the bracket 16 the guiding member 14 may be insulated at the portion where it makes contact with the metallic base element 12 and thus the guiding member 14 may be directly coupled onto the base element 12 without short circuiting obviously guiding member 14 may similarly be made with insulated side sections providing better support thereof on the base element 12 a preferred method of utilizing the assembly 2 will now be described with reference to fig4 first the base element 12 is clamped around the tooth 22 with the aid of a suitable tool the edges of which are advantageously inserted in holes 52 54 made in the legs 18 20 for this purpose after the base element 12 is clamped around the tooth 22 the guiding member 14 is screwed onto the bracket 16 and the combined unit is pressure fitted over the base element 12 the aperture 28 allows fine adjustment of the guiding member 14 above the tooth 22 in the direction of arrow a as well as allows its swivelling as indicated by arrow b so as to align the slot above the required drilling and filing point in the tooth the plug 6 is inserted in socket 8 and the assembly 2 in position is now ready to accommodate the shaft 36 of the dental instrument 38 upon drilling or filing the shaft 36 continuously or intermittently contacts the side walls of the opening 34 thereby closing an electrical circuit in the apex locator device 4 and activating same it operates on.