lead 94.

A thermocouple cable lead 104 extends from the arcuate conductor heat to a thermocouple 106 which is positioned in the flame of the pilot burner and which has its other lead connected to a ground terminal 61. A switch 110, of any suitable type such as a thermally responsive bimetal switch, has a pair of normally closed contacts 112 and 114 which are connected to the terminal assembly leads 94 and 95, respectively. The thermocouple circuit thus defines an energizing circuit for the magnet coil 72 while the switch circuit defines a controlling circuit therefor.

To place the system of FIG. 1 in operation, the manual valve 26 is rotated to an on position and temperature setting means (not shown) for the thermostatically operated valve is moved to a selected temperature, e.g., F. for conventional domesic hot water heaters. The reset button 50 is manually depressed whereby the valve member 38 is closed on valve seat 36 to prevent any fuel flow through the main flow passage 24 during lighting and whereby the lower end of the reset stem 40 moves the valve member 58, stem 60 and armature 62 as a unit against the bias of coil spring 68 to an attracted or valve open position permitting a fuel flow through the pilot flow passage 16 to the pilot burner 22 where it is ignited as by a match. As soon as the thermocouple 106 is heated sufficiently by the pilot burner flame to energize the holding electromagnetic means, core 70 and coil 72, the pushbutton may be released whereupon the armature 62 is held in its attracted position and both

valve seats

36 and 54 are open. Inasmuch as the thermostatically operated valve 28 is open, fuel flows to the main burner 34 which is ignited by the flame of the pilot burner 22.

When the water temperature reaches the selected temperature, the valve 28 is closed and the main fuel flow is cut off and the main burner 34 is extinguished. A subsequent decrease in the water temperature, as when the hot water is drawn off and replenished with cold water, causes the valve 28 to be opened again. During normal operation the main burner 34 will be cycled thermostatically as outlined above to maintain the water temperature at 140 F.

Should the flame at the pilot burner 22 be extinguished from any cause, the thermocouple 106 will cool and the thermoelectric current to the coil 72 will cease; thereupon, the armature 62 will be released from the magnet core 70 under the bias of the coil spring 68, which closes the valve member 58 on the valve seat 54 to effect 100% shut off of any fuel flow. In order to place the system in operation again, the resetting procedure outlined above must be repeated.

In this particular burner control system, the thermostatic switch 110 functions as a thermal limit control to prevent excessively high water temperature that could cause explosion of the hot water tank. In the event the water temperature increases to a dangerously high temperature, e.g., in the range above 190 F. due to some malfunction which keeps the main burner in operation, the thermostatic switch 110 will open. When the thermostatic switch 110 is closed, the thermoelectric energizing circuit may be traced as follows: from the ground terminal 108 through the thermocouple 106, the thermocouple cable lead 104, the arcuate seat 100, the terminal assembly lead 95, the closed switch contacts 114 and 112, the terminal assembly lead 94, the conductor 76 and the magnet coil 72 to the ground terminal 74. Thus, the abnormal temperature switch 110 is serially connected in the energizing circuit to define a controlling circuit for the magnet coil 72. When the thermostatic switch 110 is open, the magnet coil 72 is deenergized causing 100% shut off of any fuel flow as described above. As soon as the water temperature decreases to its normal operating range, the switch 110 will close, however, to commence operation of the system, the resetting procedure must be repeated as outlined above.

A feature of the present invention is shown in FIGS. 5-10 wherein the electric terminal assembly 90 is formed of half sections which are substantially identical in construction. As is illustrated in FIGS. 6 and 8, the terminal assembly lead 94 (95) is stripped of its insulated covering and has a circular cross section 120 (121) at its entrance in the terminal end 92, a flattened cross section 122 (123) in the terminal center 93 and a circular cross section 124 (125) in the terminal end 91. The

sections

120, 122 and 124 are a continuation of the lead 94 and constitute a conductor contact as will become apparent hereinafter. The lead 94 (95) is a tinned, stranded wire which during the forming process rearranges the strand positions without fraying or separating the strands when formed; thus, the

rounded end sections

120 and 124 are especially adapted to act as a retention means.

An electric insulator, indicated generally at 126, is disposed flatwise between the stripped leads 94 and 95 so that its side edges are disposed in recesses formed in the two

sections

150 and 152. The insulator 126 is made of any suitable flexible material into a generally rectangular flat strip that is out along its side edges to define differently dimensioned widths. As viewed in FIG. 7, the smallest width between side edges 128 and 129 is increased by the transverse edge shoulders 130 and 131 to an intermediate width between

edges

132 and 133, which in turn is increased by the sloping edge shoulders 134 and 135 to the central and largest width between

edges

136 and 137. The left end of insulator 126 is identically formed as the right end, i.e., the central Width is decreased by the sloping edge shoulders 138 and 139 to the intermediate width between

edges

140 and 141, which in turn is decreased by the transverse edge shoulders 142 and 143 to the smallest width between edges 144 and 145.

For the sake of brevity, only the structure of section 150 will be described in detail and it is to be understood that section 152 is substantially identical to and is a mirrored detail of section 150, which permits mating the half sections into the whole assembly 90. The body section 150, molded from any suitable plastic material, has a plurality of recesses of different dimensions so as to receive the different shapes of the leads 94 and and the insulator 126. As viewed on the right side of FIG. 9, the body section has spaced cut

outs

154 and 155 for the terminal lead circular sections 120 and 121, respectively. Continuing from right to left in FIG. 9, the recesses for lead 94 will be described and those for conductor 95 referenced in parentheses since the corresponding recesses have equal configurations. Accordingly, entrance cutout 154 (155) communicates with a recess 156 (157) of larger depth, which in turn leads to a shoulder rest 158 (159), thence to recess 160 (161) that is at the same depth as recess 156 (157). The recess 160 (161) communicates with the terminal center 93 so that the terminal flat 122 (123) extends from recess 160 (161) across the opening in center 93 to recess 162 (163) which has the same depth as recess 160 (161) and which is stepped up to an end recess 164 (165) that has a lesser depth.

In order to secure the insulator between the stripped leads 94 and 95, the body section 150 has mounting recesses therefor centrally disposed between the series of spaced recesses for the stripped leads 94 and 95. As is illustrated in FIG. 9, a mounting recess 168 at the same depth as recess 156 (157 and extending between the shoulder 158 (159) provides a mounting for insulator edge 128; recess 168 is stepped inwardly by a transverse shoulder 170 leading to an intermediate recess 172 which is at a greater depth than adjoining recess 160 (161) and which extends slightly into the terminal center 93 where it terminates in an abutting shoulder 174. The shoulder 174 is substantially aligned with the central part of a chordal section 176; the left side (FIG. 9) of the body section mounting recesses are similarly arranged starting with the shoulder 178 on the edge of intermediate recess 180 which is stepped upwardly at shoulder 182 to the end recess 184. The various depths of the mounting recesses are illustrated in FIG. 10.

To assemble the terminal 90, the terminal leads 94 and 95 and the insulator 126 are deformed or bowed to con form to the angularity with which the terminal ends 91 and 92 intersect the terminal center 93. The bowed insulator 126 is mounted in the central recess with edge 128 in recess 168, shoulder 130 engaging recess shoulder 170, edge 132 in the intermediate recess 172, and

central edges

136 and 137 being disposed in the terminal center 93 between the chordal sections 176 and 177 (FIG. 5); and with the

insulator sloping edges

134 and 138 engaging the recess edges 174 and 178, respectively. The left side of insulator 126 is similarly mounted with edge 140 disposed in intermediate recess 180, shoulder 142 engaging recess shoulder 182, and edge 144 disposed in end recess 184.

Terminal lead 94 (95) is assembled in its recess with its circular portion 120 (121) extending onto rest 158 (159), its central bowed, flat portion 122 (123) extending from recess 160 (161) across the terminal center 93 into the recess 162 (163), and its rounded end portion 124 (125) being disposed in the end recess 164 (165). Adjacent its lower end, the terminal center 93 has a ridge 186 upon which a dished contact 183 is mounted so that it engages the bowed fiat portion 123 of the terminal 95.

The assembly of terminal 90 is completed by mating the body section 152 to 150 which is provided with aligning

pins

190 and 192 to be received in bores provided in the body section 152. The body section 150 differs from 152 in the provision of a peripheral sealing ridge 194 on the left end 91 and similar peripheral sealing

ridges

196 and 198 on the right terminal end 92. The entire assembly is then joined as by ultrasonic welding resulting in a sealed assembly.

It should be noted that the recesses 160 (161) and 162 (163) have decreased sloping top (and bottom) walls.

Mechanical strain relief is provided by the central bowing of the assembled terminal leads about the terminal center 93 and by the larger round leads ends 124 (125) in the restraining cavity 164 (165). In addition, the oversized recess 160 (161) provides sufficient free space so that the insulation part of the lead 94 (95) is not weakened or melted during the ultrasonic welding of the assembly.

Inasmuch as the preferred embodiment of the present invention is subject to any variations, modifications and changes in detail, it is intended that all matter contained in the foregoing description or shown on the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electric terminal assembly comprising a body member made of electrical insulating material and having a center portion and end portions,

said end portions being in opposed angular relation to said center portion,

conductor means extending into said body member and being bowed to conform to the angular relation defined by said center and end portions to assure retention of said conductor means in said body member, recess means in said body member, and

protruding means on said conductor means disposed adjacent said center and end portions and being disposed in said recess means to lock said conductor means in said body member.

2. The combination as recited in claim 1 wherein said a conductor means comprises a pair of terminal leads and wherein an insulator strip is disposed between said pair of terminal leads.

3. An electric terminal assembly comprising a body member made of electrical insulating material and having a center portion and end portions, said end portions being in opposed angular relation to said center portion, conductor means extending into said body member and being bowed to conform to the angular relation defined by said center and end portions to assure retention of said conductor means in said body member, said conductor means including a pair of terminal leads having an insulator strip disposed therebetween, recess means in said body member, and means on said conductor means disposed in said recess means to lock said conductor means in said body member, said pair of terminal leads each comprising a tinned stranded wire. 4. An electrical terminal assembly comprising a body member made of electrical insulating material and having a center portion and end portions, said end portions being in opposed angular relation to said center portion, conductor means extending into said body member and being bowed to conform to the angular relation defined by said center and end portions to assure retention of said conductor means in said body member, said conductor means including a pair of terminal leads having an insulator strip disposed therebetween, recess means in said body member, and means on said conductor means disposed in said recess means to lock said conductor means in said body member, said body center portion being an open cylinder and said pair of terminal leads having flat conductor portions extending from said body end portions across said body center portion to define spaced contact surfaces in the opposite end of said open cylinder. 5. The combination as recited in claim 4 wherein each conductor portion is a continuation of its terminal lead. 6. The combination as recited in claim 4 wherein said recess means includes a series of recesses of different dimensions for each terminal lead and said body member includes a series of mounting recesses for said insulator strip to assure locking of said terminal leads and said strip in assembled relationship.

7. The combination as recited in claim 6 wherein said series of recesses includes a pair of oversized recesses defining end restraining cavities, and said pair of leads have rounded ends disposed in said restraining cavities.

8. The combination as recited in claim 7 wherein said series of recesses include a pair of enlarged recesses for entrance portions of said pair of leads and said body member comprises a pair of aligned mating half sections of plastic material, and sealing means for sealing the body half sections in assembled relationship.

9. The combination as recited in claim 8 wherein said sealing means comprises a sealing rib extending substantially around the periphery of one of said body half sections.

10. The combination as recited in claim 9 wherein said sealing means further includes an ultrasonically welded joint to integrate said body half sections into a unitary assembly.

11. An electromagnetic assembly comprising a housing,

electromagnetic means in said housing,

a supporting base for said electromagetic means having an upper portion defining a wall of said housing and a bottom portion defining a cavity with a dimension less than said bottom portion,

conductor means connected to said electromagnetic means and extending through said supporting base into said cavity,

means defining a slot in the bottom portion of said supporting base transversely cooperating with said cavity to facilitate insertion and removal of a terminal assembly relative to said cavity,

peripheral groove means on the bottom portion of said base, and

a retainer member having peripheral rib means disposed in said groove means for retention on the bottom portion of said base and having a bottom wall with an annular part covering an open end portion of said slot whereby a terminal assembly may be retained in said slot, said bottom wall having a central open ing therein with a dimension greater than the dimension of said cavity whereby a conductor may be inserted through said central opening into said cavity.

12. The combination as recited in claim 11 wherein said slot means extends completely across the bottom portion of said base.

13. An electromagnetic assembly comprising a housing,

electromagnetic means in said housing,

a supporting base for said electromagnetic means having an upper portion defining a wall of said housing and a bottom portion defining a cavity,

peripheral groove means on the bottom portion of said base, and

a retainer member having peripheral rib means disposed in said groove means for retention on the bottom portion of said base and having an annular wall part covering the open end portions of said slot whereby a terminal assembly may be retained in said slot,

said retainer member includes a cup-shaped plastic ele ment being flexible to permit removal from said peripheral groove means.

14. The combination as recited in claim 13 wherein the bottom portion of said base includes a tool receiving 75 head.

9 10 15. The combination as recited in claim 4 wherein 2,353,042 7/1944 Koch 23621 a terminal assembly is retained in said bottom portion and 2,678,774 5/ 1954 Arvin 23621 includes a center portion disposed in said cavity and end 2,987,919 6/1961 Kirby. portions disposed in said slot.

16. The combination as recited in claim 15 wherein MARVIN A. CHAMPION, Primary Examiner said terminal assembly comprises spaced conductor mem- 5 JOSEPH H. MCGLYNN, Assistant Examiner bers insulated from each other in said center portion.

US. Cl. X.R. References Cited 339 217 UNITED STATES PATENTS 10 1,950,717 3/1934 Douglas 339210 2,318,822 5/1943 Wantz.