Abstract:
System and methods are described for irreversibly destroying optical signatures that utilize optical signature defining material, for example discrete objects or fluids, disposed in optical signature chambers to create individual optical signatures that together form an optical signature for the system. By irreversibly destroying the individual optical signatures, unauthorized replication of the system optical signature is prevented ensuring the security of the optical signature and any technology relying upon or tied to the optical signature.

Description:
PRIOR APPLICATIONS 
     This application is a Continuation-in-Part of U.S. application Ser. No. 12/146,810 filed on Jun. 26, 2008, and a Continuation-in-Part of U.S. application Ser. No. 12/146,836 filed on Jun. 26, 2008, both of which are incorporated by reference in their entirety. 
    
    
     FIELD 
     This disclosure relates to optical signatures and systems and methods for destroying the optical signatures. 
     BACKGROUND 
     U.S. patent application Ser. Nos. 12/146,810 and 12/146,836 disclose systems and methods useful for creating unique optical signatures. In one described example, a unique optical signature can be created by an optical signature chamber containing a plurality of discrete, non-uniform, randomly disposed objects in relatively fixed but changeable positions with respect to each other that create a unique optical signature when light from a light source is directed through the optical signature chamber. In another described example, an optical signature chamber contains a fluid that creates an optical signature when light is directed through the optical signature chamber. 
     The unique optical signature can be used in any of a large number of applications including, but not limited to, the construction of an encryption key or uniquely identifying an object such as an electronic device. 
     In certain applications, particularly high security applications, it is important to prevent unauthorized access to the unique signature in order to prevent duplication or replication of the signature that can, for example, be used to gain unauthorized access to sensitive data or allow unauthorized use of equipment. In the case of current encryption technology, the encryption key is intended to be erased from memory if an unauthorized person tries to access the encryption key. However, this active erasure approach requires active power or a battery source, which is not necessarily always present. Additionally, the erasure timeline of an active erasure approach may be too long for some application environments. 
     SUMMARY 
     System and methods are described for irreversibly destroying optical signatures that utilize optical signature defining means, for example discrete objects or fluids, disposed in optical signature chambers to create the optical signatures. By irreversibly destroying the optical signatures, unauthorized replication of the optical signatures is prevented ensuring the security of the optical signature and any technology relying upon or tied to the optical signature. 
     The optical signature concepts described herein can be used in any of a large number of applications including, but not limited to, the construction of an encryption key or uniquely identifying an object such as an electronic device. Any application that would benefit from using a unique signature could utilize the optical signature concepts described herein, as well as the optical signature destruction concepts described herein. 
     In the disclosed examples, the destruction of the optical signature is initiated and completed without the application or presence of electrical power. Any destruction method or technique that results in an irreversible destruction of the optical signature can be utilized. For example, the destruction can be gravity dependent whereby the method relies upon gravity to cause the optical signature defining means to exit the optical signature chambers, which irreversibly alters the optical signatures. In another example, the destruction can rely upon manual application of a mechanical force that forces the optical signature defining means from the optical signature chambers, which irreversibly alters the optical signatures. In still another example, the destruction can rely upon a physical alteration of the optical signature defining means, in which case the optical signature defining means can remain in the optical signature chambers but with shifted physical positions or a physical alteration of a material property, such as a change in an optical property. 
     When electrical power is available, a destruction method that is initiated and/or completed using electrical power can be utilized. In addition, the destruction method can rely upon the application of electrical energy, such as x-rays which are often used to inspect equipment as part of reverse engineering efforts, to cause an irreversible alteration of the optical signature defining means when exposed to the x-rays, thereby irreversibly altering the resulting optical signature. 
     In one example, an optical signature system comprises a plurality of optical signature chambers, each optical signature chamber defining an interior space containing optical signature defining means that defines an optical signature of the optical signature chamber resulting from light being directed through the optical signature chamber. The system also includes means for irreversibly altering the optical signature defining means of at least one of the optical signature chambers so as to irreversibly alter the optical signature thereof. 
     In another example, an optical signature system comprises a housing defining a plurality of optical signature chambers, each optical signature chamber having an interior space, a first end and a second end. The first end of each chamber is closed by first barriers, and the second end of each chamber is closed by second barriers. Each optical signature chamber contains optical signature defining material whereby each optical signature chamber defines an optical signature resulting from light being directed through the optical signature chamber. At least one of the first barriers closing the first end of each chamber and the second barriers closing the second end of each chamber are movable or breakable to allow the interior space of each chamber to communicate with ambient space around the housing. 
     When the barriers are moved or broken, the optical signature defining material can exit the chambers. When the optical signature defining material comprises discrete, non-uniform, randomly disposed objects in relatively fixed but changeable positions with respect to each other, the objects fall out of or are otherwise discharged from the chambers, which destroys the random arrangement of the objects in each chamber, which results in an irreversible destruction of the optical signatures of each chamber. When the optical signature defining material comprises fluids, the fluids flow out of or are otherwise discharged from the chambers, where they can mix together so one does not know which fluid came from which chamber thereby preventing replication of the correct individual optical signatures and preventing replication of the correct arrangement of the individual optical signature of each chamber. 
     In another example, a method comprises designing an optical signature system having a housing defining a plurality of optical signature chambers, each optical signature chamber containing optical signature defining material whereby each optical signature chamber defines an optical signature resulting from light being directed through the optical signature chamber. The method also includes designing the optical signature system such that the optical signature defining material in the optical signature chambers can be irreversibly altered so as to irreversibly alter the optical signature of each optical signature chamber. 
    
    
     
       DRAWINGS 
         FIG. 1A  depicts an optical signature system with an array of optical signature chambers containing fluid. 
         FIG. 1B  depicts an optical signature system with an array of optical signature chambers containing discrete objects. 
         FIG. 2A  illustrates the system of  FIG. 1A  with a lid being opened to initiate destruction. 
         FIG. 2B  illustrates the system of  FIG. 1B  with a lid being opened to initiate destruction. 
         FIG. 3A  illustrates the system of  FIG. 1A  with the fluids spilled from the chambers. 
         FIG. 3B  illustrates the system of  FIG. 1B  with the objects spilled from the chambers. 
         FIG. 4  illustrates an example of a system using two movable lids. 
         FIG. 5  illustrates the system of  FIG. 4  with the lids opened to spill the contents of the chambers. 
         FIG. 6  illustrates an example of a system that utilizes piercing members to mechanically pierce the chambers. 
         FIG. 7  illustrates the system of  FIG. 6  with the piercing members piercing the chambers. 
         FIG. 8  illustrates a system that utilizes a pump mechanism for applying fluid pressure to a plurality of the chambers. 
         FIG. 9  illustrates the system of  FIG. 8  with the pump actuated. 
         FIG. 10  illustrates a system where the chambers are pressurized. 
         FIG. 11  illustrates the system of  FIG. 10  with the walls of the chambers lifted or broken. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1A , an optical signature system  10  is illustrated. The system  10  includes a housing  12  defining a plurality of optical signature chambers  14 . Each optical signature chamber  14  defines an interior space, a first end and a second end. The first end of each chamber is closed by first barriers  16 , and the second end of each chamber is closed by second barriers  18 . Each chamber  14  contains a fluid material that forms an optical signature defining means. 
     Further details on a system with chambers  14  containing fluids for creating an optical signature is described in U.S. application Ser. No. 12/146,836, which is incorporated by reference herein in its entirety. 
       FIG. 1B  illustrates an optical signature system  20  that includes a housing  22  defining a plurality of optical signature chambers  24 . Each optical signature chamber  24  defines an interior space, a first end and a second end. The first end of each chamber is closed by first barriers  26 , and the second end of each chamber is closed by second barriers  28 . Each chamber  24  contains a plurality of discrete, non-uniform, randomly disposed objects in relatively fixed but changeable positions with respect to each other that form optical signature defining means. 
     Further details on a system with chambers  24  containing objects for creating an optical signature is described in U.S. application Ser. No. 12/146,810, which is incorporated by reference herein in its entirety. 
     The systems  10 ,  20  generally function as described in U.S. application Ser. Nos. 12/146,836 and 12/146,810. Light from one or more light sources is intended to direct light, represented by the arrows  15  in  FIG. 1A , into and through the barriers  16 ,  26  of the chambers  14 ,  24 . Each chamber creates a unique optical signature as a result of refraction and/or reflection of the light as the light passes through the chambers. The optical signatures can be the resulting pattern of light or portion of the pattern emerging from each chamber  14 ,  24 , the intensity of one or more pixels of light emerging from each chamber, or any other measure of the light emerging from the chambers. One or more light detectors  17  are provided to detect the resulting unique optical signature produced by the chambers  14 ,  24  as the light emerges from the barriers  18 ,  28  of the chambers. The detectors may resolve a single point or an array of points and will accept light from at least one optical signature chamber. The detectors can be, for example, a pin diode that produces a single intensity output, or a charge-coupled device (CCD) which receives a relatively wider area of light and produces an array of values as an output, i.e. an image, each relating intensity or color. 
     As used herein, the optical signature produced by each optical signature chamber will be referred to as an individual optical signature. The individual optical signatures of the chambers together form a resulting unique optical signature for the respective systems  10 ,  20 . In some circumstances, the light waves emerging from the individual optical signature chambers may continue to interact and interfere with each other as they exit the optical chambers, and it is the result of the interfering and interacting light waves that is detected and forms the resulting optical signature of the system. 
     The number of chambers and the arrangement of the chambers in a pattern or array can vary based on a number of factors, including the particular security application and how many different individual optical signatures need to be created. The chambers are arranged adjacent to one another so as to receive the light from the light source. The chambers can be arranged into a pattern, for example an array of rows and columns, for example x rows and y columns. 
     When fluid is used as the optical signature defining means, at least two chambers  14  having differing individual optical signatures is required. In the case of randomly disposed objects, only a single chamber  24  is required. The chambers  14 ,  24  could also contain a mixture of fluid and randomly disposed objects. Likewise, the chambers in each system  10 ,  20  could comprise chambers containing fluid and chambers containing randomly disposed objects. 
     For both systems  10 ,  20 , the larger the number of chambers, the more individual optical signatures that are created and in general the more unique the resulting optical signature of the system. However, for the same number of chambers, changing the pattern of the chambers and the locations of the various chambers in the pattern alters the resulting unique optical signature for the entire system. 
     The barriers  16 ,  18 ,  26 ,  28  are designed to allow passage of light waves into and from the chambers. The barriers can be, for example, made of a material that permits passage of the light, such as clear glass. Alternatively, the barriers can be light transmissive only in the locations where they directly cover the interior spaces of the chambers. The remainder of the housings  12 ,  22  can also be wholly or partially made of light transmissive material. 
     The light source(s) and the chambers can be arranged so that the light enters through a longitudinal end of the chambers (for example, through the barriers  16 ,  26 ), and exits through an opposite longitudinal end (for example, through the barriers  18 ,  28 ). If multiple light sources are used, there can be one light source for each chamber or multiple chambers can share a common light source. The light provided by the light source is preferably visible light, more preferably monochromatic light, for example coherent light from a laser. However, in certain applications, other types of visible light, such as white light, could be used. In addition, non-visible light, for example infra-red light, could be used. Any light source that provides a light wave that can be detected by a suitable optical detector after passing through the signature chamber(s) can be used. 
     The fluid used has optical properties suitable for producing an individual optical signature. These optical properties can include, for example, indices of refraction, opacity, and wavelength filtering or combinations thereof. In one embodiment, the optical properties of the fluid in the chambers will be different whereby each chamber produces a different individual optical signature. In other embodiments, some of the chambers can contain the same fluid or dissimilar fluids having the same optical properties so that they produce the same individual optical signatures. Nonetheless, not all chambers will produce the same individual optical signature and the chambers will be arranged such that the resulting optical signature for the entire system  10  is unique. 
     Any of a large number of different kinds of fluids can be used. The fluids can be, for example, optical coupling gels available from Dow Corning Corporation of Midland, Mich. The viscosity of the fluids in the chambers can vary from very viscous fluids such as liquids including water, to less viscous fluids such as gels. In addition, the fluids can be gases having differing optical properties. 
     The fluid in each chamber can be homogenous or a mix of two or more fluids. In addition, colorant can be added to the fluid to alter the color of the fluid. 
     The chambers can be completely filled with the fluid so that reorientation of the chambers does not alter the individual optical signatures. In another embodiment, the chambers are only partially filled with fluid. Thus, if the orientation of the chambers is altered sufficiently (for example by tilting the chambers from a vertical orientation shown in  FIG. 1A  to an angled or horizontal orientation), the fluids shift within the chambers and the individual optical signatures are altered since the light path through the chambers changes. 
     Further, the fluids used are preferably optically stable so as to maintain their optical characteristics over time. Nonetheless, one or more of the fluids can be designed to optically degrade or change in some manner when exposed to certain operating conditions. For example, the optical properties of one or more of the fluids can be designed to change when the chambers are exposed to certain environmental conditions, such as temperature extremes, or when exposed to x-rays, or when exposed to certain operational conditions, such as vibration or shock extremes. 
     With respect to the chambers  24 , relatively fixed objects means that during normal use the objects retain their positions relative to each other. However, those relative positions are changeable upon the occurrence of an event including, but not limited to, attempted tampering with the chamber(s) or upon application of sufficient force to the chamber(s) that destroys the chamber(s), which thereby alters the resulting optical signature that is created by the chamber(s). For sake of convenience, any event that causes the relative positions of the objects to change will be described herein as a destructive event. 
     The objects used are preferably discrete from each other during normal use. In other words, the objects are separate or separable from one another although they may be in abutting contact, which facilitates changing of the relative positions. During normal operating condition, the objects can also be described as being separable from each other or non-fusible. Although a number of different words can be used to describe the discrete, separate objects, in one example the objects are intended to spill from the chambers during a destructive event and randomly mix with objects spilled from the other chambers. 
     The objects in each chamber  24  also have non-uniform or differing optical properties. These optical properties can include, for example, indices of refraction, opacity, and wavelength filtering or combinations thereof. The differing optical properties can be provided in a number of ways, including, but not limited to, using objects of differing sizes, shapes, materials, colors and the like. Thus, the term non-uniform can refer to non-uniformity of the optical properties, or simply non-uniformity in a structural and/or material configuration that results in the non-uniformity of the optical properties. When each chamber is filled with the objects, the resulting non-uniformity of the objects creates the unique signature when the light is passed through the chamber. The objects are preferably solid so that they do not change shape, and thus their optical properties, during normal use. 
     Further, when the chambers are filled, the objects in each chamber have random positions and orientations within the chamber. However, the chambers are filled such that the positions and orientations of the objects during normal use are maintained. Thus, in one embodiment, the chambers can be completely filled in a tightly packed configuration whereby regardless of the orientation of the chambers (i.e. vertical, horizontal, angled, etc.) the positions and orientations of the objects in the chambers are maintained. In another embodiment, the chambers are only partially filled. Thus, if the orientation of the chambers is altered sufficiently (for example by tilting the chambers from a vertical orientation shown in  FIG. 1B  to an angled or a horizontal orientation), the positions and/or orientations of the objects can change, thereby altering the resulting optical signatures. 
     The objects can comprise a number of different forms. In one embodiment, the objects are beads, such as optical beads, made of any suitable material such as glass and having any suitable shape, such as generally spherical. The objects could also be marbles or marble-like objects. Thus, in these two non-limiting examples, the chambers would somewhat resemble jars full of marbles. 
     Preferably, the objects are non-fusible over normal operating conditions to maintain their separable conditions. The objects are also preferably non-ablative, non-wearing, and have no relative motion with respect to each other during normal operating conditions. Further, the objects are preferably optically stable so as to maintain their optical properties over time. Nonetheless, the objects can be designed to degrade or change in some manner when exposed to certain operating conditions to alter their optical properties. For example, the optical properties of the objects can be designed to change when the chambers are exposed to certain environmental conditions, such as temperature or humidity extremes, or when exposed to x-rays, or operational conditions, such as vibration or shock extremes. 
     Each system  10 ,  20  is provided with means for irreversibly altering the optical signature defining means (for example, the objects or the fluid) of at least one of the optical signature chambers so as to irreversibly alter the optical signature thereof, which results in alteration of the optical signature of the system. The alteration can be initiated and completed without the application or presence of electrical power. 
     The means for irreversibly altering the optical signature defining means can cause alteration either within the chambers themselves whereby the optical signature defining means remain in the chambers, or can cause alteration as a result of the optical signature defining means exiting the chambers, or a combination thereof. 
     When the optical signature defining means exit the chambers, the exiting can be gravity dependent whereby gravity causes the optical signature defining means to exit the chambers. In another example, the exiting can rely upon manual application of a mechanical force that helps to force the optical signature defining means from the optical signature chambers. When the optical signature defining means remain in the chambers, the alteration can occur by a physical alteration of the optical signature defining means. Physical alterations include, but are not limited to, irreversible alteration of physical positions or locations of the optical signature defining means or a physical alteration of a material property, such as a change in optical property. 
     Returning now to  FIGS. 1A and 1B , an example of means for irreversibly altering the optical signature defining means in the form of a gravity dependent mechanism is illustrated. In this example, the barriers  16 ,  26  are formed by a common structure, in this case a lid  30 . The lid  30  is pivotally connected to the housing  12 ,  22  for movement from a first, closed position (shown in  FIGS. 1A and 1B ) to a second, open position (shown in  FIGS. 2A and 2B ) where one end of each chamber is open. The lid  30  can be actuated to the second, open position by a manually actuated lever or other manually actuated mechanical mechanism connected to the lid. 
     When the lid  30  is actuated to the second, open position, the chambers  14 ,  24  empty as a result of gravity as illustrated in  FIGS. 3A and 3B . In the case of the liquids as the fluids, the liquids flow out of the chambers and mix together  32  as shown in  FIG. 3A . When this happens, the individual index values of each liquid are lost. Further, with the chambers now substantially empty, it cannot be determined which chamber contained which liquid and how much of each liquid. In the case of discrete objects, the objects fall out of the chambers and can gather into a pile  34  as shown in  FIG. 3B . With the chambers empty, it cannot be determined which chamber contained which objects or how the objects were arranged in the chambers. 
       FIGS. 4 and 5  illustrate another example of means for irreversibly altering the optical signature defining means in the form of a gravity dependent mechanism. In this example, the barriers  16 ,  18 ,  26 ,  28  are formed by first and second lids  40 ,  42 . The lids  40 ,  42  are pivotally mounted adjacent their bottom ends for movement between first, closed positions (shown in  FIG. 4 ) and second, open positions (shown in  FIG. 5 ). The lids include facing ramp surfaces  44 ,  46  adjacent their upper ends. A wedge  48  or other member is movably disposed adjacent the upper end and is movable downward into engagement with the ramp surfaces  44 ,  46 . Engagement between the wedge and the ramp surfaces forces the lids  40 ,  42  to pivot outwardly to the second, open position, uncovering the ends of the chambers and allowing the optical signature defining means to flow or fall out of the chambers.  FIGS. 4 and 5  illustrate the optical signature defining means as being fluid, but the optical signature defining means could be discrete objects as well. 
       FIGS. 6 and 7  illustrate another example of means for irreversibly altering the optical signature defining means in the form of a mechanical force mechanism that forces the optical signature defining means from the optical signature chambers. In this example, gravity also helps alter the optical signature defining means. The barriers  16 ,  18 ,  26 ,  28  are formed by breakable material  50 ,  52  that can be pierced by piercing members  54  to mechanically pierce the chambers. The piercing members  54  are disposed to the left in  FIG. 6  and are movable to the right to initially pierce the barriers  16 ,  26 , with the piercing members continuing through the chambers to pierce the barriers  18 ,  28 . 
     As shown in  FIG. 7 , the piercing members  54  force the optical signature defining means, which in the illustrated example is fluid, from the chambers where the fluid collects in a pool  56 . The piercing members can be generally conical shaped, and the chambers can be cylindrical shaped, with the diameter of the piercing members generally equaling the diameter of the chambers at one end, so that the fluid can only flow out from one end of the chamber. The reduction in volume of the chambers caused by the piercing members forces the fluid to flow from the chambers. In addition, gravity will aid the fluid in flowing from the chambers. If the optical signature defining means are discrete objects, the piercing members  54  will force the objects from the chambers. 
       FIGS. 8 and 9  illustrate another example of means for irreversibly altering the optical signature defining means in the form of a pump mechanism for applying fluid pressure to a plurality of the chambers to force the optical signature defining means from the optical signature chambers. In this example, the barriers  16 ,  26  are formed by a lid  60  that is pivotally connected to the housing for movement from a first, closed position (shown in  FIG. 8 ) to a second, open position (shown in  FIG. 9 ) where one end of each chamber is open. The housing also defines a pump chamber  62  that is filled with, for example, a fluid. Flow channels  64  extend from each chamber  14 ,  24  to the pump chamber  62 . Only two channels  64  are illustrated in  FIGS. 8 and 9 , it being understood that each chamber could have a flow channel connecting to the pump chamber  62 . 
     A manually actuated piston mechanism  66  is disposed in the pump chamber. The piston mechanism  66  includes a lid engaging end  68  that can project from the housing as shown in  FIG. 9  to actuate the lid to the open position. The end  68  also closes off the ends of the flow channels  64  as shown in  FIG. 8  to retain the chambers  14 ,  24  filled with the optical signature defining means, which in the illustrated example is fluid. The piston mechanism  66  also includes a push button end  70  that extends from the housing to be manually actuated by a user. 
     When a user pushes the push button end  70  in the direction of the arrow, the piston mechanism  66  is moved to the left. The engaging end  68  moves with the push button end  70 , pushing the lid to the open position. At the same time, the flow channels  64  are uncovered and the volume of the pump chamber  62  is decreased. The fluid in the pump chamber  62  is thereby forced through the flow channels and into the chambers  14 ,  24 , which forces out the fluid from the chambers as shown in  FIG. 9 . A similar principle will work when the optical signature defining means are the discrete objects, where the fluid forced from the pump chamber into the chambers  24  will force the objects out of the chambers. 
       FIGS. 10 and 11  illustrate another example of means for irreversibly altering the optical signature defining means in the form of pressurized chambers  14 ,  24 . Each chamber  14 ,  24  is pressurized, while the area around the housing is at a lower pressure for example atmospheric pressure or at vacuum. Alternatively, the chambers can be kept under vacuum while the area around the housing is at a higher pressure. Any pressure differential between the chambers and the area around the housing that is suitable to result in alteration of the optical signature defining means can be used. The barriers  16 ,  26  are formed by breakable material, or the barriers can be formed by a movable lid. When the barriers  16 ,  26  are broken or lifted, the internal pressure of the chambers forces out the optical signature defining means as illustrated in  FIG. 11 . 
     In the specific example illustrated herein, fluid or objects can be used as the optical signature defining means. Therefore, for example, illustration of objects in  FIGS. 10 and 11  is not intended to limit the concepts disclosed in  FIGS. 10 and 11  to be limited to use with objects. 
     Other means for irreversibly altering the optical signature defining means can be used. The optical signature defining means could be stirred or mixed within the chambers, either by themselves or with added material, so as to alter the optical signature. The optical signature defining means can also be altered if one manually opens the lid  30  or  40 ,  42  or the lid is somehow physically destroyed. Further, if the housing that forms the chambers is destroyed, the optical signatures would be destroyed. In addition, in the case of optical fluids, a pressurized system can be used to dissolve a gas into the optical fluid, and when the barriers are removed, the fluid outgases or bubbles, changing the optical properties. This embodiment would be useful, for example, when two non-mixing fluids are introduced into the same chamber and the fluids are kept apart by surface tension of the fluids. The outgassing would force some mixture of the fluids, thereby changing the optical properties. 
     In addition, as discussed above, the optical signature defining means can remain in the chambers, and the alteration can occur by a physical alteration of the optical signature defining means. For example, the optical signature can be altered by changing the orientation of the housing, which causes the optical signature defining means to shift positions. The optical signature defining means can be designed to physically alter when exposed to x-rays. The optical signature defining means can also be exposed to an acid that can be introduced into the chambers to cause alteration of the optical signature defining means thereby resulting in alteration of the optical signature. 
     The side walls of the chambers may also be breakable or movable such that when broken or moved, the chamber contents mix while the contents remain in the housing. 
     In the systems described herein, if the optical signature defining means are spilled, forced or otherwise leave the chambers, the optical signatures the chambers are destroyed. Due to the nature of the optical signature defining means and the chambers, one cannot determine which optical signature defining means came from which chamber, in what amounts, nor in what arrangement. Therefore, the optical signature cannot be recreated. Likewise, if the optical signature defining means remain in the chambers, but are physically or positionally altered, the correct optical signature cannot be recreated. If physical or positional alteration does occur, that can be used as an indicator of out-of specification treatment of a device incorporating the described system, which can, for example, void a warranty or act as a “switch” for shutting off an abused device requiring interface with an authorized entity to reset the device. 
     The examples disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.