US3206701A

US3206701A - Variable resistance key switch

Info

Publication number: US3206701A
Application number: US233529A
Authority: US
Inventors: Marion B Gregory
Original assignee: DH Baldwin Co
Current assignee: DH Baldwin Co
Priority date: 1962-10-29
Filing date: 1962-10-29
Publication date: 1965-09-14
Anticipated expiration: 1982-09-14

Description

Sept. 14, 1965 M. B. GREGORY 3,206,701

VARIABLE RESISTANCE KEY SWITCH Filed Oct. 29, 1962 5 Sheets-Sheet 1 Fig. 4

INVENTOR. MARION B. GREGOR ATTO R N IY".

Sept. 14, 1965 M. B. GREGORY 3,206,701

VARIABLE RESISTANCE KEY SWITCH Filed Oct. '29, 1962 5 Sheets-Sheet 2 a) o) 25 w 33 @527 5: -25 U 1 i l x if; F1g-6 I I I I r 30 3/ 26 I 4 I, 33 27 a? I m I'I m I r j i Fig.7 I I ai 30 31 33 2a 28 INVENTOR. MARION B. GREGORY,

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Sept. 14, 1965 M. B. GREGORY VARIABLE RESISTANCE KEY SWITCH 3 Sheets-Sheet 3 Filed Oct. 29. 1962 Fig.14 42 47 4 5 Fig.15 4 0 f INVENTOR.

m w w M m G in B. m M n m M a m Fig.19

Patented Sept. 14, 1965 3,206,701 VARIABLE RESISTANCE KEY SWITCH Manon B. Gregory, Cincinnati, Ohio, assignor to D. H. Baldwin Company, a corporation of Ohio Filed Oct. 29, 1962, Ser. No. 233,529 15 Claims. (Cl. 338-69) The invention relates to variable resistance key switches for use in electric musical instruments and the like. In musical instruments of the class in which oscillations are generated continuously, and wherein tones are produced by connecting the sources of electric oscillations to voicing, output and sound reproducing devices through corresponding key actuated switches, there is an electrical surge when the key switch is opened or closed. As a result the tone comes on abruptly, and is generally accompanied by a thump at the onset and sometimes by a click upon cessation, greatly detracting from the beauty and quality of the tone. It has been understood that this result can be overcome by the use of variable resistance key switches, such as the one described in the U.S. Patent No. 2,215,124 to Kock et al. It is an object of this invention to provide improved forms of variable resistance key switches.

It is an object of the present invention to provide a variable resistance key switch which is simpler and less expensive to manufacture.

It is an object of the present invention to provide a variable resistance key switch which will operate fully with a minimum of movement of the switch parts.

It is an object of the present invention to provide a variable resistance key switch with sufficiently low operating force to permit actuation of many switch elements per key, with an overall cost which permits such multiplication.

Another object of the present invention is to provide a switch which will yield a minimum rate of change in decibel level from first contact to full on position, while controlling the onset and cessation tone envelopes to the end set forth above.

Another object of the invention is to provide a key switch as described which will operate satisfactorily after as many as ten million cycles of operation.

These and other objects of the invention which will be described hereinafter or will be apparent to those skilled in the art upon reading this specification are ac complished by that construction and arrangement of parts of which certain exemplary embodiments will now be described. Reference is made to the accompanying drawing wherein:

FIG. 1 is a perspective view of a plunger element and stop means therefor.

FIG. 2 is a plan view of the plate support means for one or a plurality of resistance key switch gangs.

FIG. 3 is an end view of the support means as seen from the right side of FIG. 2.

FIG. 4 is a plan view of an insulative panel showing five resistance key switch assemblies mounted thereon.

FIG. 5 is an end view of the insulative panel of FIG. 4 with the contact elements shown.

FIGS. 6, 7 and 8 show in elevation an insulative panel with a resistance key switch assembly mounted thereon, illustrating the various positions assumed by the contacting element.

FIG. 9 is a partial fragmentary view illustrating two resistance key switch assemblies operated by a single key.

FIG. 10 is a perspective view illustrating the tandem mounting of two insulative panels and their respective gang resistance key switch assemblies.

FIG. 11 is a plan view of another embodiment of the present invention showing an insulative panel and resistance key switch assemblies mounted thereon.

FIG. 12 is a plan view of still another embodiment of the present invention showing an insulative panel and resistance key switch assemblies mounted thereon.

FIGS. 13, 14 and 15 are cross-sectional views taken along the section line 1313 of FIGURE 12 illustrating the various positions assumed by the contacting element.

FIG. 16 is a partial fragmentary view illustrating two resistance key switch assemblies, of the type illustrated in FIG. 12, in tandem mounting for operation by a single key.

FIGS. 17, 18 and 19 are cross-sectional views similar to FIGS. 13, 14 and 15 illustrating the various positions assumed by the contacting element when two strips of resistive material are used.

FIG. 20 is a perspective view of the spring element shown in FIG. 16.

Resistive switches of the type to be described are readily formed in gangs of varying length containing a considerable number of individual switches. These switch gang structures can readily be used in tandem so that the depression of one key will actuate a plurality of switches for the purpose of feeding harmonically related oscillations into separate headers, as in the organization of that type of electrical musical instrument which is set forth in U.S. Patent No. 2, 233,948 to Kock, issued March 4, 1941.

Briefly, in the practice of the invention a gang switch is provided with a unitary base plate or support which may be of sheet metal. The metal base plate serves to hold the gang switches in place in the instrument, and to mount a series of plungers for generally vertical re ciprocation, these plungers adapted to be associated individually with the keys of the instrument, there being at least one switch in each gang. The base plate is illustrated at 1 in FIGS. 2 and 3. It may be stiffened by turning up its edge portions as at 2 and 3 and may be of any length suitable for convenient mounting in the instrument. The base plate is adapted to be supported either at its ends or upon a rail 4 as shown in FIG. 9 to which it may be attached by screws 5.

It will be understood by one skilled in the art that the base plate 1 may support as many decks or gangs of resistive switch assemblies as required, and has a plurality of spaced perforations 6 for mounting such decks as will be more fully described hereinafter. The base plate is provided with an upper support means for vertically reciprocating plungers, which comprises a strip 7 of metal, plastic or other suitable material held in parallel spaced relationship to the base plate 1 by means of bolts 8 with suitable spacing sleeves 9 thereon. The strip 7 has a plurality of spaced slots 10 adapted to serve as upper bearings for the reciprocating plungers. The base plate itself is provided with a series of corresponding slots 11 (see FIG. 10) serving as lower bearings for the plungers.

FIG. 1 illustrates an exemplary plunger. The plunger, generally indicated at 12, comprises a flat elongated strip of insulative material of such width as to be slidable freely in the slots 10 and 11. The lower end of the plunger 13 is of somewhat greater width than the slot 11. The plunger is provided with

notches

14 and 15 for retention of a washer or ring-like structure 16. The ring 16 may be made of any suitable resilient insulative material; and as will be understood from FIG. 9 will serve as a quietly operating stop for the plunger when it is actuated upwardly to its fullest extent. A second similar ring 19 is held in the

notches

17 and 18 in the plunger and will serve as a quietly operating stop when the plunger is in its lowermost position. The plunger is also provided with

angular notches

20 and 21 for the engagement of switch elements more fully described hereinafter. It will be understood by one skilled in the art that the length of the plunger, the number of angular notches, and the height of the strip 7 53 from the base plate 1 will be dependent upon the number of gangs of switch elements to be used.

FIGS. 4 and 5 illustrate a switch deck with a number of switch assemblies mounted thereon. The switch deck comprises an insulative panel 22. The insulative panel has a plurality of perforations 23 which correspond to the perforations 6 on the base plate 1. FIGS. 9 and 10 illustrate how one or more of the insulative decks may be affixed in parallel spaced relationship to the base plate 1 by means of bolts 24 extending through coaxial sets of perforations 6 and 23 and bearing spacing sleeves indicated at 25. The insulative deck 22 is provided with a plurality of spaced grommets 26 corresponding to the number of key switch assemblies to be mounted thereon. The rearmost edge of the insulative deck is provided with a series of connectors 27 in alignment with the grommets 26 and affixed to the insulative panel or deck in any suitable manner as for example by a punched out central portion extending through the insulative panel and bent over much like the grommets.

Resistors 28 may be soldered between each adjacent pair of grommets 26 and connectors 27. These resistors act as isolating resistors for the several generators and are connected respectively to the appropriate generators through the connectors 27 A common electrical collector for all the switches of the panel 22, hereinafter referred to as the bus, extends the length of the insulative panel and is illustrated at 29. The bus 29 is surrounded by a resistive material 30. The resistive material may be formed of any suitable conductive plastic material, as for example vinyl, buna and hypalon, with the inclusion of carbon and/ or graphite in their formulation. It has been found, however, that excellent results are achieved when the resistive material is made of an inorganic silicone elastomer. As will be noted in FIG. 5, the resistive material has a flat upper surface which slants downwardly toward the front edge of the insulative deck at an angle of approximately 10 from the horizontal, that is, to the surface of the deck. The assembly of the common connector or bus and its resistive coating may be formed by extruding the plastic material about a wire or strip of metal.

A bar of insulative material, hereinafter referred to as the fulcrum bar, is located on the insulative panel parallel to and in spaced relationship with the bus 29. The bus within the resistive material 30 and the fulcrum bar 31 are held to the insulative panel 22 in spaced relationship by means of clips 32. It will be noted from FIG. 5 that the fulcrum bar has a generally rectangular cross section.

A plurality of contact elements in the form of resilient wires 33 are located on the insulative deck 22, by soldering one end of each such element to one of the grommets 26. This will insure an electrical connection between the contact element 33 and that resistor 23 which is connected to the same grommet. The free end of each contact element will be engaged in one of the angular notches in a plunger 12. For example, as shown in FIG. 10, when two decks of switch assemblies are employed, the ends of the contact elements will be engaged in the slanted notches and 21 of the plunger.

The contact elements 33 may be made of any suitable resilient conductive material as for example straight round silver clad wires and the like. It has been found that wires of circular cross section are economical to produce and require less dimensional control than do elements of other geometric cross section.

FIGS. 6, 7 and 8 illustrate the various positions of the contact element ranging from its normal position shown in FIG. 6 to its fully depressed condition shown in FIG. 8. As will be noted from FIG. 6 when the contact ele ment is in its normal position it rests on the fulcrum bar 31 and does not contact the resistive material surrounding the bus. As the contact element is depressed, the fulcrum bar acts as a fulcrum between that end of the contact member soldered to the grommet 26 and the resistive material 30. The contact elements may have different natural curvatures; but through the combined action of the plungers and the fulcrum bar they are fully controlled. As a contact element is depressed, it first contacts the uppermost corner of the resistive material 30. As the contact element is further depressed its initially point contact with the resistive material becomes an area contact. There may at the outset be some slight compressive distortion of the edge of the resistive material; but the 10 included angle in the upper surface of the resistive material permits the wire to conform to its surface as contact progresses.

It will be understood by one skilled in the art from FIGS. 6 through 9, that the insulative or fulcrum bar places a pre-load on the wire between the fulcrum bar and the plunger, thus compensating to a large degree for possible lack of straightness in the wire and thereby enhancing uniformity of the operation. Since the fulcrum bar now becomes the fulcrum or pivot bearing and is relatively close to the resistive material 30, the approach of the contacting element to the resistive material is slower in proportion to the travel of the plunger. When the plunger is fully depressed, however, the contacting element load that begins on the fulcrum bar is totally transferred to the resistive material 30. It will be understood also that the fulcrum bar assures contact of the contacting element with the resistive material at the same plunger position for all switches.

FIG. 9 is an exemplary showing of one embodiment of the switch of the present invention, illustrating the use of several insulative panels with switch elements thereon and means for actuating the plunger. In a multiple switch assembly and a gang switch assembly, each of the plungers may be actuated by a corresponding playing key 34. The playing key 34 is pivotally mounted on a fulcrum 35 and biased to a non-depressed position by the spring 36, causing the end of the key to rest on a stop 37. The key is provided with a leaf spring 38 and a controlling screw 39 to regulate its action on the plunger 12. It will be clear to the one skilled in the art that as the key is depressed the rear end of it will move upwardly in the direction of the arrow and the leaf spring 38 will contact the plunger 12 which in turn will actuate two contacting elements simultaneously. Other key constructions may, of course, be used. For example, the key construction described in the copending application entitled, Electronic Organ Construction, Serial No. 831,984 in the name of Donald W. Elbrecht, filed August 6, 1959, may be used. When such a key construction is used, the switch structure as illustrated in FIG. 9 will be inverted.

FIGS. 11 and 12 illustrate two additional embodiments of the switch of the present invention. The embodiment of FIG. 11 comprises an insulative panel 4t) with a plurality of spaced grommets 41 and corresponding connectors 42, with resistors 43 soldered therebetween. A resilient wire contact element 44 is soldered to each of the grommets 41. The structure thus far described, is similar to that illustrated in FIG. 4. However, in this instance, a plate 45 is affixed to that side of the insulative panel opposite the contact elements 44. The plate extends beyond the edge 46 of the insulative panel. A strip of resistive material 47 is held in place on the outermost edge of the plate 45 by means of bent over tabs 48 which may be integral with the plate 45. The resistive strip 47 is similar to the resistive strip 30 in FIG. 4 except that it does not carry within it a common electrical collector or bus. In this instance, the plate 45 acts as the bus. This embodiment further eliminates the need for a fulcrum bar (as shown in FIG. 4 at 31), since the edge 46 of the insulative panel 40 serves as the fulcrum.

FIG. 12 illustrates a modified construction, and those parts which are similar to the structure of FIG. 11 are given like index numerals. The structure of FIG. 12 differs from that of FIG. 11 only in that the insulative panel 40 is extended with the result that the edge 49 thereof abuts the resistive strip 47. The edge 49 of the insulative panel is provided with a plurality of spaced notches 50, one for each contact element. The rearmost edge 51 of each notch serves as the fulcrum for the contact element, in the same way as does the edge 46 in FIG. 11. An advantage of the structure of FIG. 12 lies in the fact that the abutment of the edge 49 with the resistive strip 47 reduces tolerance and alignment problems in the manu facturing process.

FIGS. 13, 14 and 15 are sectional views along the section line 14-14 of FIG. 12 and illustrate the various positions of the contact element 44 ranging from its normal position (FIG. 13) to its fully depressed position (FIG. 15). It will be understood by one skilled in the art that the relationship of the rearmost edge 51 of the notch 50 of FIG. 12, and the edge 46 of the insulative panel of FIG. 11 are such that FIGS. 13 through 15 may be considered as showing the various positions of the contact element 44 in the embodiment of FIG. 11 as well. It will be noted that that portion of the plate 45 hearing the resistive strip 47 is angled slightly downwardly from the plane of the insulative panel so that the contact surface of the resistive strip 47 will be properly oriented with respect to the contact element 44 and the fulcrum edge 51. FIG. 13 illustrates the contact element 44 in its normal position wherein it rests on the insulative panel edge 51. It is therefore maintained out of contact with the resistive strip 47. As the outermost end of the contact element 44 is depressed, the contact element pivots about the fulcrum 51 and contacts the uppermost corner of the resistive strip. As the contact element is further depressed (FIG. 15) its initial point contact with the resistive strip becomes an area contact and the contact element load that began on the fulcrum edge 51 is totally transferred to the resistive strip 47.

Thus it will be seen that the structures of FIGS. 11 and 12 accomplish the purposes and advantages outlined above with respect to the structure of FIGS. 1 through 9. In addition, they afford a simpler structure by the elimination of the fulcrum bar and the bus within the strip of resistive material.

FIG. 16 is similar to FIG. 9 but illustrates the tandem mounting of switch assemblies of the type illustrated in FIG. 12. Insulative panels 40 are mounted in parallel spaced relationship to a base plate 52 by means of bolts 53 with suitable spacing sleeves 54 thereon. As above described, the plate will have a series of slots therein serving as lower bearings for plungers, one of which is shown at 55. The plunger 55 is provided with resilient insulative rings 56 serving as quietly operating stops for the plunger 55. As described above, the plunger 55 engages the contact elements 44 and causes them to contact the resistive strips 47 when the plunger is raised by a playing key assembly (not shown).

FIGS. 16 and 20 illustrate a structure for providing support for the upper end of the plunger 55, while at the same time biasing the plunger to its key-off posi tion. A plate 57, of angle iron cross section, is supported in spaced relationship with the base 52 at the upper ends of those bolt spacer assemblies 53, 54 nearest the plungers. The plate 57 serves as a support for a strip of re silient material 58 having a plurality of outwardly eX- tending arms 59, each of which engages the upper end of a plunger 55. As will be noted in FIG. 20, the upper end of each plunger 55 is provided with a centrally located notch 60, and the outwardmost portion of each resilient arm 59 is provided with inwardly extending

notches

61 and 62. The cooperation of the

notches

61 and 62 in the resilient arm and the notch in the plunger 55 forms an inter-engagement between the arm and the plunger such that the arm will not only bias the plunger to its key-01f position, but also will maintain the plunger in its proper vertical orientation. The strip 58 will be held in place on the plate 57 by the passage of the bolt 53 therethrough and the nut 63.

FIGS. 17 through 19 are similar to FIGS. 13 through 15, and differ only in that two

respective strips

64 and 65 are substituted for the single restrictive strip 47. As shown in FIG. 17, the contact element 44, when in its normal position, rests on the fulcrum edge 51 of the insulative panel 40, out of contact with the resistive stri s 64 and 65. As the element 44 is moved downwardly (FIG. 18) its initial point of contact will be with the highest corner of the resistive strip 65. As the contact element continues in its downward motion, it will contact the surface of the resistive strip 65 and then the uppermost corner of the resistive strip 64. In its fully depressed position (FIG. 19) the element 44 will contact the surfaces of both resistive strips. It has been found that this structure gives a better starting characteristic for the tone created. This structure exends the range of reisstance from the initial point of contact of the first strip to the fully depressed position. The resistance letdown is more gradual as is the build up of the tone. It has also been found that the contact surface of the strip 65 may be scored or roughened by any suitable means which tends to make the initial resistance as high as desirable. The two strips, if desired, may have different specific resistivities.

Modifications may be made in the invention without departing from the spirit of it. For example, when a direct contact switch is desired, the above described embodiments may be provided with a bus without resistive material. Reference is made to the copending application in the name of Walter Munch, Jr., entitled Switching System for Electrical Organs, Serial No. 233,560 filed October 29, 1962. The coaction of the contact elements and a fulcrum bar or fulcrum edge of an insulative panel will compensate to a large degree for possible lack of straightness in the wire contact elements, will assure contact between the bus and wire elements at the same plunger position for all switches, and will thus enhance uniformity of operation.

The invention having been described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

1. In a musical instrument wherein each tone is controlled by an electric switch actuated by a playing key, a switch assembly for the purpose described comprising a first elongated stationary contact element, a series of second movable contact elements, said first and second contact elements mounted on an insulative base substantially at right angles to each other, said second contact elements being resilient wire-like elements one end of which, remote from said first contact element is affixed to said base, means in connection with said base near said first contact element to act as a fulcrum for said second contact elements to determine the time of contact of said second contact elements with said first contact element as the outer ends of said second contact elements are drawn downwardly, said firs-t contact element having a surface of resilient conductive substance which surface lies aslant to the plane of the outer ends of said second movable contact elements when the latter are first brought into contact with said first contact element, whereby further movement of said second contact elements will cause them to contact said first contact element along a line of increasing length transversely of said first contact element.

2. The structure claimed in claim 1 wherein said first contact element comprises a metallic strand encased in a body of resistive substance.

3. The structure claimed in claim 1 wherein said first contact element comprises a metal plate attached to said insulative base near one edge thereof, said plate bearing a strip of resistive substance in a position to be contacted by said second contact element.

4. The structure claimed in claim 3 wherein said plate holds said strip of resistive substance in substantial abutment with said edge of said insulative base but below the top surface thereof and wherein said edge of said base is notched to provide said fulcrum means.

5. The structure claimed in claim 3 wherein said plate supports two strips of resistive substance in side-by-side relationship, and in which the respective positions of the surfaces of said strips is such that they will be contacted seriatim by said second contact elements.

6. In a musical instrument wherein each tone is controlled by an electrical switch actuated by a playing key, a switch for the purpose described comprising a first stationary contact element, a second movable contact element, said first and second contact elements mounted on an insulative base at right angles to each other, insulative means in connection with said base to serve as a fulcrum for said second contact, resistive means surrounding said first contact and having a flat contact surface angled slightly downwardly and away from the direction of motion of said second contact, said second contact being operatively connected with a playing key whereby in closing said switch said second contact coacts .'ith a continuously increasing area of said contact surface to close said switch first through a high resistance and then through a continuously lowering resistance as said key is depressed.

7. In a musical instrument wherein each tone is controlled by an electrical switch actuated by a playing key, a switch for the purpose described comprising an insulative base, a stationary contact means, a resistive means associated with said stationary contact means, means for mounting said stationary Contact and said resistive meansnear and parallel to one edge of said base, a connector mounted on the opposite edge of said base, an insulative bar afiixed to said base between said connector and said stationary contact, said insulative bar being near and parallel to said stationary contact, a movable contact element, one end of said movable contact element fixedly mounted to said base, means for connecting said fixed end to said connector, said resistive means having a flat contacting surface angled slightly downwardly and away from said insulative bar, means actuable by said playing key engaging the free end of said movable contact causing said contact to coact with said insulative bar and with resistive means to establish a circuit when said key is depressed, whereby in closing said switch said insulative bar acts as a fulcrum about which said movable contact pivots to contact said resistive means, the area of contact between said movable contact and said flat contact surface increasing to close said switch first through a high resistance and thence through a continuously lowering resistance upon progressive key depression.

8. The structure claimed in claim 6 wherein said resistive means is an extruded member surrounding said stationary contact and is composed of a material chosen from a class consisting of an elastomer containing conductive material.

til

9. The structure claimed in claim 8 wherein said movable contact is a resilient wire member.

10. The structure claimed in claim 9 wherein said means for joining said fixed end of said movable contact element to said connector is an isolative resistance.

11. The structure claimed in claim 9 wherein said angularity of said fiat contact surface of the resistive means is of the order of 10.

12. The structure claimed in claim 11 in which said resistive substance is a silicone elastomer containing carbon.

13. In a musical instrument wherein each tone is controlled by an electrical switch actuated by a playing key, a switch for the purpose described comprising an insulative base member, an elongated stationary contact element mounted thereon and having a surface at an angle thereto, an elongated movable contact element having a free end and an end fixed to said base and at right angles to said stationary element, insulative fulcrum means on said base between the fixed end of said movable element and said stationary contact, means connecting said free end of said movable contact to said key, whereby upon depression of said key, said movable contact is caused to meet said stationary contact substantially at a point thereon, thereafter contacting an increasing area as said key is further depressed.

14. The combination claimed in claim 13, wherein said fulcrum means comprises an edge of said base member.

15. In combination a metal plate having a plurality of aligned slits adjacent one edge thereof, guide means having a similar plurality of aligned slits supported in spaced relationship from said plate, a plurality of flat, plunger-like elements operating respectively in the slits of said plate and guide means, at least one deck comprising an insulative panel supported in spaced relationship from said plate, wire elements each having mounting means adjacent one edge of said deck, a resistive element comprising a metal bus and a coating thereon of resilient substance containing conductive material, said bus being supported on said deck adjacent the other edge thereof, insulative fulcrum means associated on said deck and lying in parallelism to said bus, said wire elements extending beyond the last mentioned edge of said deck and engaging in slits in said plunger-like elements.

References Qited by the Examiner UNITED STATES PATENTS 2,558,659 6/51 Mork 338-69 2,559,077 7/51 Johnson et al.

2,931,877 4/60 Henley 338-114 X 2,959,693 11/60 Meyer 33869 X o RICHARD M. WOOD, Primary Examiner.

Claims

15. IN COMBINATION A METAL PLATE HAVING A PLURALITY OF ALIGNED SLITS ADAJCENT ONE EDGE THEREOF, GUIDE MEANS HAVING A SIMILAR PLURALITY OF ALIGNED SLITS SUPPORTED IN SPACED RELATIONSHIP FROM SAID PLATE, A PLURALTIY OF FLAT, PLUNGER-LIKE ELEMENTS OPERATING RESPECTIVELY IN THE SLITS OF SAID PLATE AND GUIDE MEANS, AT LEAST ONE DECK COMPRISING AN IN SULATIVE PANEL SUPPORTED IN SPACED RELATIONSHIP FROM SAID PLATE, WIRE ELEMENTS EACH HAVING MOUNTING MEANS ADJACENT ONE EDGE OF SAID DECK, A RESISTIVE ELEMENT COMPRISING A METAL BUS AND A COATING THEREON OF RESILIENT SUBSTANCE CONTAINING CONDUCTIVE MATERIAL, SAID BUS BEING SUPPORTED ON SAID DECK ADJACENT THE OTHER EDGE THEREOF, INSULATIVE FULCRUM MEANS ASSOCIATED ON SAID DECK AND LYING IN PARALLELISM TO SAID BUS, SAID WIRE ELEMENTS EXTENDING BEYOND THE LAST MENTIONED EDGE OF SAID DECK AND ENGAGING IN SLITS IN SAID PLUNGER-LIKE ELEMENTS.