referring now to the drawings and in particular to fig1 there is shown a plug - in relay , generally characterized by numeral 1 , which is unplugged from its mounting sub - base which will be described in greater detail hereinafter . the vital plug - in relay 1 basically includes an insulative relay base 2 , an electromagnetic structure 3 , an electrical contact assembly 4 , a protective cover 5 , a name plate 6 , a handle 7 and a latching arrangement including a push rod 8 . the relay supporting base member 2 is molded of suitable insulative material , such as , fiber glass filled resin , or the like , into a rectangular box - like member which has the necessary mechanical strength and the desired electrical insulative characteristics . the relay mounting base functions as a supporting means for the electrical contacts and the electromagnet as well as guiding and retaining means for the push rod . as shown in fig1 and 2 , the relay mounting base includes a contact mounting block portion 2a which carries a plurality of movable and stationary spring contacts which are disposed in superimposed cooperative association with each other . the forward ends of the stationary front and back contacts 4a include a silver contact point 4a &# 39 ; while the front end of the movable heel contact 4b includes a dual nonweldable contact point 4b &# 39 ; which is disposed in cooperative relationship with the front and back contact points . further , the bottom six conductive elements , namely , the lower three elements 4c in each row , are employed to provide electrical connections for the ends of the six coil leads of the three coil electromagnets . it will be appreciated that the front , back and heel contact springs 4a and 4b and six conductive elements are provided with rearwardly extending male terminal portions which are adapted to be plugged into female receiving connectors carried by a mounting sub - base which will be described in greater detail hereinafter . an insulative barrier partition 2b is situated between the contact assembly 4 and the electromagnet 3 to provide the necessary isolation between the two sub - components . as shown , the electro - magnetic structure 3 is mounted to the front of the relay base 2 by an appropriate bolt and screw which pass through holes 9 and 10 , respectively , formed in the relay base 2 . thus , the thread portions of the bolt and screw are inserted in openings 9 and 10 and are screwed into tapped holes formed in the back side of c - shaped backstrap 3a and u - shaped metallic frame 3b . the electromagnet includes the backstrap member 3a to which is staked the upper ends of the magnetic cores 3c carrying a pair of bobbins and coils 3c &# 39 ;. the downwardly extending legs of the c - shaped backstrap member 3a and the upwardly extending legs of the u - shaped metallic frame 3b are secured by suitable fasteners , such as , flat head screws ( not shown ). in viewing fig1 it will be noted that a flat spring biased pivotal armature member 3d has its rearward end inserted into a slot 3e formed in an upstanding portion 3f of the u - shaped frame 3b . in practice , an undercut slot 11a and 11b is formed on each side near the back end of the armature 3d which cooperates with entrapment portions 12a and 12b formed of the upstanding wall portion 3f so that a hinge coupling is created therebetween . thus , the armature 3d is attracted upwardly when the electromagnet 3 is energized to open the back contacts and to close the front contacts . conversely , when the electromagnet 3 is deenergized the biased armature 3d will release and will result in the opening of the front contacts and in the closing of the back contacts by the movable heel contacts 4b &# 39 ;. after the attachment of the electromagnet 3 to the relay base 2 , the elongated push rod 8 is the next element to be assembled in relation to the other element of the relay 1 . initially , a helical coil spring 13 is slipped onto the push rod 8 and is slid to the forward end which includes a finger button or knob 8a . next , the push rod 8 is threaded through a hole 15 formed in the apex portion of the name plate 6 . as shown , the rearward end of the push rod includes two ninety degree ( 90 °) offset portions 8b and 8c . the rearward end is located in an apertured slot 2c formed laterally across the intermediate portion of the relay base 2 . as shown in fig2 and 3 , a pair of upper and lower ribs 2d and 2e are located in slot 2c to provide low frictional contact surfaces for engaging the lateral portion 8b of the push rod 8 . the tip of offset portion 8c fits in a hole 2f formed in the backside of base 2 . the push rod 8 freely passes through a semicircular hole 2g located at the right edge of the front face of the barrier and contact cover plate portions of the mounting base 2 . after the push rod 8 is situated in place , the molded plastic transparent cover 5 is slipped over the contact assembly and electromagnetic structure . the rear edge of cover 5 fits into a peripheral notch 2h formed around the front of insulative base 2 . thus , the cover 5 and the mating base 2 form a weather - tight enclosure which protects the operating parts from dust , dirt and other foreign substances . as shown in fig1 the right side of the cover member 2 is provided with a channel or trough 2g for accommodating the intermediate portion of the push rod 8 . the cover 5 , the name plate 6 and the handle 7 each includes a pair of apertures or holes which are aligned with a pair of tapped holes formed in the front side of the electromagnet 3 . thus , a pair of machine screws 16 are screwed into the tapped holes and are tightened to securely hold the parts in proper relationship as shown in fig1 . hence , the spring 13 is under compression and is trapped between the finger button 8a and the front side of the name plate 6 so that push rod 8 is biased outwardly away from the top of cover 5 . in viewing fig3 and 5 , it will be noted that the back of the relay base 2 is recessed or hollowed at 2j to receive the insulative sub - base member 20 which is mounted to the relay racks in the instrument casings or housing or to panel boards of the control apparatus . the mounting sub - base 20 is preferably molded on suitable insulative material , such as , fiber glass filled resin or the like , into a rectangular box - like member . as shown in fig1 the left side of the rectangular sub - base 20 includes a raised portion 20a having two rows of orthogonal openings 20a &# 39 ; for accepting the male terminals of the fixed and stationary contacts as well as the coil energizing male terminals of the relay 1 . the openings 20a &# 39 ; are adapted to accommodate suitable female connectors which are hard wired to appropriate external circuits which are to be controlled by the relay 1 . as shown in fig4 a recessed channel or trough 20b is formed in the front intermediate portion of the insulative mounting sub - base 20 . the recessed channel 20b extends from the top to the bottom of the front surface of sub - base 20 and starts at an upper notch 20c removed from the top edge of sub - base 20 and ends at a lower notch 20d formed on the bottom edge of sub - base 20 . the upper and lower notches 20c and 20d are aligned with and are adapted to accept an upper and lower inclined inwardly depending ears 2k and 2m formed on the upper and lower inner surfaces of recess 2j of base 2 . in viewing fig4 and 5 , it will be seen that a rectangular thru opening 20e is formed in the middle of the sub - base 2 . further , an upper rectangular opening 20f and a lower rectangular opening 20g are centrally located in the area of the recessed channel 20b . in practice , a flat elongated leaf spring 21 is placed into the recessed slot 20b with the ends or tips protruding slightly beyond the upper and lower notches 20c and 20d as shown in fig4 and 5 . as shown in fig5 the ends of the leaf spring 21 are turned back upon themselves to strengthen them . after the ends of the leaf spring are bent back they may be soldered or welded to add to their rigidness and sturdiness . the spring 21 is made of resilient metal , such as , beryllium copper , which may be repeatedly bent or deflected but will return to its original shape due to its pliancy and resiliency . as shown in fig5 a pair of top knock - out tabs 22a and 22b are punched out of the upper middle section of the beryllium copper spring 21 , and a similar pair of bottom knock - out tabs 23a and 23b are punched out of the lower middle section of the spring 21 . the tabs 22a and 22b fit into the rectangular opening 20f while tabs 23a and 23b fit into the rectangular opening 20g . the tab 22a engages the upper surface of opening 20f while the tab 23a engages the lower surface of opening 20g to center the spring 22 in the slot 20b when it is in its extended or normal position as shown in full lines in fig5 . the leaf spring 21 and right side of the sub - base 20 is covered by an indexing plate 24 which is held in place by four screws threaded into the tapped holes 25 located in the respective corners . the indexing plate 24 includes two columns of circular holes 26a - i into which preselected indexing pins , such as , pins 11 and 12 , of the appropriate relay may be inserted . the indexing arrangement is employed to prevent the installation of another relay not having the exact characteristics of the original relay 1 by plugging all the indexing holes except 26a and 26d . the indexing plate 24 also has a thru hole 27 formed in it intermediately between the top and bottom edge overlying the middle of the leaf spring 21 which rests over the rectangular aperture 20e . let us now assume that the relay 1 is completely assembled , that the sub - base 2 is fixedly mounted to the panel or rack , and that a maintainer or installer wishes to plug the relay 1 into the sub - base 20 . the individual merely grasps the handle 7 and aligns the box recess 2j of relay base 2 with the outer perimeter of the box - like sub - base 20 . this will result in the alignment of the electrical male terminal portions with the female connectors and also in the lining up of the indexing pins 11 and 12 with the unplugged indexing holes 26 so that relay 1 may be pushed unto sub - base 20 by applying force to handle 7 . it will be appreciated that as the relay 1 is pushed onto the sub - base 20 , the inclined ears 2k and 2m force the tips of the latching spring 21 inwardly and results in the bending of its intermediate portion as shown in phantom in fig5 . when the tips of the deflected spring 21 reach the end of the inclined surfaces , they are snapped back behind the ears 2k and 2m to positively lock the relay 1 onto the sub - base 20 as illustrated by the full lines in fig5 . thus , the front , back and heel contacts are appropriately connected to the external electrical circuits , and vibrations or accidental bumping of the relay case will not loosen the electrical connections because of the positive latching of the ends of the spring 21 with the inclined ears 2k and 2m . let us know assume that a maintainer or inspector desires to unplug the relay 1 for inspection , maintenance or replacement purposes . the individual simply depresses the spring biased push rod button 8a so that the lateral portion 8b easily slides inwardly along ribs 2d and 2e and causes the tip of the offset portion to enter hole 27 in the indexing plate 24 . as the tip of offset portion 8c moves forward , it engages the intermediate portion of spring 21 and causes it to become deflected or bent as shown in phantom in fig5 . the deflection of the spring 21 by the tip of the push rod 8 causes the ends of the spring 21 to become retracted from behind the ears 2k and 2m so that the relay 1 is effectively unlatched . it will be appreciated that the amount of retraction is limited by the tabs 22b and 23b which engage the lower face of hole 20f and the upper face of hole 20g , respectively , which also function as centering means for ensuring that the tips are retracted an equal amount when the spring 21 is deflected by the tip of the push rod or by the inclined surfaces of ears 2k and 2m . while depressing the finger button 8a , the individual simply pulls on the handle 7 to unplug the relay 1 from the sub - base 20 to break the electrical connection with the external circuits . thus , the relay 1 may be readily plugged and latched onto the mounting sub - base 2 and also may be quickly unlatched and unplugged from the sub - base with a minimum of effort and time and without the need of ancillary equipment or tools . hence , the unique latching arrangement has the advantage of easy attachment and detachment of a plug - in relay to a mounted sub - base . it will be appreciated that while the present invention finds particular utility in plug - in electromagnetic relays , it is readily understood that the subject latching arrangement may be used in other types of relays and the like . further , it is understood that regardless of the manner in which the invention is used , it is apparent that various changes and modifications may be made by persons skilled in the art without departing from the spirit and scope of the invention . for example , the tips of spring 21 may be reinforced by having separate pieces of metal attached thereto rather than having the ends turned back upon themselves and soldered thereto . the shape and length of the push rod 8 may be varied and may pass internally through the cover 5 rather than along the outside thereof . other slight modifications may also be made , and therefore , it will be evident that all variations , alterations and equivalents falling within the bounds of the present invention are herein meant to be included in the appended claims .