An electrochemical cell capable of operating in pressure differentials exceeding about 2,000 psi, using a porous electrode. The inner and outer polepieces are provided with mutually communicating bores via which the backscattered electron detector can be withdrawably inserted into the electron beam path within the objective lens. A wiper element on at least one of the rows of projections and alternating recesses includes a tapered surface extending away from the axis, and an engagement face substantially perpendicular to the axis and adjoining the tapered surface. 3 mm, and has approximately parallelepiped meshes, the dimensions of which range from 1. The composition comprises 30-99.