Abstract: A cartridge has a case of an electrically non-conductive material. A head of the case is typically formed with a coaxial through bore seating a terminal assembly comprising an annular electrode within which is an annular insulator holding a central electrode. A spark gap is defined between inner ends of the central electrode and annular electrode. The outer end of the central electrode terminates in a recess in the outer face of the head of the case in axially spaced relation to the outer end of the annular electrode. A hammer in operative association with the cartridge has spaced contact points in alignment with the annular electrode and central electrode, the contact points having the forementioned axial spacing. The entrance to the recess of the outer face of the head of the case is covered by an insulator which is displaceable by a boss on the opposed face of the hammer.