"The invention will now be described by way of example with reference to the accompanying drawings, in which: apparatus; FIG. 2 is a detail on a larger scale of the apparatus shown in FIG. 1; 7 FIG. 3 is a'circuit diagram for the apparatus of FIGS. 1 and 2; FIG. 4 is a diagramatic cross-section of another part of the apparatus; and FIG. 5 is a circuit diagram for the apparatus of FIG. 4; while FIG." . . . .