Abstract:
An improved structure of an electric connector of a gas sensor is provided which is designed to establish a firm electric connection between each electrode terminal formed on a sensing element and one of plural leads extending outside of the gas sensor for connection with an external device. The connector has a stopper which is placed in contact with an end wall of an insulating member to keep the connector on the insulating member without the connector dropping into a hole through which a lead extends from the sensing element when the lead is coupled to the connector. This allows the connector to be pressed and welded to secure the lead in a constant position relative to the insulating member, thus improving the stability of the sensor signal to be received at the external device.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine for air-fuel ratio control, and more particularly to an improved structure of an electric connector used in such a gas sensor. 
     2. Background Art 
     EP0506897 discloses a typical laminated gas sensor. This sensor has the disadvantage that a structure for providing a sensor signal to an external device is complex and results in instability of the sensor signal. The present invention is directed to an improved structure of a gas sensor which is capable of outputting a sensor signal accurately. 
     SUMMARY OF THE INVENTION 
     An oxygen concentration measuring gas sensor, as will be described below in detail, may be proposed as being installed in an exhaust system of an internal combustion engine for air-fuel ratio control. 
     The gas sensor includes a sensor element, a sensor element side insulation porcelain, an atmospheric side insulation porcelain, a hollow cylindrical housing, and an atmospheric cover. The sensor element side insulation porcelain is made of a cylindrical member and retains therein the sensor element. The atmospheric side insulation porcelain is disposed in alignment with the sensor element side insulation porcelain to cover a base portion of the sensor element. The housing retains therein the sensor element side insulation porcelain. The atmospheric cover is disposed on a base end of the housing to surround the atmospheric side insulation porcelain. 
     The atmospheric side insulation porcelain has disposed therein a plurality of leads  16 , as shown in FIG. 17, connected electrically with the sensor element. Each lead  16  is coupled through a hollow connector  92  electrically to a lead  17  extending outside of the gas sensor. 
     Each lead  16  is, as clearly shown in FIG.  16 ( b ), inserted into one of the connectors  92  from an end  920 , while each lead  17  is, as shown in FIG. 17, inserted into the connector  92  from the other end in alignment with the lead  16 . Each of the connectors  92  is, as clearly shown in FIG.  16 ( a ), made of a flat plate bent to a rectangular shape in cross section which is smaller than holes  322  formed in the atmospheric side insulation porcelain  32 . 
     The above structure, however, encounters the drawback in that the connectors  92  may, as shown in FIG. 17, drop partly into the holes  322 , which leads to a difficulty in pressing and welding desired portions of the connectors  92  to join them to the leads  16 . This problem may be avoided by decreasing the diameter of the holes  322 , but it will result in a difficulty in passing the leads  16  through the holes  322 . The holes  322  also work as a reference gas induction passage for supplying a reference gas (i.e., air) into a reference gas chamber. The decrease in diameter of the holes  322 , thus, also results in a decrease in amount of the reference gas admitted into the reference gas chamber. 
     It is therefore an object of the invention to provide an improved structure of an electric connector of a gas sensor which is designed to ensure the reliability of electric connection between a sensing element and an external device. 
     According to one aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a sensing element having a length which includes a sensing portion and a base portion, the sensing element being retained in the housing with the base portion projecting from the housing; (c) a hollow insulating member provided so as to surround the base portion of the sensing element, the insulating member having a wall in which holes are formed; (d) a cover covering the insulating member; (e) first leads each having a first end portion and a second end portion, each of the first end portions being in electric contact with the sensing element within the insulating member, each of the second end portions passing through one of the through holes and projecting outside the insulating member; (f) second leads each having a first end portion and a second end portion, the first end portion extending outside of the gas sensor for electric connection with an external device; and (g) connectors establishing electric connections between the first leads and the second leads, respectively, each of the connectors having a lead joint portion and a stopper, each of the lead-joint portions electrically joining the second end portion of one of the first leads and the second end portion of a corresponding one of the second leads, each of the stoppers being placed in contact with the wall of the insulating member. 
     In the preferred mode of the invention, each of the stoppers is provided by an extension formed on an end of one of the connectors. 
     Each of the stoppers may be provided by a portion of one of the connectors which extends outward. 
     Each of the stoppers may have a tapered end wall for ease of insertion of the second end portion of one of the first leads. 
     Each of the stoppers may have walls which define a rectangular shape in cross section and have ends expanding outward so that the ends are placed outside one of the holes of the insulating member in contact with the wall of the insulating member. 
     Each of the connectors may have a longitudinal slit formed therein. 
     The first leads and the second leads are joined to the connectors by pressing the lead-joint portions of the connector to plastically deform them or welding or soldering the lead-joint portions of the connectors and the first and second leads together. 
     According to the second aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a sensing element having a length which includes a sensing portion and a base portion, the sensing element being retained in the housing with the base portion projecting from the housing; (c) a hollow insulating member provided so as to surround the base portion of the sensing element, the insulating member having a wall in which holes are formed; (d) a cover covering the insulating member; (e) first leads each having a first end portion and a second end portion, each of the first end portions being in electric contact with the sensing element within the insulating member, each of the second end portions passing through one of the through holes and projecting outside the insulating member; (f) second leads each having a first end portion and a second end portion, the first end portion extending outside of the gas sensor for electric connection with an external device; (g) connectors establishing electric connections between the second end portions of the first leads and the second end portions of the second leads, respectively; and (h) stoppers provided on one of the second end portions of the first leads in contact with the wall of the insulating member. 
     In the preferred mode of the invention, each of the connectors may have a longitudinal slit formed therein. 
     The first leads and the second leads are joined to the connectors by pressing the lead-joint portions of the connector to plastically deform them or welding or soldering the lead-joint portions of the connectors and the first and second leads together. 
     According to the third aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a sensing element having a length which includes a sensing portion and a base portion, the sensing element being retained in the housing with the base portion projecting from the housing; (c) a hollow insulating member provided so as to surround the base portion of the sensing element, the insulating member having a wall in which holes are formed; (d) a cover covering the insulating member; (e) first leads each having a first end portion and a second end portion, each of the first end portions being in electric contact with the sensing element within the insulating member, each of the second end portions passing through one of the through holes and projecting outside the insulating member; (f) second leads each having a first end portion and a second end portion, the first end portion extending outside of the gas sensor for electric connection with an external device; (g) connectors establishing electric connections between the second end portions of the first leads and the second end portions of the second leads, respectively; and (h) stoppers provided in contact with the wall of the insulating member to hold the connectors above the holes of the insulating member. 
     In the preferred mode of the invention, each of the connectors has a longitudinal slit formed therein. 
     The first leads and the second leads are joined to the connectors by pressing the lead-joint portions of the connector to plastically deform them or welding or soldering the lead-joint portions of the connectors and the first and second leads together. 
     According to the fourth aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a sensing element having a length which includes a sensing portion and a base portion, the sensing element being retained in the housing with the base portion projecting from the housing; (c) a hollow insulating member provided so as to surround the base portion of the sensing element, the insulating member having a wall in which holes are formed; (d) a cover covering the insulating member; (e) first leads each having a first end portion and a second end portion, each of the first end portions having a shoulder and being in electric contact with the sensing element within the insulating member, each of the second end portions passing through one of the through holes and projecting outside the insulating member; (f) second leads each having a first end portion and a second end portion, the first end portion extending outside of the gas sensor for electric connection with an external device; (g) connectors establishing electric connections between the second end portions of the first leads and the second end portions of the second leads, respectively; and (h) stoppers each provided by an extension formed on an end of one of the connectors, placed in contact with the shoulder of the first end portion of one of the first leads to hold the one of the connector. 
     In the preferred mode of the invention, each of the connectors has a longitudinal slit formed therein. 
     The first leads and the second leads are joined to the connectors by pressing the lead-joint portions of the connector to plastically deform them or welding or soldering the lead-joint portions of the connectors and the first and second leads together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
     FIG. 1 is a longitudinal sectional view which shows a gas sensor according to the first embodiment of the invention; 
     FIG. 2 is a partially sectional view which shows electric connections between a sensing element and leads extending outside a gas sensor; 
     FIG.  3 ( a ) is a perspective view which shows the structure of a connector; 
     FIG.  3 ( b ) is a longitudinal sectional view which shows the connector of FIG.  3 ( a ); 
     FIGS.  4 ( a ) and  4 ( b ) are views which show a sequence of steps of joining leads to connectors; 
     FIGS.  5 ( a ) and  5 ( b ) are longitudinal sectional views which show modifications of the connector of FIG.  3 ( a ); 
     FIG. 6 is a partially perspective view which shows a modification of the connector of FIG.  3 ( a ); 
     FIGS.  7 ( a ),  7 ( b ),  7 ( c ),  7 ( d ),  7 ( e ), and  7 ( f ) are sectional views taken along the line A—A in FIG. 2 which show variations in sectional shape of a connector; 
     FIG. 8 is a longitudinal sectional view which shows a gas sensor according to the second embodiment of the invention; 
     FIG. 9 is a partially sectional view which shows electric connections between a sensing element and leads extending outside a gas sensor according to the third embodiment of the invention; 
     FIG. 10 is a partially perspective view which shows a connector of the third embodiment; 
     FIG. 11 is a partially sectional view which shows a gas sensor according to the fourth embodiment of the invention; 
     FIG.  12 ( a ) is a perspective view which shows a lead with a stopper in the fourth embodiment; 
     FIGS.  12 ( b ) and  12 ( c ) are perspective views which show modifications of the lead of FIG.  12 ( a ); 
     FIG. 13 is a partially sectional view which shows a gas sensor according to the fifth embodiment of the invention; 
     FIG.  14 ( a ) is a perspective view which shows a stopper in the fifth embodiment; 
     FIG.  14 ( b ) is a perspective view which shows a modification of the stopper in the fifth embodiment; 
     FIG. 15 is a partially sectional view which shows a gas sensor according to the sixth embodiment of the invention; 
     FIG.  16 ( a ) is a perspective view which shows the structure of a connector mounted in a gas sensor which may be proposed to alleviate problems of the prior art; 
     FIG.  16 ( b ) is a longitudinal sectional view which shows the connector of FIG.  16 ( a ); and 
     FIG. 17 is a partially longitudinal sectional view which shows a gas sensor in which the connector of FIG.  16 ( a ) is installed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIGS. 1 and 2, there is shown a gas sensor  1  according to the first embodiment of the invention which may be employed in a burning control system for automotive vehicles to measure concentrations of components such as NOx, CO, HC, and O 2  contained in exhaust gasses of the engine. 
     The gas sensor  1  generally includes a sensor element  159 , a first insulation porcelain  31 , a second insulation porcelain  32 , a hollow cylindrical housing  10 , and an air cover  11 . The sensor element  159  is made of a laminated plate. For example, U.S. Pat. No. 5,573,650, issued on Nov. 12, 1996 to Fukaya et al. teaches a typical laminated sensor element, disclosure of which is incorporated herein by reference. The first insulation porcelain  31  is fitted within the housing  10  and holds therein the sensor element  159 . The second insulation porcelain  32  is mounted on the first insulation porcelain  31  in alignment with each other and surrounds a base portion of the sensor element  159 . The air cover  11  is installed at an end thereof on the housing  10  to cover the second insulation porcelain  32 . 
     The second insulation porcelain  32  is made of a hollow cylindrical insulating member and has disposed therein four leads  16  (only two are shown for the simplicity of illustration) each of which is made of a wire folded elastically to make an electric contact at one end with an electrode terminal (not shown) formed on the sensor element  159 . The leads  16  extend at the other end through holes  322  formed in an end of the second insulation porcelain  32  and connect with four leads  17  through connectors  2 , respectively, for transmission of sensor signals between the sensor element  159  and an external device and supply of electric power to a heater installed on the sensor element  159 . 
     Each of the connectors  2  retains therein the end of one of the leads  6  projecting from the upper end of the second insulation porcelain  32 , as viewed in FIG. 1, and the end of one of the leads  17  in alignment with each other to make an electric connection therebetween. Specifically, each of the connectors  2 , as will be described later in detail, includes a lead-clamping portion for clamping one of the leads  16  and a connector-holding portion placed in contact with the upper end of the second insulation porcelain  32  to hold the connector  2  on the second insulation porcelain  32 . The lead-clamping portion includes a press portion  201 . The connector-holding portion is provided by a stopper  212 . 
     Each of the connectors  2  includes a hollow body  20  having the press portion  201  and a head portion  21  having an extension which expands outward to define the stopper  212 . 
     Referring back to FIG. 1, the gas sensor  1  also includes a protective cover assembly  15  consisting of an outer cover  151  and an inner cover  152 . The protective cover assembly  15  is installed on a head of the housing  10  to define a gas chamber into which a gas to be measured is admitted through gas holes  150  formed in the outer and inner covers  151  and  152 . 
     The air cover  11  is, as described above, mounted on the base end of the housing  10 . An outer cover  12  is provided around the air cover  11  and staked or crimped to retain a water-repellent filter  13  on the periphery of the air cover  11 . 
     The first insulation porcelain  31  is retained within the housing  10  hermetically through a metallic packing ring  101  placed on an annular shoulder  100  formed on an inner wall of the housing  10  and holds therein the sensor element  159  through a glass sealing member  310 . 
     The second insulation porcelain  32  is, as described above, mounted directly on the base end  311  of the first insulation porcelain  31  and surrounded by the air cover  11 . The air cover  11  has an upper small-diameter portion, as viewed in the drawing, to form a shoulder  110 . A disc spring  119  is disposed between the shoulder  110  and an end of the second insulation porcelain  32  to elastically urge the second insulation porcelain  32  into constant engagement with the first insulation porcelain  31 . 
     The second insulation porcelain  32  has formed therein a chamber  320  which communicates with the four through holes  322  formed in the base end thereof. The chamber  320  opens at the front end of the second insulation porcelain  32  and thus establishes communication between the holes  322  and the outside of the front end of the second insulation porcelain  32 . 
     In the chamber  320 , four ribs  325  are provided which work to fix locations of the leads  16  and insulate the leads  16  from each other. 
     The sensor element  159 , as described above, has a heater built therein which heats the sensor element  159  up to a temperature required for the sensor element  159  to be sensitive to a gas to be measured correctly. The sensor element  159  has formed thereon four electrode terminals two of which are used for outputting sensor signals and the others for supply of electric power to the heater. The electrode terminals are connected electrically with ends of the leads  16  in an illustrated manner, respectively. The leads  16  extend through the holes  322  and are inserted into the connectors  2 , respectively. 
     An insulating holder  14  made of rubber is, as clearly shown in FIG. 2, disposed inside the small-diameter portion of the air cover  11 . An air chamber  140  is defined between the bottom of the insulating holder  14  and the base end of the second insulation porcelain  32 . The insulating holder  14  has formed therein four through holes  141  into which the leads  17  are inserted. The holes  141  have defined therein large-diameter bores  142  in which the leads  17  are joined to the connectors  2 , respectively. 
     Each of the connectors  2  is, as can be seen from FIG.  3 ( a ), made by folding a metallic plate longitudinally into side-to-side contact so as to have a rectangular shape in cross section and consists of the body  20 , the head portion  21 , and the base portion  23 . The lead  17  is retained firmly in the base portion  21 . The lead  16  is clamped tightly by the press portion  201  and an opposite inner wall of the body  20 . 
     The body  20  of the connector  2  has a window  22  formed between the press portion  201  and the base portion  23 . The press portion  201  is, as clearly shown in FIG.  3 ( b ), defined by a dimple which may be formed by a press. The distance r between the press portion  201  and the opposite inner wall of the connector  2  is, thus, shorter than the distance R between opposed inner walls of the head portion  21 . In this embodiment, r=0.4 mm, and R=0.6 mm. 
     The head portion  21  has the stopper  212  and an extension  211 . When the lead  16  and the connector  2  are joined together in an assembling process of the gas sensor  1 , at least the stopper  212  is placed, as clearly shown in FIG. 2, in contact of an end thereof with an end wall  323  of the second insulation porcelain  32  to hold the connector  2  on the end wall  323  of the second insulation porcelain  32 . 
     How to join each of the leads  17  and  16  to one of the connectors  2  will be described below in detail. 
     First, the lead  17  (i.e., a bundle of wires from which insulation is removed) is inserted into the base portion  23  of the connector  2  until it reaches a given position. The base portion  23  is clamped or pressed to hold the lead  17  firmly. 
     Next, the lead  16  is inserted into the second insulation porcelain  32  from the front end thereof (i.e., the lower end as viewed in FIG. 1) and drawn, as shown in FIG.  4 ( a ), at an end thereof out of the hole  322 . 
     The connector  2  to which the lead  17  is joined is put on the end of the lead  16  projecting from the hole  322  of the second insulation porcelain  32  in contact of the stopper  212  and the extension  211  with the end wall  323  of the second insulation porcelain  21 , so that the end of the lead  16  is, as shown in FIG.  3 ( b ), inserted into the connector  2  over the press portion  201 . The pressure is applied to the press portion  201  to deform it to hold the end of the lead  16  firmly in the connector  2 . Subsequently, a laser beam is, as shown in FIG.  4 ( b ), radiated to a given portion of the peripheral wall of the connector  2  using a laser welding device  4  to weld the connector  2  to the lead  16 . The connector  2  and the lead  16  may alternatively be soldered together. 
     Finally, the base of the sensor element  159  fitted in the first insulation porcelain  31  is inserted into the center of the chamber  320  of the second insulation porcelain  32  to make electric connections between the electrode terminals formed on the sensor element  159  and the leads  16 . 
     As apparent from the above discussion, the stopper  212  extends outward from the head portion  21  of the connector  2  so that the end of the connector  2  may be kept in contact with the end wall  323  of the second insulation porcelain  32  when the lead  16  is coupled to the connector  2 , thus avoiding an undesirable drop of the connector  2  into the through hole  322 , which allows the connector  2  to be pressed and laser-welded to secure the lead  16  in a constant position. 
     Both the extension  211  and the stopper  212  of each of the connectors  2  may alternatively be, as shown in FIG.  5 ( a ), expanded outward. This facilitates ease of insertion of the lead  16  into the connector  2 . 
     The extension  211  and the stopper  212  may also be, as shown in FIG.  5 ( b ), chamfered or tapered. This provides for smooth insertion of the lead  16  into the connector  2 . 
     The head portion  21  may also, as shown in FIG. 6, have four extensions  213  expanding outward to work as stoppers. This absorbs shifts in location of the lead  16  from the end of the connector  2  in all directions and also facilitates ease of insertion of the lead  16  into the connector  2 . 
     The connectors  2  may be formed to various shapes in cross section, as shown in FIGS.  7 ( a ) to  7 ( f ). 
     Specifically, the connector  2  of FIG.  7 ( a ) has a longitudinally extending slit  209  formed in a side wall thereof. The connector of FIG.  7 ( b ) is of C-shape in cross section defined by three side walls. The connector of FIG.  7 ( c ) is of circular shape in cross section and has a longitudinally extending slit  209 . The connector of FIG.  7 ( d ) is, like FIG.  7 ( c ), of circular shape in cross section, but has abutting ends  208  welded to each other. The connector of FIG.  7 ( e ) is of rectangular shape in cross section and has abutting ends  208  welded to each other. The connector  2  of FIG.  7 ( f ) is of U-shape in cross section. 
     FIG. 8 shows a gas sensor  3  according to the second embodiment of the invention which is equipped with a cup-shaped sensing element  35 . 
     The sensing element  35  consists of a cup-shaped solid electrolyte body  358  and a bar-shaped heater  359 . The solid electrolyte body  358  is retained in a hollow cylindrical housing  31 . The heater  359  is disposed within the solid electrolyte body  358 . 
     The sensing element  35  and the housing  31  are hermetically sealed by a packing ring  311  placed on an annular shoulder formed on an inner wall of the housing  30 , a sealing powder  312 , and a ceramic cylinder  313 . 
     A metallic ring  314  is disposed on an end of the ceramic cylinder  313  and urged downward by a curved edge of the housing  31 , as viewed in the drawing, to hold the sensing element  35  within the housing  31  firmly. 
     A lead  361  is provided for electric connection between one of the connectors  2  and the sensing element  35 . The lead  361  is made of a conductive strip which has formed at an end thereof a ring fitted on the base end of the sensing element  35  leading to a gas measuring electrode (not shown) formed on an outer wall of the solid electrolyte body  358 . A lead  362  made of a conductive strip is provided for electric connection between the heater  359  and one of the connectors  2 . The lead  362  has formed at an end thereof a cylindrical heater holder  365  which is fitted on the end of the heater  359  to hold it within the solid electrolyte body  358  and placed in electric contact with a reference gas electrode (not shown) formed on an inner wall of the solid electrolyte body  358 . The gas measuring electrode and the reference gas electrode are known, for example, in European Patent Application EP 0918215 A2 assigned to the same assignee as that of this application, disclosure of which is incorporated herein by reference. 
     The leads  361  and  362  extend through the holes  322  formed in an end surface of a hollow cylindrical insulation porcelain  32  and connect with the leads  17  through the connectors  2 , respectively. The heater  359  is, like the solid electrolyte body  358 , connected to the leads  17  through the connectors  2 . 
     Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 9 show a gas sensor according to the third embodiment of the invention which is different from the first embodiment only in that each of the connectors  2  has a stopper  41  formed on a side wall thereof. Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     Each of the connectors  2  is, as clearly shown in FIG. 10, made of a hollow rectangular conductive member which has a longitudinally extending slit  40  formed on one side wall thereof and a strip sheared out from an opposite side wall to define the stopper  41 . Each of the connectors  2  is, as shown in FIG. 9, inserted partially into one of the holes  322  in contact of the stopper  41  with the end wall  323  of the second insulation porcelain  32 , thereby avoiding an undesirable drop of the connector  2  into the hole  323 . 
     FIG. 11 shows a gas sensor according to the fourth embodiment of the invention which is different from the first embodiment only in structure of the leads  16 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     Each of the leads  16  is, as clearly shown in FIG.  12 ( a ), made of a conductive plate which has a stopper  42  formed by shearing out a strip from a base portion of the conductive plate to be joined to the connector  2 . The joining of the head  16  to the connector  2  is accomplished by inserting the base portion of the lead  16  into an end of the connector  2  as long as the stopper  42  advances and pressing the connector  2  in the same manner as described in the first embodiment. When the lead  16  is joined to the connector  2 , the stopper  42  extends out of the hole  322  of the second insulation porcelain  32 , thereby avoiding an undesirable drop of the connector  2  into the hole  322 . 
     The stopper  42  may alternatively be, as shown in FIG.  12 ( b ), made by shearing out half a width of the base portion of the lead  16  or pressing, as shown in FIG.  12 ( c ), the base portion of the lead  16  to form a protrusion. 
     FIG. 13 shows a gas sensor according to the fifth embodiment of the invention which is different from the first embodiment only in that a stopper  43  is provided independently from each of the connectors  2 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     The stopper  43  is made of a material different from that of the lead  16  and formed by a hollow cylinder, as shown in FIG.  14 ( a ), which has a slit  400  formed therein. The stopper  43  is disposed beneath each of the connectors  2  in partial contact with the end wall  323  of the second insulation porcelain  32 , thereby avoiding an undesirable drop of the connector  2  into the hole  322  when the lead  16  is coupled to the connector  2 . The stopper  43  may alternatively, as shown in FIG.  14 ( b ), be made of a hollow member having a rectangular shape in cross section. 
     FIG. 15 shows a gas sensor according to the sixth embodiment of the invention. 
     Each of the connector  2  has a hollow cylindrical extension  219  which passes through the hole  322  and engages a shoulder  169  of the lead  16 . Specifically, the extension  291  is placed in contact with the shoulder  169  of the lead  16 , thereby keeping the press portion  201  at a constant level from the end wall  323  of the second insulation porcelain  32 . This allows the connector  2  to be pressed and laser-welded to secure the lead  16  in a constant position. 
     While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.