of the front face where it aligns with a boss 76 that projects forwardly toward the grounding disk 12 each boss 76 is slightly smaller both in height and width than the groove 74 that lies behind it and is connected to the main body of the disk 64 along regions of weakness 78 fig3 if the disk 64 is urged forwardly against the grounding disk 12 with enough force the leading disk 64 will fracture at the regions of weakness 78 allowing the entire plunger 10 to move forwardly toward and against the grounding disk 12 while the bosses 76 which are detached at this point are received in the grooves 74 fig6 thus the plunger 10 has the capability of moving from its initial position to a displaced position the trailing disk 66 is very similar to the leading disk 64 only it does not have the bosses 76 in particular the trailing disk 66 has a cylindrical configuration which is the same diameter as the leading disk 64 it also has bores 80 fig5 that align with and are the same size as the bores 72 in the leading disk 64 moreover it has grooves 82 fig3 which align with and are the same size as the grooves 74 in the disk 64 but these grooves extend the full length of the disk 66 so as to in effect form continuations of the grooves 74 in the leading disk.