the invention is described below with reference to embodiments . fig1 is a vertical cross sectional view of a circuit - breaking apparatus or current cut - out device showing a first embodiment of the invention . the circuit - breaking apparatuses 901 , 902 , 903 , 904 in fig7 all have the same structure , so the circuit - breaking apparatus 901 is only described . in this figure , the circuit - breaking apparatus 901 comprises an insulating cylinder 93 , a conductor 96 housed in the insulating cylinder 93 , and two electrodes 97 , 99 electrically and mechanically fixed to both sides of the conductor 96 . the conductor 96 has a circuit - breaking rod 96a in a middle , a barrel section or barrel 96l located at the left of the circuit - breaking rod 96a , and a fixed portion 96r located at the right thereof . the circuit - breaking rod 96a can pass through the barrel 96l . the fixed portion 96r has a threaded portion described below , into which the first electrode 99 is screwed and fixed , and the barrel 96l also has at its left end a threaded portion , into which the second electrode 97 is screwed and fixed . a connecting portion between the circuit - breaking rod 96a and the fixed portion 96r is constricted or narrowed so as to form a narrow portion 96b , and a connecting portion between the circuit - breaking rod 96a and the barrel 96l is similarly constricted or narrowed to form a narrow portion 96c . the first electrode 99 has , at a side of the circuit - breaking rod 96a , a built - in detonator 94 in which an explosive is filled to explode by an electric current supplied via a lead 501 . the second electrode 97 has a through hole along a central axis thereof , into which an operation - indicating rod 98 is inserted . a right - end head of the operation - indicating rod 98 protrudes into the barrel 96l , while a left end is flush with the end surface of the second electrode 97 . an arc - extinguishing insulating cylinder 95 is provided around the circuit - breaking rod 96a so as to contact an inner surface of the insulating cylinder 93 . the second electrode 97 has a terminal 91 connected to an external circuit , and the first electrode 99 has a terminal 92 . fig2 is a cross sectional view of the conductor shown in fig1 . in this figure , the conductor 96 comprises the barrel 96l on the left , the circuit - breaking rod 96a in the middle , and the fixed portion 96r on the right . the fixed portion 96r has a threaded portion 96g in which the first electrode 99 is mounted , and a hollow portion in which the detonator 94 is located . in the detonator , an explosive is filled and the lead 501 is introduced as described above . the barrel 96l has a threaded portion 96h in which the second electrode 97 is mounted , and constitutes a cylinder with an inner diameter slightly larger than the outer diameter of the circuit - breaking rod 96a . the barrel 96l has , near its left tip and at the right of the threaded portion 96h , a large - diameter portion 96f which operates as a locking section and which has an inner diameter larger than the inner diameter of the barrel 96l so that a space enlarged outwardly is produced therein . the circuit - breaking rod 96a is shaped like a round bar , and is connected to the fixed portion 96r via the narrow portion 96b and to the barrel 96l via the narrow portion 96c . the circuit - breaking rod 96a has at its left end a recess 96e and a protruding portion 96d surrounding the recess 96e . the recess 96e has a cross section shaped to fit the head of the operation - indicating rod 98 and the protruding portion of the electrode 97 . specifically , the recess 96e has a conical shape with a flat bottom surface . the axial dimension l4 of the large - diameter portion 96f is set so as to be almost the same as the axial dimension l5 ( fig3 ( a )) of the protruding portion of the second electrode 97 described below . the conductor 96 may be formed by integrally uniting the barrel 96l , the circuit - breaking rod 96a , and the fixed portion 96r together , or these parts may be separately produced and connected together by means of brazing or diffusion bonding while the narrow portions 96b , 96c are simultaneously formed . fig3 ( a )- 3 ( c ) show the second electrode 97 and the operation - indicating rod 98 penetrating the electrode 97 , as shown in fig1 . fig3 ( a ) is a cross sectional view of the second electrode 97 , fig3 ( b ) is a side view of the operation - indicating rod 98 , and fig3 ( c ) is a cross sectional view of the second electrode and the operation - indicating rod when assembled together . in these figures , the second electrode 97 has a through hole 97b along its central axis , and also has a screw section 97c screwed into the threaded portion 96h of the barrel 96l in the conductor 96 and a threaded section 97d on which the terminal 91 is mounted . the second electrode 97 has at its right end a protruding portion 97a with the same axial dimension and inclination as the conical portion of the recess 96e ( see fig2 ) of the conductor 96 . the operation - indicating rod 98 is formed of a material harder than the second electrode 97 , and has two diameter portions . one diameter portion is slightly smaller than the inner diameter of the through hole 97b which allows easy insertion , and the other is a large - diameter portion 98b with a larger outer diameter . the operation - indicating rod 98 has at its right end a head 98a with a larger - diameter which contacts the bottom surface of the recess 96e of the conductor 96 when the circuit - breaking rod 96a is urged thereto . the larger - diameter portion 98b of the operation - indicating rod 98 is produced by means of knurling , and is press - fitted in the through hole 97b with a smaller diameter . the presence of the large - diameter portion 98b causes the operation - indicating rod 98 to be held in position after press - fitting , so that the operation - indicating rod 98 is preventing from moving easily in the through hole 97b . adequate press - fitting and retaining forces can be obtained by selecting appropriate values for the diameter and axial dimension l6 of the large - diameter portion 98b . the circuit - breaking or current cut - out operation of the circuit breaking - apparatus shown in fig1 is described below . if an impact sufficient to activate an air bag in an electric car occurs upon an accident , an output signal from the impact detector built in the controller 400 shown in fig8 causes an electric current to flow to the detonator 94 via the lead 501 , thereby exploding the explosive . the internal pressure of the detonator 94 increases rapidly by the explosion to generate a force that pushes the circuit - breaking rod 96a toward the second electrode 97 . the narrow portions 96b and 96c are cut off by this force , thus releasing the circuit - breaking rod 96a to move through the barrel 96l and to hit the second electrode 97 . consequently , the conductor 96 is cut off at the circuit - breaking rod 96a , thereby eliminating the current path and interrupting the electric current passing through the conductor 96 . fig4 is a vertical cross sectional view of the circuit - breaking apparatus at the moment when the bottom surface of the recess 96e of the conductor 96 contacts the head 98a of the operation - indicating rod 98 . in this figure , the distance l1 between the left end surfaces of the first electrode 99 and the fixed portion 96r , and the right end surface of the circuit - breaking rod 96a is set at a value larger than the insulating distance required therebetween . due to the presence of the large - diameter portion 98b as described above , a large force is required to move the operation - indicating rod 98 by the circuit - breaking rod 96a from the position shown in this figure until the left end of the rod 98 protrudes from the second electrode 97 . as described above , the dimension l1 is large enough not to cause insulation breakdown between the first electrode 99 and the circuit - breaking rod 96a to prevent failure of the circuit - breaking function even if the operation - indicating rod 98 is not pushed enough by failure and the circuit - breaking rod 96a remains in the position shown in fig4 . fig5 is a vertical cross sectional view of the circuit - breaking apparatus after the circuit - breaking operation has been completed . the circuit - breaking rod 96a is moved further from the position shown in fig4 so as to push the operation - indicating rod 98 in such a way that the left end protrudes as shown in the figure . due to the large - diameter portion 98b described above , the operation - indicating rod 98 is held in position once it protrudes from the second electrode . thus , it is possible to visually confirm whether the circuit - breaking apparatus has performed its circuit - breaking function . in addition , a very large force is applied to the circuit - breaking rod 96a in order to press and move the operation - indicating rod 98 leftwardly . therefore , after the circuit - breaking rod 96a hits the second electrode 97 , its protruding portion 96d is plastically deformed when it is pressed against the large - diameter portion 96f , and then enters into the portion 96f . as a result , even after the pressure has been eliminated , the plastically deformed portion serves as a stopper for holding the circuit - breaking rod 96a in contact with the second electrode 97 . this prevents the circuit - breaking rod 96a from returning to its original position by any reason to re - form a conductive path . in case an electric current is flowing through the narrow sections 96b and 96c when the narrow sections are cut off and the circuit - breaking rod 96a starts moving leftwardly , an arc may be generated in the gap at the narrow portion 96b which has been formed when it has been cut off . this arc is usually cut off and extinguished due to the rapid movement of the circuit - breaking rod 96a in order to complete the circuit - breaking action , but extinction of the arc may be difficult if a large electric current , such as a short - circuit current , is present . in view of this point , an arc - extinguishing insulating cylinder 95 is provided to improve the arc - extinguishing performance . a material for the arc - extinguishing insulating cylinder 95 should have good circuit - breaking characteristics and be able to sufficiently maintain the insulating characteristic after circuit - breaking . also , the material should possess a large ratio of hydrogen to carbon in the decomposed gas that has been generated by thermal decomposion of the material , so that a large amount of hydrogen gas is generated upon circuit - breaking to thus effectively cool the arc for extinction . furthermore , decomposed gas without a benzene derivative is effective in maintaining insulation resistance after the circuit - breaking , and polyacetal resin is suitable for the material . the materials , however , are not limited to these resins . the arc - extinguishing insulating cylinder 95 also serves to protect the insulating cylinder 93 . fig6 is a cross sectional view of the circuit - breaking apparatus showing a second embodiment of the invention . this embodiment differs from the first embodiment in that an arc - extinguishing material 95a is located at the end of the detonator 94 . when the explosive is exploded , the narrow portions 96b and 96c are cut off to move the circuit - breaking rod 96a , causing an arc to generate . the arc then impinges on the arc - extinguishing material 95a to generate gas , which cools and extinguishes the arc rapidly . the arc - extinguishing material 95a is preferably a polyacetal resin powder suitable for the above arc - extinguishing insulating cylinder 95 , but other materials are also suitable . although the structure other than the arrangement in fig6 which relates to the arc - extinguishing material 95a , has been described in the first embodiment , the structures including the installation of the operation - indicating rod 98 , the arc - extinguishing cylinder 95 and the large - diameter portion 96f for retaining the position of the circuit - breaking rod 96a by using plastic deformation need not be necessarily incorporated in the circuit - breaking apparatus . that is , if one or more of these elements is applied to the circuit - breaking apparatus , such element or elements will produce the respective effects . consequently , four items including the installation of the arc - extinguishing material 95a may be applied to the actual circuit - breaking apparatus by combining them as appropriate . as described above , according to the first aspect of the circuit - breaking apparatus of the invention , since the locking section in the barrel ensures the locking of the circuit - breaking rod that has been shifted to the second electrode , the circuit - breaking condition can be reliably maintained without subsequent movement of the circuit - breaking rod . in the second aspect of the invention , the protruding portion at the end of the circuit - breaking rod is plastically deformed and enters into the large - diameter portion provided in the barrel . the portion that the circuit - breaking rod has entered acts as a stopper to lock the rod . according to the third aspect of the circuit - breaking apparatus , at the time of circuit - breaking , if an arc is generated when the circuit - breaking rod is shifted , the arc is reliably extinguished , because the arc is cooled by decomposed gas generated when the arc contacts the arc - extinguishing cylinder or the arc - extinguishing material . according to the fourth aspect of the circuit - breaking apparatus , at the time of circuit - breaking , since the circuit - breaking rod contacts the operation - indicating rod to protrude the operation - indicating rod externally from the second electrode , this protrusion can be externally seen to easily confirm whether or not a circuit - breaking operation has been performed . in addition , since the operation - indicating rod is press - fitted into the electrode , it is prevented from being moved easily either before or after a circuit - breaking operation , so that an error in confirming whether a circuit - breaking operation has been performed is prevented . furthermore , since the moving distance of the circuit - breaking rod until it contacts the operation - indicating rod is set so as to be longer than the insulating distance required between the circuit - breaking rod and the first electrode , even if the circuit - breaking rod stops at that position , the required insulating strength is ensured to perform the reliable circuit - breaking function .