reference will now be made in detail to the preferred embodiments of the present invention , examples of which are illustrated in the accompanying drawings . fig1 shows a partial cross - sectional view according to an embodiment of the present invention . fig2 shows a lay - out of the present invention in fig1 . the present invention prevents the food from being burnt inside a cooking container as well as from being cooled down abruptly by forming a container 1 to have a double structure providing a cavity 116 having a hollow pocket 1 a and a heat conductive medium 2 . in the present invention , the heat conductive medium 2 is preferably silicon oil or other suitable medium known to one of ordinary skill in the art . the hollow pocket 1 a is preferrably filled with gaseous medium , such as ambient air . the container 1 of the cooking apparatus 100 according to the present invention includes an outer shell 111 and an inner shell 112 disposed adjacent the outer shell 111 . edges of the outer and inner shells 111 and 112 are preferably rolled together to form a rolled joint 117 . as a result , the cavity 116 partially filled with the silicon oil 2 and air 1 a is provided between the outer and inner shells 111 and 112 . the air pocket 1 a and silicon oil 2 are sealed and are not in fluid or gaseous communication with outside unless the pressure controlling member 3 is opened . the cavity 116 is preferably filled with ambient air and silicon oil during the manufacturing process . in the preferred embodiment , the distance between the outer and inner shells 111 and 112 is approximately 15 to 25 mm , and preferably about 20 mm . preferably , the outer and inner shells 11 and 12 are made of such materials as aisi304 stainless steel that has a thickness of about 0 . 6 mm . alternatively , instead of using a single - layered stainless steel , a multiple - layered composite materials may be used . typically , three or more layered stainless steel or combination of stainless copper and steel are preferred . in the preferred embodiment of the present invention , both the outer and inner shells 111 and 112 are jointly rolled about 2 to 8 times , and preferably 3 times , to form the rolled joint 117 . the tightly rolled joint 117 prevents the air or silicon oil contained in the cavity 116 from escaping through the rolled joint 117 even when the cooking apparatus 100 is subjected to intense heat . according to the preferred embodiment of the cooking apparatus 100 , the pressure controlling member 3 is provided to control the pressure in the cavity 116 formed between the outer and inner shells 111 and 112 . in particular , the pressure controlling member 3 releases the excessive pressure generated by the air 1 a and silicon oil 2 stored in the cavity 116 . referring to fig1 and 2 and in the first embodiment of the present invention , there is provided an electrical heating element 4 , such as a resistive heating coil , for providing heat to the container 1 . the heating element 4 is preferably placed inside the cavity 116 , and more particularly , the silicon oil 2 . the heating element 4 has a pair of plugs 5 extending through the submerged in outer shell 111 . the plugs 5 are connected to an electrical heat controller to provide electricity to the heating element 4 . the heating element 4 is placed either against the upper surface of the outer shell 111 or co - planarly between the outer and inner shells 111 and 112 without touching either one of the shells . referring to fig1 the pressure controlling member 3 , which is connected to the container 1 through a clamping hole l b , includes a spring housing 6 which is afixed to the outer shell 111 of the container 1 . according to one embodiment of the present invention , the spring housing 6 has a shape of a screw or bolt which is securely afixed to the outer shell 111 using a fastening nut 7 . the spring housing 6 defines an elongated spring device hole 6 a at one end and a pressure controlling hole 6 b at opposite end , thus sharing the same center axis . on the outer circumference the spring housing 6 that defines the spring device hole 6 a , there provide are threads 6 c for receiving a spring suspension member 10 . at the other end of the spring housing 6 , a screw head 6 d is formed to abut against the inner surface of the outer shell 111 . preferably , a washer or packing 12 may be provided between the screw head 6 d and the outer shell 111 to secure the sealing thereof . referring to fig1 in the spring device hole 6 a , a pressure control valve 8 made of heat - resistant rubber or other suitable material is inserted . the pressure control valve 8 preferably comprises a needle valve . a spring 9 is placed in the rear of the pressure control valve 8 and the spring suspension member 10 is coupled to the spring housing 6 . the spring 9 biases the pressure control valve 8 against the pressure controlling hole 6 b to substantially block the same . the spring suspension member 10 has inner threads matching that of the outer threads 6 c of the spring housing 6 to fastened thereto . thus , any excess pressure generated from the cavity 116 is released through the pressure controlling member 3 by elastic movement of the pressure control valve 8 . when sufficient pressure is created in the cavity 116 , the pressure control valve 8 is pushed back by the perssure and the pressure is relieved through the pressure controlling hole 6 b , the spring device hole 6 a and the orifice 10 a defined by the spring suspension member 10 . in the preferred embodiment , the lengthwise location of the spring suspension member 10 may be rotatably adjusted with respect to the spring housing 6 . the location of the spring suspension member 10 adjusts the tension in the spring 9 to control the opening and closing of the pressure control valve 8 . during the manufacturing process of the cooking apparatus 100 , the silicon oil 2 may be injected into the cavity 116 before sealing the cavity 116 by rolling the inner and outer shells 112 and 111 . alternatively , the silicon oil 2 may be introduced in the cavity 116 by injecting through the pressure controlling hole 6 b . furthermore , a protection cover to absorb mechanical shock as well as to improve heat insulation may be attached to the bottom of the cooking apparatus 100 for convenience . the cooking apparatus 100 shown in fig1 may be fitted with a heat insulating cover , such as a dome - shaped glass lid 51 shown in fig4 . the lid 51 is preferably made of a tempered glass disk of which the edge is protected by a stainless steel ring . moreover , the cooking apparatus 100 may have a heat insulation cover , similar to the stainless steel lid 41 shown in fig4 . fig3 shows a cross - sectional view of another embodiment of a pressure controlling member 30 according to the present invention . the pressure generated from the heated silicon oil 2 in the cavity 116 is controlled by elastically suspending a pressure control valve 38 . a spring housing 36 having a spring device hole 6 a and a pressure controlling hole 6 b in a clamping hole 1 b of the outer shell 111 is afixed to the outer shell 111 by welding or other suitable process . the pressure controlling member 30 may be used with the container 1 shown in fig1 or that shown in fig4 . the welding of the spring housing 36 to the outer shell 111 of the container 1 enhances assembly and productivity . according to fig3 the spring housing 36 of the pressure controlling member 30 is shaped like screw or a bolt which is securely afixed to the outer shell 111 . the spring housing 36 defines an elongated spring device hole 36 a at one end and a pressure controlling hole 36 b at opposite end , thus preferably sharing the same center axis . on the outer circumference of a portion of the spring housing 36 that defines the spring device hole 36 a , there provide is a circumferencial outer groove 36 e for receiving therein a step ring 91 . at the other end of the spring housing 36 , a screw head 36 d is formed to be placed against the inner surface of the outer shell 111 . referring to fig3 in the spring device hole 36 a , a pressure control valve 38 made of heat - resistant rubber or other suitable materials is inserted . the pressure control valve 38 preferably comprises a needle valve . a spring 39 is placed in the rear of the pressure control valve 38 and a spring suspension member 90 is coupled to the spring housing 36 . in particular , the inner wall of the spring suspension member 90 has a matching inner groove 90 b as that of the outer groove 36 e . the step ring 91 is positioned between the outer groove 36 e and the inner groove 90 b to releasably couple the spring suspension member 90 to the spring housing 36 . the step ring 91 is made of metal or other suitable materials . the spring 39 biases the pressure control valve 38 against the pressure controlling hole 36 b . thus , any excessive pressure generated from the cavity 116 is released through the pressure controlling member 30 by elastic movement of the pressure control valve 38 . when sufficient pressure is created in the cavity , the pressure is relieved through the pressure controlling hole 36 b , the spring device hole 36 a and the orifice 90 a defined by the spring suspension member 90 . when the heating element 4 of the cooking apparatus 100 shown in fig1 is energized , the heating element 4 generates heat which in turn heats the silicon oil 2 stored in the cavity 116 . due to the placement of the silicon oil 2 , the inner shell 112 of the container 1 is evenly heated , thereby providing even heat distribution to the food contained therein . as a result , the scorching of the container 1 and sticking of the food inside the container 1 is substantially minimized . moreover , the cooking apparatus 100 according to the present invention has excellent heat retension because even if the heating element 4 is no longer energized , the hot silicon oil 2 maintains the food warmer for a longer period of time . the present invention provides the pressure controlling member 3 or 30 to evenly discharge the increased inner pressure in the cavity 116 . as the inner pressure rises , the pressure control valve 8 is pushed outward to release the inner pressure . further , when the pressure in the cavity 116 returns to a predetermined nominal value , the spring 9 biases the pressure control valve 8 to close the valve . thus , substantially even pressure is maintained in the cavity 116 of the cooking apparatus 100 . when cleaning is necessary , the double - layered cooking apparatus 100 may be simply washed similar to any other kitchen cooking apparatus . in room temperature , the pressure control valve 8 is biased to block the pressure controlling hole 6 b to prevent any silicon oil 2 from escaping therethrough . the plug 5 portion of the heating element 4 extending through the outer shell 111 is also sealed with silicon to prevent silicon oil 2 from escaping therethrough . fig4 shows a cross - sectional view of a second embodiment of the cooking apparatus 200 . according to the second embodiment of the cooking apparatus 200 , the inner lid 50 is constructed with a dome - shaped disk 51 of which edge is surrounded by a safety ring 52 preferably made of stainless steel or other suitable materials . the safety ring 52 is attached to the edges of the disk 51 , thereby preventing damages to the disk 51 . however , the inner lid 50 may be used without the ring 52 . preferably , the disk 51 is made to form a slight convexed surface with respect to the container 60 of the cookware . the disk 51 of the inner lid 50 is preferrably made of tempered glass of approximately 4 mm thick . alternatively , the disk 51 may be made of stainless steel , alumunium , aluminum alloy or other suitable materials known to one of ordinary skill in the art . a handle 53 is attached to the center of the dome - shaped disk 51 by , for example , piercing the central portion of the disk 51 . alternatively , the handle 53 may be affixed to the disk 51 by using adhesives or other fasteners . the thermal insulating cover 40 is preferably made of a stainless steel plate 41 being about 0 . 5 to 0 . 7 mm thick , and preferably about 0 . 6 mm thick , and has a dome like figure . the insulating cover 40 and the inner lid 50 form a horizontal air cavity 44 which acts as an insulating layer to trap and hold heated air therein . as a result , once the heat source is removed from underneath the cooking apparatus 200 , the air cavity 44 and the vertical cavity 66 filled with ambient air insulate the inner shell 62 from outside and reduce heat loss , thus keeping the food contained in the inner shell 62 warmer for a longer period of time . more importantly , the structure of the present invention allows the food to be cooked even after removing heat from the cooking apparatus 200 . alternative to a single sheet structure , the cover 40 may comprise a plurality of layers of stainless steel and / or aluminum for increased durability and preservation of heat . in the thermal insulating cover 40 , a knob 42 is attached to the center of the thermal insulating cover 40 . in the present invention , the knob 42 may include a built - in temperature gauge for measuring the inside temperture of the cookware . the temperature gauge provides the user to remove the cooking apparatus 200 from the heat source once a desired temperature has been reached . the container 60 of the cooking apparatus 200 according to the present invention includes an outer shell 61 and an inner shell 62 disposed adjacent the outer shell . edges of the outer and inner shells 61 and 62 are preferably rolled together to form a rolled joint 67 . as a result , a vertical cavity 66 partially filled with air and a heat conductive medium , such as silicon oil 68 , is provided between the outer and inner shells 61 and 62 . the cavity 66 is sealed and is not in fluid or gaseous communication with outside . the cavity 66 filled with air and silicon oil 68 is preferably filled with ambient air during the manufacturing process . in the preferred embodiment , the distance between the outer and inner shells 61 and 62 is approximately 15 to 25 mm , and preferably about 20 mm . preferably , the outer and inner shells 61 and 62 are made of such materials as aisi304 stainless steel that has a thickness of about 0 . 6 mm . alternatively , instead of using a single - layered stainless steel , a multiple - layered composite materials may be used . typically , three or more layered stainless steel or combination of stainless copper and steel are preferred . in the preferred embodiment of the present invention , both the outer and inner shells 61 and 62 are jointly rolled about 2 to 8 times , and preferably about 3 times , to form the rolled joint 67 . the tightly rolled joint 67 prevents the air or silicon oil 68 contained in the cavity 66 from escaping even when the cooking apparatus 200 is subjected to intense heat . referring to fig4 there is provided a first heat conduction plate 64 placed between the outer and inner shells 61 and 62 . the first heat conduction plate 64 is preferably made of an aluminum disk , copper or other suitable materials known to one of ordinary skill in the art . the first heat conduction plate 64 is preferably flushly affixed to the bottom of the outer shell 61 . the first heat conduction plate 64 is about 1 . 5 to 2 . 5 mm thick , and is preferably about 2 mm thick . to provide speedy transmission of heat to the food contained in the cooking apparatus 200 , the first heat conduction plate 64 also abuts against the bottom of the inner shell 62 . due to the presence of the first conduction plate 64 , there is no space or cavity 66 between the bottom of the inner shell 62 and that of the outer shell 61 . in the preferred embodiment , a second heat conduction plate 65 is disposed below the bottom surface of the outer shell 61 preferably below the first heat conduction plate 64 . similar to the first heat conduction plate 64 , the second heat conduction plate 65 is preferably made of an aluminum disk or other suitable materials known to one of ordinary skill in the art . the second heat conduction plate 65 is about 2 to 4 mm thick , and preferably about 3 mm thick . the second heat conduction plate 65 is securely affixed to the bottom of the outer shell 61 by brazing or other suitable method known to one of ordinary skill in the art . the second heat conduction plate 65 is covered with a support cover 63 . the support cover 63 is attached to an outer bottom surface of the outer shell 1 fully surrounding and in contact with the second heat conduction plate 65 . the support cover 63 is preferably made of the same material as that of the container 60 of the cooking apparatus 200 . in particular , the support cover 63 is made of aisi304 stainless steel that has a thickness of about 0 . 5 mm . in the container 60 , the first heat conduction plate 64 , the bottom wall of the outer shell 61 , the second heat conduction plate 65 and the support cover 63 are in thermal communication with each other . in the cooking apparatus 200 , the cavity 66 is partially filled with silicon oil 68 up to about 40 to 70 % by volume . preferably , the silicon oil is filled up to about 55 % of the cavity 66 to accommodate for increased pressure . for example , while cooking , the pressure in the cavity 66 varies from 200 mmag to 600 mmag due to expansion of the silicon oil 68 . according to fig4 the container 60 has a pair of knobs 75 attached to opposite ends of the outer shell 61 . a pressure controlling member 3 is installed near the upper portion of the outer shell 61 . the construction and explanation of the pressure controlling member 3 have been provided above with respect to fig1 and 3 and therefore will not be repeated here . moreover , the pressure controlling member 30 shown in fig3 may be used with the container 60 . the heat conducting medium or silicon oil 68 may be injected into the cavity 66 before the the outer and inner shells 61 and 62 are sealed at the joint 67 . alternatively , silicon oil may be introduced into the cavity 66 through the pressure controlling hole of the pressure control valve 3 after the container 60 has been manufactured . once the bottom of the cooking apparatus 200 is in contact with a heat source , the content contained in the inner shell 62 is heated by the heat transmitted through the support cover 63 , second heat conduction plate 65 , outer shell 61 , first heat conduction plate 64 and finally the inner shell 12 , successively . because the heat source generally provides for uneven heat distribution to the bottom surface , the bottom of the cooking apparatus 200 is unevenly heated . however , due to the presensence of the support cover 63 , second heat conduction plate 65 and first heat conduction plate 64 , the heat from the heat source is more evenly distributed to the bottom of the inner shell 62 allowing even distribution of heat to the food contained therein . the first and second conduction plates 64 and 65 also serve to heat the vertical cylindrical walls of the cooking container 60 . although the most of the heat transmitted to the bottom of the outer shell 61 is conducted to the first heat conduction plate 64 , the rest of the heat from the first and second conduction plates 64 and 65 are transferred to the side walls of the outer and inner shells 61 and 62 , respectively . then , the temperature of the air in the space 66 is gradually increased and the heat energy of the air is transmitted to the inner shell 62 and spread out throughout the entire wall of the inner shell 62 . therefore , the temperature along the inner shell 62 is substantially constant at any region thereof . in other words , the temperature of the bottom of the inner shell 62 is about the same as that of the cylindrical wall . the presence of silicon oil 68 and air in the cavity 66 delays heat transfer between the sides of the outer and inner shells 61 and 62 as well as provides even heat distribution as the heat conducted through the outer shell 61 of the cooking apparatus 200 primarily is not transferred to the inner shell 62 of the pot but is partially conveyed through the air . the heat distributed uniformly in the inner shell 62 heats up the content in the cooking apparatus 200 . once heated , the double - layered structure of the cooking apparatus 200 according to the present invention traps the heat , even after the heat source is removed , for a prolonged period . the heat loss through the sidewalls of the inner and outer shells 61 and 62 are slowed due to the presence of the cavity 66 . the present invention provides the pressure controlling member 3 or 30 to evenly discharge the increased inner pressure in the cavity 66 . as the inner pressure rises , the pressure control valve 8 is pushed outward to release the inner pressure . further , when the pressure in the cavity 66 returns to a predetermined nominal value , the spring 9 biases the pressure control valve 8 to close the valve . thus , substantially even pressure is maintained in the cavity 66 of the cooking apparatus 200 . as an alternative embodiment of the present inventio , the second conduction plate may have a built - in heating element embeddd in the aluminum disk . as a result , in lieu of using externally applied heat , the heating element , similar to one shown in fig1 can produce heat for purpose of heating the first and second conduction plates 64 and 65 . it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions . thus , it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and equivalents .
a cooking apparatus in a form of a container includes an outer shell and an inner shell wherein a cavity is provided by coupling the edges of the outer and inner shells . the cavity is partially filled with a heat conductive medium , such as silicon oil . the cooking apparatus further includes an electric heating element installed under the container between the outer and inner shells . when energized , the heating element produces heat which in turn heats the silicon oil . the hot silicon oil heats the inner shell and thus cooks the food in the container . the outer shell of the container also has a pressure controlling member that is opened when a cavity pressure exceeds a predetermined value . the pressure controlling member comprises a spring housing securely affixed to the outer shell and defining a first through hole . the spring housing has a valve biased to open when a cavity pressure exceeds a predetermind value . the pressure controlling member also includes a spring disposed in the first through hole to bias the valve in a closed position and a spring suspension member connected to the spring housing to support the spring to bias the valve in the closed position .