Patent Publication Number: US-6712649-B2

Title: Plug

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a plug structure for connecting a pair of lead wires comprising a signal wire and a ground wire to an apparatus such as an audio apparatus. 
     2. Description of the Related Art 
     In an apparatus employing a lead wire for wiring, for example, an audio apparatus, an amplifier and a speaker are connected to each other by a pair of lead wires comprising a signal wire and a ground wire. Connection between a lead wire and such an apparatus is usually established as follows. As shown in FIG. 11, a lead wire is, at its skinned end, connected to a connector, called a lever terminal  103 , attached to a terminal plate  101  provided on the back-surface side of the apparatus. 
     That is, a lever  105  is rotated in a clockwise direction viewing the figure against a force of a torsion spring  107 , and a skinned end of a lead wire  109  is inserted into an insertion hole  111  of the lever terminal  103 . Upon a return of the lever  105  to its original position, the lead wire  109  is caught in between a metal contactor  113 , which is formed integrally with the lever  105 , and a metal contactor  117  formed inside a housing  115  of the lever terminal  103 , and is thereby connected to the metal contactor  117 . 
     However, modem audio apparatuses, in contrast to conventional ones having a 2-channel system, have come to have a 4- or 6-channel system, i.e. require a larger number of speakers. This requires that the lead wire  109 , formed by using a signal and a ground wire in combination, be separately connected to each of the speakers. Consequently, as shown in FIG. 12, the terminal plate  101  on the back-surface side of the amplifier is provided with a multiplicity of lever terminals  103  arranged in line. 
     Workers find difficulty in performing connection operations in the terminal plate  101  on the back surface of the amplifier, because the visibility of the work area is poor from the worker position. It is also extremely difficult to connect a multiplicity of lead wires  109  to their respective lever terminals  103  without fail. 
     Due to the difficulty of the connection operations, the lead wire  109  cannot be connected to the terminal with its skinned portions kept in a deeply-inserted state. Consequently, adjacent portions are brought into contact with each other, causing short circuits. Moreover, due to the poor visibility of the work area, the signal and ground wires, although they are usually distinguished from each other by their skin colors (red and black), might be inadvertently connected to reverse positions. 
     To overcome such inconveniences, a connection method as shown in FIG. 13 has been proposed that employs a plug  125 . Specifically, a signal wire  121  and a ground wire  123 , used in pairs as a lead wire  109 , are attached to the plug  125 , and this plug  125  is inserted into a socket  127  provided on an amplifier side. 
     That is, the end of each of the lead wires  121  and  123  is fixed in advance to its respective metal-made connector terminal  129 . The connector terminal  129  is barrel-shaped, and its rear-end portion (left-end portion viewing the figure) has a U-shaped section. Upon insertion of the skinned end of the lead wire  109 , the barrel is subjected to caulking, thereby fixing the lead wire  109  to the connector terminal  129 . The connector terminal  129  has its substantially cylindrical front-end portion formed as a contact terminal  133 , into which a pin terminal acting as an amplifier-side terminal  131  is inserted so as to be in contact therewith. These two connector terminals  129  with the lead wires  109  fixed thereto are received in their respective housing portions  135  formed in a single plug  125  so as to be retained. 
     Formed in a terminal plate  137  disposed on the back surface of the amplifier is a connection recess  139  constituting the socket  127  which is engaged with the plugs  125 . Two apparatus-side terminals, or pin terminals  131 , are formed inside the connection recess  139  protruding therein, these corresponding to the signal wire  121  and the ground wire  123 , respectively. 
     To prevent the plug  125  from being inserted in a 180-degree inverted position, the connection recess  139  has a rib  141  formed on its inner side along the insertion direction so as to be protruded, and the plug  125  has, on its outer surface, a groove  143  which receives the rib  141 . The plug  125  and its corresponding socket  127  have the same skin color. 
     In the connector shown in FIG. 13, 
     (1) By providing the concave groove  143  and the rib  141  for the plug  125  and the socket  127 , respectively, the plug  125  can be inserted in a proper position, thereby preventing the signal and ground wires from being connected in reversed polarity. 
     (2) By giving the plug  125  and its respective socket  127  the same skin color, the lead wire  109  can be connected to the speakers for a multi-channel system with no possibility of wrong wiring. 
     (3) Since the skinned portion of the lead wire  109  is not exposed, adjacent lead wires  109  do not make contact with each other. Consequently, short circuits can be prevented. 
     (4) In this connector, unlike the lever terminal  103  shown in FIG. 111 which requires two operations: rotation of the lever  105 ; and insertion of the lead wire  109 , the connection operation can be completed simply by inserting the plug  125 . Therefore, even though the back surface of the amplifier is not exposed to the worker view, the connection operation can be easily achieved with one hand. 
     However, the connector shown in FIG. 13 lacks versatility. For example, in a case where an audio enthusiast tries to connect a specially-designed thick lead wire to the speaker, whereas the lever terminal  103  shown in FIG. 11 conforms to such a requirement, the connector in question is unusable. That is, only the lead wire  109  attached in advance to the plug  125  may be acceptable. 
     To achieve both of the convenience of the plug  125  shown in FIG.  13  and the versatility of the lever terminal  103  shown in FIG. 11, two types of connectors, i.e. the socket  127  and the lever terminal  103 , need to be provided in a single back-surface panel of an amplifier or the like. This leads to an undesirable increase in the number of constituent components, causing space shortage in the back-surface panel. 
     The above description deals only with problems associated with connection between an amplifier and a lead wire provided in a speaker. However, similar problems tend to occur when a lead wire, realized by using a signal and a ground wire in combination, is connected between another device constituting an acoustic or other apparatus. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention solves the above-described problems, and accordingly its object is to provide a plug that prevents reverse insertion of a signal and a ground wire arranged in pairs, prevents occurrence of short circuits in adjacent lead wires, facilitates connection of the lead wires, and allows connection of lead wires of any other type, i.e. offers versatility. 
     To achieve the above object, according to a first aspect of the present invention, a plug for connecting a pair of lead wire comprising a first lead wire and a second lead wire to an apparatus, comprising: 
     a plug housing configured so as to be insertable into and withdrawable from a connection recess provided on an apparatus side; 
     a pair of plug-side contact terminals disposed inside the plug housing, the plug-side contact terminals being contacted by a pair of apparatus-side terminals corresponding to the first and second lead wires; 
     reverse insertion preventive means for providing the plug housing with an asymmetrical profile so that it is inserted into and withdrawn from the asymmetrically-configured connection recess only in a predetermined position; and 
     connecting means for connecting a pair of plug-side contact terminals to the first and second lead wires, respectively, 
     the connecting means being clamp means detachably attached to the first or second lead wires. 
     Being configured so as to be insertable into and withdrawable from the connection recess on the apparatus side and provided with the reverse insertion preventive means, the plug housing can be engaged with and connected to an apparatus-side socket, which is engaged with and connected to the plug with the lead wire attached thereto, the lead wire being, at its end, fixed to a connector terminal. 
     Being formed as clamp means, the connecting means is capable of connecting a lead wire of an arbitrary thickness to the plug-side contact terminal. 
     According to a second aspect of the present invention, in the plug according to the first aspect, the pair of lead wires are connected between an amplifier and a speaker in an audio apparatus. A socket provided in the amplifier or speaker constitutes the connection recess. The reverse insertion preventive means is composed of a recess and a projection fitted into the recess, the recess is formed in one of the connection recess and the plug housing and the projection is formed in the other of the connection recess and the plug housing. 
     It is possible to use a pair of lead wires of an arbitrary thickness for connection between the amplifier and the speaker. 
     The reverse insertion preventive means allows the plug to be inserted into the connection recess only in such a state that the recess and the projection are engaged with each other, thereby preventing the amplifier and the speaker from being connected to each other in reversed polarity. 
     According to a third aspect of the present invention, in the plug according to the first or second aspect, the clamp means comprises: 
     a plate spring formed integrally with the plug-side contact terminal; and 
     a rotary lever rotatably supported by the plug housing, the rotary lever pressing the plate spring so that when the first or second lead wires are inserted into the plug housing, they will be clamped between the plate spring and the inner wall of the plug housing. 
     As the rotary lever is rotated, the plate spring is pressed, whereby the first or second lead wires are clamped between the plate spring and the inner wall of the plug housing, and is thereby electrically connected to the plug-side contact terminal which is integrally formed with the plate spring. 
     By rotating the rotary lever in a reverse direction, the pressure on the plate spring is released, whereby the first or second lead wires are unclamped and thus can be pulled out. 
     According to a fourth aspect of the present invention, the plug according to the third aspect further comprises: 
     a pressing pawl disposed in the rotary lever for applying a pressing force; and 
     a pressing recess formed in a portion of the plate spring at which pressing force is applied, 
     wherein the pressing pawl crosses over the pressing recess only when the rotary lever is rotated into an unclamped state. 
     In the clamped state, the pressing recess is pressed by the pressing pawl, whereby the rotary lever is settled in a rotation position to achieve clamping. Moreover, since the rotary lever is inhibited from rotating until the press pawl crosses over the pressing recess, it never occurs that the rotary lever is rotated into an unclamped state by an inadvertent external force. 
     According to a fifth aspect of the present invention, in the plug according to the first or second aspects, the clamp means comprises: 
     a plate spring formed integrally with the plug-side contact terminal; and 
     a rotary lever rotatably supported by the plug housing, the rotary lever allowing the first or second lead wire, inserted into the plug housing, to be gripped and clamped between the plate spring and the plug housing. 
     As the rotary lever is rotated, the first or second lead wire is clamped between the plate spring and the inner wall of the plug housing, and is thereby electrically connected to the plug-side contact terminal which is formed integrally with the plate spring. 
     By rotating the rotary lever in a reverse direction, the first or second lead wire is unclamped and thus can be pulled out. 
     According to a sixth aspect of the present invention, in the plug according to the fifth aspect, the rotary lever is provided with an engagement portion. The rotary lever is, at its engagement portion, fitted into the plug housing when set in a rotation position so as for the first or second lead wire to be gripped between the plate spring and the plug housing, thus attaining a clamped state. 
     The rotary lever is engaged with the plug housing in a rotation position for achieving clamping, and is thus prevented from being rotated into an unclamped state by an inadvertent external force. 
     According to a seventh aspect of the present invention, in the plug according to the fifth or sixth aspect, a pair of clamp means have an integrally-formed rotary lever, in which by the rotation of the rotary lever, the first and second lead wires are clamped or unclamped concurrently. 
     The first and second lead wires can be concurrently clamped or unclamped by the rotation of the rotary lever. 
     According to an eighth aspect of the present invention, in the plug according to the first or second aspect, the clamp means comprises: 
     a contact plate formed integrally with the plug-side contact terminal; 
     a plate spring arranged opposite to the contact plate, the plate spring being loaded with a force that tends to move it toward the contact plate so as for the first or second lead wire to be gripped and clamped between the contact plate and the plate spring; and 
     a push button for achieving unclamping by pressing the plate spring in a direction reverse to a force-loading direction. 
     The first or second lead wire is gripped between the contact plate and the plate spring which is loaded with a force that tends to move it toward the contact plate, and is thereby electrically connected to the plug-side contact terminal. When the push button is pressed, the plate spring is moved away from the contact plate, whereby the first or second led wire is unclamped and thus can be pulled out. 
     According to a ninth aspect of the present invention, in the plug according to the first or second aspect, the clamp means comprises: 
     a contact plate formed integrally with the plug-side contact terminal; 
     a pressing member arranged opposite to the contact plate, the pressing member allowing the first or second lead wire to be gripped and clamped between the contact plate and the pressing member; 
     elastic means for loading the pressing member with a force that tends to move it toward the contact plate; and 
     a push button for achieving unclamping by pressing the pressing member in a direction reverse to a force-loading direction. 
     The first or second lead wire is gripped between the contact plate and the pressing member which is loaded by the elastic means with a force that tends to move it toward the contact plate, and is thereby electrically connected to the plug-side contact terminal. When the push button is pressed, the pressing member is moved away from the contact plate, whereby the first or second led wire is unclamped and thus can be pulled out. 
     According to a tenth aspect of the present invention, in the plug according to the ninth aspect, the contact plate, the pressing member, and the push button are each arranged in pairs corresponding to each of the first and second lead wires. Of these components, the pressing members and the push buttons are each disposed at both ends of an U-shaped member serving as the elastic means. 
     The elastic means is realized as an U-shaped member and thus serves also as a paired clamp means. 
     A pair of push buttons are grippingly pressed to unclamp the first and second lead wires concurrently. 
     The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a first plug embodiment according to the present invention, wherein the plug  1  is connectable to a socket  127  designed for use with a conventional plug  125 . 
     FIG. 2 is a cross sectional view showing the plug  1 , in its unclamped state, i.e., the rotary lever  39  being in its non-clamping position. 
     FIG. 3 is a cross sectional view showing a case where the plug  1 , which has been driven from the state shown in FIG. 2 to a clamped state, is inserted into the socket  127 . 
     FIG. 4 is a vertical sectional view of the plug  1  and the socket  127  shown in FIG. 3, sectioned along a terminal insertion hole  23  of the plug  1 . 
     FIG. 5 is a plan view of a plug  2  of a second plug embodiment according to the present invention. 
     FIG. 6 is a side view of the plug  2 . 
     FIG. 7 is a cross sectional view showing the plug  2  in an unclamped state, taken along line A—A of FIG.  5 . 
     FIG. 8 is a cross sectional view wherein the plug  2 , in a clamped state, is inserted into the socket  127 . 
     FIGS. 9A and 9B are views of a third plug embodiment according to the present invention, FIG. 9A showing a cross sectional view of the plug taken in a position of a push button  55 , and FIG. 9B showing a perspective view of a plug-side contact terminal  25 , a contact plate  57 , and a plate spring  59  that are formed integrally with one another. 
     FIG. 10 is a vertical sectional view showing yet another plug embodiment according to the present invention. 
     FIG. 11 is a vertical sectional view showing a conventional lever terminal  103 . 
     FIG. 12 is a front view of a terminal plate  101 , provided on the back surface of an audio apparatus, in which a multiplicity of lever terminals  103  as shown in FIG. 11 are disposed. 
     FIG. 13 is a perspective view showing the structure of a connector composed of conventional plug  125  and socket  127 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 1 to  4 . 
     As shown in FIG. 1, a lead wire  3  to be connected to a plug  1  is realized by using a signal wire  5 , serving as a first lead wire, and a ground wire  7 , serving as a second lead wire, in combination. The lead wire  3  is connected between an amplifier A and a speaker S of an audio apparatus as shown in FIG.  1 . 
     That is, a single plug  1  is provided to receive both of the ends of the signal wire  5  and the ground wire  7  for connection to the speaker side. The plug  1  is inserted into a connection recess  139  constituted by a socket  127  which is formed in a terminal plate  137  provided on the back surface of the amplifier. As shown in FIG. 1, the socket  127  can also be fitted into the above-described plug  125  of the prior art example so as to be connected therewith. Hence, the socket  127  will be identified with the same reference number and detailed descriptions thereof will be omitted. 
     A plug housing  15  has a rectangular vertical cross section which is substantially identical with but is slightly smaller than the vertical cross section of the connection recess  139 . The plug  1 , just as with the plug  125 , is so configured as to be insertable into and withdrawable from the connection recess  139 . Moreover, the connection recess  139  has a projection, as a rib  141 , formed on its inner surface. The plug housing  15  has a recess, as, e.g., a concave, a rectangular, or like cross section groove  19 , formed on its outer surface. The groove  19  receives the rib  141  during insertion. Providing the rib  141  and the groove  19  makes asymmetrical the configuration of each of the connection recess  139  and the plug housing  15 . 
     The connection recess  139  has two pin terminals, as apparatus-side terminals  131 , fixed to its inner part. The apparatus-side terminals  131  are so formed as to protrude toward the opening of the connection recess  139 . 
     As shown in FIG. 2, the plug housing  15  has, on its front surface, a terminal insertion hole  23  into which the apparatus-side terminal  131  is inserted. Inside the terminal insertion hole  23  is situated a substantially cylindrical portion  27  of a plug-side contact terminal  25 . That is, the substantially cylindrical portion  27  is split in half, and its elasticity maintains the elastic engagement of the pin terminal acting as the apparatus-side terminal  131 . 
     The plug-side contact terminal  25  has an engagement pawl  29  engageable with an engagement window  31  of the plug housing. The engagement pawl  29  is obtained by cutting and raising part of the plug-side contact terminal  25 . When the plug-side contact terminal  25  is inserted into the plug housing  15  from the rear (left-hand side viewing the figure), the engagement pawl  29  is engaged in the engagement window  31 , thereby preventing the plug-side contact terminal  25  from being slipping off. 
     The plug-side contact terminal  25  has a plate spring  33  formed integrally and contiguously therewith that extends toward the rear side of the inner part of the plug housing  15 . Formed on the rear surface of the plug housing  15  is an insertion hole  35  for insertion of the lead wire  3 . A skinned portion  37  of the lead wire  3  is inserted through the insertion hole  35  to reach a position where it is arranged side by side with the plate spring  33 . Then, as will be described later, the lead wire  3  is sandwiched between the elastically deformable plate spring  33  and an inner wall  51  of the plug housing  15 . 
     The plug housing  15  has a rectangular section elongated in a direction in which the two plug-side contact terminals  25  are arranged. Rotatably on a side wall  15   a  of the plug housing  15  is attached a rotary lever  39  for pressing the plate spring  33 . The entire rotary lever  39  has the shape of an inverted L. Of two arm portions defining the reversed L configuration, an arm portion  39   a  has, at its end, a pressing pawl  43  for applying a pressing force, and has, on both sides, a rotary shaft  41  which is protrudingly formed so as to rotate with respect to the side wall  15   a.    
     The other arm portion  39   b  defining the L configuration comes out of the plug housing  15  through a lever window  45  piercingly formed on the side wall  15   a . On the surface of its exposed portion is formed a non-slip portion  47 . 
     As shown in FIG. 3, the plate spring  33  has, in its portion to be pressed by the pressing pawl  43 , a pressing recess  49  which acts to position the rotary lever  39  in a state where the lead wire  3  is clamped, to pull on the lead wire  3 , and to prevent the rotary lever  39  from being rotated by an unexpected external force. Thus, the depth of the pressing recess  49  is determined such that the rotary lever  39 , on the one hand, cannot be easily rotated by an unexpected external force, and, on the other hand, is rotated to allow the pressing pawl  43  to cross over the pressing recess  49  when some operating force is exerted thereon. 
     The above-described constituent components including the terminal insertion hole  23 , the plug-side contact terminal  25 , the insertion hole  35 , and the rotary lever  39  are each provided in pairs corresponding to each of the signal and ground wires  5  and  7 , and the two components constituting each pair are disposed in the upper and lower portions of the plug housing  15 , respectively, in the lengthwise direction. 
     To connect the lead wire  3 , i.e. the signal wire  5  or the ground wire  7 , to the plug  1  and as shown in FIG. 2, the outer arm portion  39   b  of the rotary lever  39  is raised by rotation to attain an unclamped state. Then, the plate spring  33  is moved away from the inner wall  51  of the plug housing  15  under its own elastic force, thereby creating an insertion space  38  for inserting the lead wire  3  between the inner wall  51  and the plate spring  33 . 
     In this state, the skinned portion  37  of the lead wire  3  is inserted through the insertion hole  35  to reach the insertion space  38 . Thereafter, the outer arm portion  39   b  of the rotary lever  39  is lowered by rotation (rotated in a clockwise direction viewing the figure) to attain a clamped state. Then, the pressing pawl  43  of the rotary lever  39  presses the plate spring  33 , thereby clamping the lead wire  3  between the plate spring  33  and the plug housing inner wall  51 . 
     As shown in FIG. 3, as the rotary lever  39  is fully rotated to be parallel with the side wall  15   a , the pressing pawl  43  is located in the pressing recess  49  of the plate spring  33 , so that the clamped state is maintained. 
     With such a clamp structure, the signal wire  5  and the ground wire  7  are respectively fixed to the plug  1 , thereby completing fixation of the lead wire  3 . Subsequently, the plug  1  is inserted into the connection recess  139  of the socket  127 . The insertion of the plug  1  can be successfully achieved only when the rib  141  of the connection recess  139  is fitted into the groove  19  of the plug  1 . This structure prevents the plug  1  from being inserted in a 180-degree inverted position, so that the signal and ground wires  5  and  7  cannot be connected to the pin terminals  131  in reversed polarity. 
     Moreover, the plug  1  and its corresponding socket  127  may have the same skin color (red, yellow, blue, green, black, or white), so that wiring cannot be carried out on the wrong speaker. 
     Given that an audio enthusiast makes a modification to the signal and ground wires  5  and  7  to make a special, thick lead wire for connection. In this case, the rotary lever  39  is raised to remove the existing lead wire  3 , then the new lead wire, which has already been skinned, is inserted, and then the rotary lever  39  is lowered. Thereupon, the special lead wire can be connected to the plug with ease, and the plug  1  with the lead wire is connected to the apparatus-side socket  127 . 
     The connection between the lead wire  3  and the plug  1 , unlike the case of a conventional lever terminal, need not be performed in the back-surface panel of the amplifier, and therefore workers can perform the connection operation smoothly without suffering from poor visibility. Consequently, the lead wire  3  can be connected to the plug  1  with its skinned portion  37  kept in a deeply-inserted state, thereby preventing short circuits caused by the contact among adjacent skinned portions. 
     Moreover, as shown in FIG. 1, to the socket  127 , the conventional plug  125  can be fitted to be connected therewith. Attached to the conventional plug  125  is the lead wire  109  press-fitted to the connector terminal  129 . Thus, connection of the plug  1 , attached to the lead wire  3  having been modified to suit the user preferences, can be achieved without changing the socket  127  structure which constitutes the conventional connection structure together with the plug  125 . 
     Although, in the above embodiment, the clamped state is maintained by the pressing recess  49  formed on the plate spring  33 , the pressing recess  49  may be omitted by forming another securing means in the rotary lever  39 . 
     FIGS. 5 to  8  show a second embodiment of the plug  2  according to the present invention. The second embodiment differs from the first embodiment in the clamp means for clamping the lead wire  3 . In the second embodiment, the lead wire  3  is gripped and clamped between a rotary lever  11  and a plate spring  13 . The rotary lever  11  is rotatably supported with respect to a plug housing  9 . 
     In the second embodiment, the components that play the same or corresponding roles as in the first embodiment will be identified with the same reference numbers, and overlapping descriptions will be omitted. 
     The plug housing  9  is formed in the shape of rectangular parallelopiped and is insertable into and withdrawable from the connection recess  139  provided in the socket  127 . The plug housing  9  has an insertion hole  35  drilled in its rear surface  9   a . The plate spring  13 , which is contiguous to the plug-side contact terminal  25 , is folded to a generally U-shape in the direction toward the rear of the contact terminal, and its free end lies within the insertion hole  35 , as shown in FIG.  7 . 
     The rotary lever  11  has, at the front of its side surface, a rotary shaft  12  formed so as to protrude vertically. The rotary shaft  12  is rotatably supported on a side wall  9   b  of the plug housing  9 , so that the rotary lever  11  is openable and closeable with respect to the side wall  9   b . At the rear of the rotary lever  11 , a pair of protruding pressing projections  14  are formed. When the rotary lever  11  is rotated parallely with the side wall  9   b  to be set in a clamp position, the pressing projections  14  are inserted through lever windows  45 , respectively drilled in upper and lower parts of the side wall  9   b , to protrude into upper and lower insertion holes  35 . When protruding into the insertion hole  35 , the front end of the pressing projection  14  abuts against and presses the plate spring  13  which also lies in the insertion hole  35 . 
     The pressing projection  14  has, on the basal-end side of its rear surface, an engagement projection  16  which is engaged with part of the side wall  9   b  acting as the rear edge of the lever window  45 . As shown in FIG. 8, the engagement projection  16  keeps the rotary lever  11  in the clamp position. 
     According to the second embodiment, as shown in the figures, the constituent components including the plate spring  13 , the insertion hole  35 , and the pressing projection  14  are each provided in pairs corresponding to each of the signal and ground wires  5  and  7 , and the two components constituting each pair are disposed in the upper and lower portions of the plug housing  9 , respectively, in the lengthwise direction. On the other hand, a single rotary lever  11  can be shared, so that the signal and ground wires  5  and  7  are clamped or unclamped concurrently. 
     As shown in FIG. 7, as the rotary lever  11  is rotated to be set in an unclamp position, enough space can be secured in the insertion hole  35  for the insertion of the lead wire  3 . Thus, the skinned front end of the lead wire  3  can be inserted into the insertion hole  35  until it reaches the side surface of the plate spring  13 . 
     Subsequently, the rotary lever  11  is rotated with the side wall  9   b  to be set in the parallel clamp position. Then, as shown in FIG. 8, the inserted lead wire  3  is sandwiched and clamped between the plate spring  13  and the pressing projection  14  of the operation lever  11 . As a result, the lead wire  3  is electrically connected to the plug-side contact terminal  23 . 
     In this clamped state, the engagement projection  16  is engaged with the rear edge of the lever window  45 . This prevents the rotary lever  11  from rotating inadvertently, so that the clamped state is maintained. Next, upon insertion of the plug  2 , to which the lead wire  3  is connected, into a connection recess  13  of the socket  127 , the paired lead wires  5  and  7  are each electrically connected via the plug-side contact terminal  25  to the apparatus-side terminal  131 . 
     As shown in FIG. 8, during the time the plug  2  is inserted into the connection recess  139  of the socket  127  for connection, the rotary lever  11  is inhibited from further rotation by the inner wall of the connection recess  139  while being set in the clamp position. This, in addition to the action of the engagement projection  16 , prevents the rotary lever  11  from rotating into the unclamp position more reliably, so that the lead wire  3  never slips off inadvertently. 
     In the first and second embodiments, clamping is achieved by rotating the rotary lever ( 39  and  11 ) toward the plate spring ( 33  and  13 ). However, as in a third embodiment shown in FIG. 9, it is also possible to achieve clamping and unclamping at the touch of a push button  55 . 
     That is, in the third embodiment, a plate spring  59  is arranged opposite to a contact plate  57  formed integrally with the plug-side contact terminal  25 , so that the lead wire  3 , i.e. the signal or ground wire  5  or  7 , is clamped between the contact plate  57  and the plate spring  59 . The push button  55  is so arranged as to face the contact plate  57  and is made rotatable about a rotary shaft  61 . Part of the push button  55  protrudes through a button window  63  of the plug housing  15  to allow a finger to be placed thereon from outside. The push button  55  has a pressing projection  65  formed in its portion located on the inner side of the plug housing  15 . The pressing projection  65  presses the plate spring  59 . The plate spring  59  is made longer than the contact plate  57  and extends outward beyond the edge of the contact plate  57 , and its extended portion  67  is pressed by the pressing projection  65 . The pressing projection  65  is so arranged as to face the end of the plate spring  59  so as not to stand in the way of the lead wire  3  within the insertion space  38 . 
     According to the third embodiment, as the pressure on the plate spring  59  is released, the lead wire is clamped under the resilient force of the plate spring  59  per se. Thus, even though part of the clamp means is broken, the clamped state, i.e. the connection of the lead wire  3 , can be maintained. 
     FIG. 10 shows yet another plug embodiment of the present invention having clamp means configured differently from those of the above-described embodiments. A contact plate  71 , formed integrally with the plug-side terminal  25 , is disposed along upper and lower inner walls  73  of the plug housing  15 . A pressing member  75  is arranged opposite to the contact plate  71 . The skinned portion  37  of the lead wire  3  is clamped between the pressing member  75  and the contact plate  71 . 
     A push button  77  is formed from an insulative plastic material integrally with the pressing member  75 . An operation portion  77   a  of the push button  77  protrudes through a button window  79 , which is drilled in the front and bottom surfaces of the plug housing  15 , to allow a finger to be placed thereon from outside. 
     The plug-side contact terminal  25 , the contact plate  71 , the pressing member  75 , and the push button  77  are each provided in pairs corresponding to each of the paired lead wires  5  and  7  for connection. Of these components, the pressing member  75  and the push button  77  are disposed on both sides of a U-shaped member  81 . The U-shaped member  81  is formed from a plastic material integrally with the push button  77  and the pressing member  75 . Being formed in the shape of the letter the U-shaped member  81  possesses elasticity and serves as elastic means for loading the pressing member  75  with a force that tends to move it vertically (outwardly of the plug housing  15 ). 
     Both of the pressing members  75  are also loaded with a force by an auxiliary spring  83  serving as auxiliary elastic means. 
     To connect the lead wire  3 , at first, the push buttons  77  are taken with the operator fingers so as for the pressing member  75  to move toward the inner side of the plug housing  15 . By doing so, an insertion space  38  can be secured between the pressing member  75  and the contact plate  71  for insertion of the lead wire  3 . In this state, after the lead wire  3  is inserted into the insertion hole  35 , the push button  77  is released. Then, the push button  77  and the pressing member  75  try to return to their original positions under the elasticity of the U-shaped member  81  or the auxiliary spring  83 , whereby the lead wire  3  is clamped between the pressing member  75  and the contact plate  71 . 
     In this case, the U-shaped member  81  serves as elastic means and thus the auxiliary spring  83  does not necessarily have to be provided. In a case where a spring like the auxiliary spring  83  is used, there is no need to make the U-shaped member  81  elastic means. By making the central portion of the U-shaped member  81  a hinge-connected portion, the U-shaped member  81  acts merely as positioning means for the push button  77  and the pressing member  75 . 
     According to this embodiment, since the unclamped state is attained in such a condition that the push button  77  is released to activate the U-shaped member  81  and the auxiliary spring  83 , it is possible to achieve clamping with stability. Moreover, by the two push buttons  77 , pressing force is applied from above and below to the plug housing  15  formed in the shape of rectangular parallelopiped, thereby obtaining a large press allowance. With this arrangement, a sufficiently wide insertion space  38  can be secured, and further, the width of the plug housing  15  can be reduced, so that the plug is made slimmer. 
     In the above-described embodiment, the plug-side contact terminal  25  is formed in a substantially cylindrical shape and is fitted into a pin terminal acting as the apparatus-side terminal  131  so as to be connected therewith. However, the plug-side contact terminal  25  may have any given shape so long as it is electrically connected. 
     Moreover, in the above-described embodiment, the apparatus-side terminal  131  is built as a pin terminal, and the plug-side contact terminal  25  is substantially cylindrical in shape and split in half. However, they do not necessarily have to have such configurations. For example, the following design may be adopted. Of the apparatus-side terminal  131  and the plug-side contact terminal  25 , one is formed as a flat terminal, and the other is formed as a terminal having elasticity. Upon insertion of the plug  1  (and  2 ), these two terminals make elastic contact with each other. 
     Further, in the above-described embodiment, the connection recess  139  and the plug housing  15  each have an asymmetrical section. This is achieved by providing the former with the rib  141  and the latter with the groove  19 . However, the other embodiments may have such a structure that the connection recess  139  is provided with the groove  19  and the plug housing  15  is provided with the rib  141 . 
     Further, in the above-described embodiment, the connection recess  139  and the plug housing  15  each have a substantially quadrangular section. To impart asymmetricity to their sections, the rib  141  and the groove  19  are provided. However, in the other embodiments, the asymmetricity can be realized by any other manner than is described above. For example, the section may be shaped to be in the form of a scalene triangle, instead of a quadrangle. 
     Still further, although, in this specification, the lead wire is illustrated as a combination of the signal wire  5  and the ground wire  7 , the pair to the lead wire is not limited to a ground wire. 
     As described heretofore, according to the present invention, a lead wire, such as a signal or ground wire, can be connected to a plug by means of attachable/detachable clamp means. This allows easy connection of a specially-designed, for example, a thick lead wire, achieving versatility. 
     Since the plug is so configured as to be fitted into and connected with the socket designed for use with a conventional plug to which a pair of lead wires is connected, it is possible to use the plug, which allows connection of any type of lead wire, while maintaining the structure for providing connection between the conventional plug and socket. 
     Further, in conformity with the asymmetrical configuration of the connection recess, the plug housing is given an asymmetrical profile. This prevents the signal and ground wires from being connected in reversed polarity. 
     Still further, the signal and ground wires, taken as a pair, are connected to the plug in advance. Thus, connection can be achieved simply by inserting the plug into the connection recess. That is, the connection operation can be easily achieved with one hand. 
     The lead wire can be connected to the plug at any given location regardless of the position of the socket. This enables workers to smoothly perform connecting operations without suffering from poor visibility. This prevents occurrence of wrong connection, as well as short circuits caused by the contact among the skinned portions of the lead wire. 
     While there has been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modification as fall within the true spirit and scope of the invention.