Abstract:
Miniature bulbs are secured in their holders against twisting by first forming holders with tapered cavities smaller at their bottoms than the bulb, then heating the holders to soften them prior to insertion of the bulb. When the bulb is fully seated and the holder has cooled and firmed up, the holder will conform to the exterior shape of the bulb including the irregularities in its surface caused by the bulb formation process. The snug fit of the holder to the irregularities in the bulb resist the twisting of the bulb.

Description:
CROSS REFERENCE TO RELATED PATENTS 
       [0001]    The priority benefit of U.S. provisional patent application Ser. No. 61/085,694, filed Aug. 1, 2008, is claimed, which application is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In a single series light string of 100 miniature lights, there are numerous potential causes for the failure of any individual light. When several such strings are placed on a single Christmas tree, and that hundreds of millions of sets of lights are manufactured every year, reduction of the mechanisms that can cause light failure quickly becomes a high priority. 
         [0003]    Miniature bulbs have two conductive wires called Dumet wires that extend from inside the glass bulb, where they connect to the ends of a filament, to the outside. These Dumet wires pass through holes in a bulb holder and are bent back around the outsides of the holder. The holder is then inserted into a lamp socket carrying two electrical terminals in opposing relation on the inside of the socket wall. When the holder is properly seated in the socket, the Dumet wires are in electrical contact with those electrical terminals and pass current from one terminal through the filament to the opposing terminal. If, however, the bulb is twisted with respect to the holder, the Dumet wires may break or may be retracted far enough back into the holder to be out of electrical contact with the terminals and to thereby open the circuit. 
         [0004]    The twisting of the bulb with respect to the holder may happen accidentally, as when a person who is accustomed to bulbs that are threaded unconsciously attempts to twist the bulb in order to withdraw it from the socket, or intentionally, as when vandals deliberately twist individual bulbs to cause them to fail. Outside lighting displays are especially subject to vandalism, as vandals will twist a number of bulbs until the whole string fails. 
         [0005]    There has been one known attempt to solve this problem of open circuits caused when bulbs are twisted with respect to the holder, namely, the use of adhesives to glue the bulbs to their holders. However, this solution has proved unsatisfactory as the thermal expansion coefficient of the plastic, particularly when the bulb is part of an outdoor display in winter, eventually causes the bulb to loosen from its holder notwithstanding the adhesive. Accordingly, there remains a need for a way to prevent inadvertent or intentional twisting of bulbs. 
       SUMMARY OF THE INVENTION 
       [0006]    According to its major aspects and briefly recited, the present invention is a method of securing a miniature lamp of the type used in strings of lights for holiday decorating to its holder. The lamp includes a glass or plastic bulb with an internal filament and two electrically conducting Dumet wires extending from the interior of the bulb to its exterior, and a holder with a pair of through-holes that receive the Dumet wires and the bottom portion of the glass bulb. The method comprises the steps of forming a transparent bulb with irregular dimensions on the end of the bulb where the Dumet wires exit the bulb, forming a thermoplastic holder with a tapered interior cavity so that the largest part of the tapered cavity, the opening, is approximately the same size as the nominal diameter of the glass bulb to be inserted into it, and with two holes at the bottom of the cavity for the Dumet wires to extend through; heating the holder to soften the plastic; then inserting an end portion of the bulb from which the Dumet wires extend into the cavity of the holder while the holder plastic is still soft so that the bulb can easily deform the plastic and become fully seated with the Dumet wires extending through holes formed in the holder. As the plastic is allowed to cool, it conforms to the irregular surface of the bulb, securing the holder to the bulb. Then the Dumet wires are bent around the outsides of the cooled holder. 
         [0007]    An important feature of the present invention is the interaction between the irregular exterior surface of the bulb and the tapered cavity of the holder that is narrower, for most of its length, than the bulb. The advantage of this feature is that the plastic holder, on cooling, conforms tightly to the irregular exterior surface of the bulb and thereby resists being twisted with respect to the holder. The irregularities of the bulb are departures from cylindrical symmetry. When the bulb is then seated in the tight grip of the cooled holder, and a torque is applied to the bulb, the irregularities interfere with material of the holder to thereby resist the rotation of the bulb. 
         [0008]    Still another important feature of the present invention is the use of heat to facilitate insertion of the bulb into the holder. By heating the tapered plastic holder until it is soft, the plastic will be sufficiently pliable to give way as the bulb is inserted but to grip it tightly upon cooling as the plastic cools and conforms to the bulb. The heat thus allows the plastic to do what thermoplastic material does best, namely, deformably alter their shape to accommodate an irregular shape such as, in this case, the bulb both as it is being inserted into the smaller end of the holder cavity and as the warmed plastic flows back against the bulb during cooling. 
         [0009]    These and other features and advantages of the present invention will be readily understood by those skilled in the art of manufacturing miniature bulbs from a careful reading of the Detailed Description of Preferred Embodiments, accompanied by the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the figures, 
           [0011]      FIG. 1  is a side view of a bulb for a miniature lamp, according to a preferred embodiment of the present invention; 
           [0012]      FIG. 2  is a side, cross-sectional view of a holder for a bulb, according to a preferred embodiment of the present invention; 
           [0013]      FIG. 3  is a schematic view of the holder in  FIG. 2  being subjected to a source of heating to soften the lower portion thereof, according to a preferred embodiment of the present invention; and 
           [0014]      FIG. 4  is a side view of the miniature lamp with the holder shown in cross section, according to a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    The present invention is a method for making a miniature lamp for use in the sockets of a string of lights, such as holiday decorative lighting used on Christmas trees. The present method results in a much higher torque being required to twist the bulb once it is in the socket, thereby essentially making it all but impossible for a person to accidentally or intentionally twist the bulb to cause an electrical short. 
         [0016]    Referring now to the figures, the present bulb  10 , best seen in  FIG. 1 , is at least translucent and preferably transparent, and has an irregular first end  12 . Irregular means that the cross-section of bulb  10  along the major axis is not uniformly circular but departs from rotational symmetry by a few thousands of an inch around its circumference at at least one point along its axis. Although the bulb has a nominal diameter, that is, is made to a pre-specified diameter, it has small variations from perfect rotational symmetry from point to point along that axis, particularly near the ends from which a pair of Dumet wires  14 ,  16 , extend. Near Dumet wires  14 ,  16 , where they exit first end  12  of bulb  10 , molten glass may have collected and cooled so that small bulges appear around each Dumet wire  14 ,  16 . Indeed, first end  12  may also have a bulge to one side of the axis or a lean away from the axis or have some other variation in its nominal diameter so that its geometry departs from an otherwise perfect cylinder through most of the length of bulb  10  but that ends in a perfect hemisphere at first end  12 . The extent of the departure from nominal diameter need only be a few thousandths of an inch for the present invention to resist twisting of bulb  10 . 
         [0017]    Bulb  10  is made by beginning with a short length of tubing made of glass or other transparent or at least translucent material. Dumet wires  14 ,  16  are positioned with respect to each other and held in spaced relation by applying a non-conducting bead  18 , preferably a glass bead applied as a molten drop of glass to the spaced Dumet wires. The Dumet wires are then attached to a filament  20  so as to run from one Dumet wire  14  to the other Dumet wire  16 . Dumet wires  14 ,  16 , bead  18  and filament  22  are inserted into first end  12  of the tube far enough so that Dumet wires  14 ,  16 , are partially in the tube but also extend partially outside the tube at its first end  12 . Then, the ends of the tube are sealed by melting the ends to thereby form bulb  10  with portions of Dumet wires  14 , 16  extending from first end  12 . 
         [0018]    In this process, the irregularities are formed in first end  12  of bulb  10 . These irregularities in the shape of first end  12  is a natural and inevitable result of the manufacture of making bulb  10 , especially the melting and cooling of the ends of the tube around Dumet wires  14 ,  16 , and is on the order of a few thousandths of an inch, sufficient for the present invention so that no additional irregularities are needed. To close the tube, the glass must be melted as additional glass added to first end  12  to seal it will cause a globular-shaped first end  12  of bulb  10  to form. The metal Dumet wires  14 ,  16 , dissipate heat more quickly in their immediate vicinity and, together with imperfections in the glass such as microscopic bubbles and variations in the thickness of the glass tube inevitably cause sufficient deformities in the rotational symmetry of the end of the glass tube near Dumet wires  14 ,  16 . Additional or greater irregularities do not have to be formed other than those that will be formed in the current process of making miniature bulbs. 
         [0019]      FIG. 2  shows a cross-sectional view of a holder  24  made of plastic material and having a cavity  24  formed therein. Cavity  26  ends in two holes  28 ,  30 , dimensioned to receive the portions of Dumet wires  14 ,  16 , that extend outside of bulb  10 . Cavity  26  is preferably smaller in diameter toward the bottom than the diameter of bulb  10 , and most preferably has a larger first diameter B at its opening  32  than a second diameter at its bottom  34 , most preferably having first diameter about the same size as the nominal diameter of bulb  10 , and then tapering gradually from opening  32  to bottom  34  of cavity  26  so that the walls of cavity  26  are inclined at an angle A, preferably in the range 3-5 degrees off the major axis of cavity  26 . The exterior dimensions of holder  24  may be the same as prior art holders. 
         [0020]      FIG. 3  is a schematic view of holder  24  being warmed by a heat source  40  to soften the thermoplastic material of which it is made prior to insertion of bulb  10 . Source  40  can be any type of heat that softens thermoplastic materials in a controlled manner to a temperature at which they are pliable, such as a water bath, a source of radiative heat, or a source of warm air. 
         [0021]    Once holder  24  is sufficiently softened, that is, softened to the point where it will deform when pressure is applied to it, first end  12  of bulb  10  is inserted into tapered cavity  26  so that Dumet wires  14 , 16 , extend through holes  28 ,  30 , respectively, and first portion  12  is fully seated on bottom  34  of tapered cavity  26 . Holder  24  will deform easily as the bulb  10  is inserted and will conform to the irregular features of first portion  12  of bulb  10 . Upon cooling, holder  24  will shrink as it firms up and thereafter hold bulb  10  tightly in a clinch fit in part because of its original taper but also because of the irregularities of the surface of bulb  10  to which holder  24  material will conform. See  FIG. 4 . Dumet wires  16 ,  18 , where they have emerged from holes  28 ,  30 , are then bent toward the outsides of holder  24 . 
         [0022]    A torque applied to bulb  10  with respect to holder  22  will have to be sufficiently large, probably large enough to break bulb  10 , in order to twist bulb  10  from holder as each irregularity interferes with the material of holder  24 . The amount of torque required would be too great for any accidental or intentional twisting to result in the shorting of bulb  10 . 
         [0023]    Those skilled in the art of manufacturing miniature lighting will understand that many substitutions and modifications can be made in the foregoing preferred embodiment without departing from the spirit and scope of the invention, as defined by the appended claims.