in the drawings and the descriptions below , similar components of the embodiments are labeled with similar or the same reference symbols . in addition , the sliding mechanism is formed by two identically shaped sliding blocks cooperating with each other , and in some figures reference symbols are provided for one of the sliding blocks . referring to fig1 , a tamper resistant receptacle incorporating a safety shutter according to embodiments of the present invention includes a cover 1 and a base 2 removably coupled to the cover . the cover 1 has hot and neutral holes 11 and a ground hole 12 . the base 2 includes hot and neutral conductors 21 and ground conductors 22 corresponding to the hot and neutral holes 11 and the ground hole 12 . the safety shutter is disposed between the cover 1 and the base 2 , preferably aligned with the hot and neutral holes 11 . the safety shutter includes two identically shaped sliding blocks 3 and corresponding biasing member . fig2 a - 2 c show the detailed structure of the safety shutter including the sliding blocks and the biasing members . as shown in fig2 a , the sliding block 3 includes a base 31 having an opening 38 . the opening 38 can have any suitable shape as long as it sufficiently exposes the conductor 21 of the base 2 so that the plug can go through the opening to contact the conductor 21 in a normal connection . at one end of the base 31 , a platform 32 extends upward and outward from the upper surface of the base 31 , the platform 32 having a slanted surface 321 facing upward and outward to form a wedge shape . when an object is inserted into the corresponding hole of the receptacle , the object moves downward along the slanted surface 321 , thereby pushing the sliding block 3 backward ( i . e . the slanted surface 321 moves toward the rear surface 323 of the platform 32 ). in one embodiment , the slanted surface has a curved shape . because the platform 32 is raised above the upper surface of the base 31 , when two sliding blocks 3 are coupled together , the lower surface 322 of the platform 32 of one sliding block 3 will move above the upper surface 311 of the base 31 of the other sliding block , as shown in fig2 b and 2 c . to limit the range of motion of the platform 32 relative to the other sliding block 3 , a blocking portion extends from the upper surface 311 of the base 31 to contact against the slanted surface 321 of the other sliding block . in one implementation , the blocking portion is a blocking wall 35 facing the rear surface 323 of the platform 32 . in another embodiment , the blocking portion may also be a blocking block 36 disposed on a side of the base 31 . to cooperate with the blocking block 36 , the platform 32 of other sliding block 3 has a notch 37 corresponding in position with the blocking block 36 of the first sliding block 3 , such that when the two sliding blocks contact each other , the contact surface 361 of the blocking block 36 of the first sliding block is in contact against the contact surface 371 of the notch 37 . in a preferred embodiment , both the blocking wall 35 and the blocking block 36 are employed . to assemble the two sliding blocks , the platform 32 of one sliding block is disposed above the upper surface 311 of the base 31 of other sliding block . the biasing members such as springs 33 are disposed at an end of the base 31 opposite the end where the platform 32 is located . various types of springs and various mounting methods may be used . in a preferred embodiment , the spring is a helical torsion spring . such springs are compact and each to mount and replace . a post 34 is disposed outside of the blocking wall 35 and extends upward from the base 31 and the torsion spring 33 is disposed around the post 34 . one leg 332 of the torsion spring 332 is pressed against the blocking wall 35 , while the other leg 331 is pressed against an inner surface 13 of the cover 1 when assembled ( see fig5 a - 7 b ). the springs 33 provide the biasing force for the sliding blocks to urge the sliding blocks against each other , such that in a normal state , the lower surface 322 of the platform 32 covers the upper surface 311 of the other sliding block 3 . as a result , the shutter is closed and the conductors of the receptacle are not accessible . also as shown in fig2 b and 2 c , a protrusion 351 is formed on the outside of the blocking wall 35 such that it is located above the leg of the spring 33 that is pressed against the blocking wall to further secure the spring in its place . in the above - described preferred embodiment , the slanting surfaces 321 face the outside of the sliding blocks 3 such that the rear surfaces 323 of the platform 32 on the two sliding blocks face each other . this allows the sliding blocks to have a greater range of travel , so that even plugs with relatively thick prongs can be plugged into the receptacle , enhancing the applicability of the receptacle . further , having the two rear surfaces 323 face each other can prevent the sliding blocks from over traveling when oversized prongs are inserted . it also prevents over compression of springs 33 . fig3 shows the sliding blocks 3 and the cover 1 from a different perspective than fig2 a - 2 c . from fig3 , it can be seen that the platform 32 extends upward and outward from the upper surface 311 of the base 31 . ( note that the sliding blocks 3 are turned upside - down in fig3 relative to fig2 a - 2 c .) it can also be seen that the base 31 having an opening 38 generally has a u shape , so that when the two sliding blocks are assembled , the opening correspond to the hot and neutral holes 11 of the receptacle , and the ground hole 12 does not have a shutter . this can be further seen in fig4 . in other words , the safety shutter of this embodiment emphasizes the protection of the hot and neutral holes 11 with a relatively compact and simple structure . the operation of the safety shutter is explained now with reference to fig5 a - 7 b . in the normal state , the two sliding blocks are urged by the two springs 33 toward the center to stable positions . in this state , the lower surface 322 of the platform 32 of one sliding block 3 rests on the upper surface 311 of the base 31 of the other sliding block 3 , as shown in fig5 b . thus , in this state , the conductors inside the receptacle are isolated from the outside environment , minimizing the adverse effects of the environment ( dust , moisture , etc .) on the conductors . when a foreign object 4 such as a metal wire , nail , a child &# 39 ; s finger , etc . is inserted into a single hole of the receptacle , as shown in fig6 a and 6 b , the foreign object 4 slides downward along the slanted surface 321 of the platform 32 of the first sliding block 31 below that hole 11 . this pushes the platform 32 toward the center , increasing the torsion on the spring 33 for the first sliding block 3 . however , the blocking wall 35 of the second sliding block 3 is urged by its spring 33 and moves in the same direction as the platform 32 of the first sliding block , so that the lower surface 322 of the platform 32 of the first sliding block 3 is always positioned above the upper surface 311 of the base 31 of the second sliding block 3 . as a result , the foreign object 4 cannot reach the conductors 21 , effectively preventing electrical shock . the normal operation of the receptacle is shown in fig7 a and 7 b . when a plug 5 is properly inserted , i . e . the two prongs 51 are inserted simultaneously , and move down along the slanted surfaces 321 on both sliding blocks . the sliding blocks 3 move toward each other against the bias of the springs 33 , such that the lower surface 322 of one block no longer overlaps the upper surface 311 of the base 31 of the other sliding block . thus , the opening 38 of the sliding blocks are exposed , allowing the prongs 51 to reach the conductors 21 to establish an electrical connection . moreover , because the blocking walls 35 are urged by the springs 33 to return to the closed position , it makes the prongs 51 more securely inserted in the receptacle . fig8 a - 8 c show several receptacles incorporating the safety shutter according to embodiments of the present invention . as seen in these figures , the structures of the hot and neutral holes and the ground hole may be varied according to the practical applications , and the receptacle maybe a wall mounted receptacle or a receptacle on an extension cord , while the structures of the safety shutter may be used regardless of these variations . due to the compact structures of the safety shutter , the overall size of the receptacle is not substantially increased . for these reasons , the receptacle has wide applications . it will be apparent to those skilled in the art that various modification and variations can be made in the power receptacle of the present invention without departing from the spirit or scope of the invention . thus , it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents .