This investigation grew out of the need to design a piece of electronic test-equipment. The purpose of the equipment was to study the effect of varying capacity upon the behaviour of a number of electric circuits, and it consisted basically of an automatically-stepping switch which presented at its output terminals a controlled sequence of values of capacity. The nature of the test demanded that the set of capacity values should as nearly as possible cover a given range in a logarithmic scale, and to economise in components the equipment was to generate the set of values from a basic set of condensers, by creating all possible parallel-connected combinations. The problem was, how to choose the basic set? The analysis has a number of points of combinatorial interest, and illustrates fairly well the ad hoc approach of the industrial mathematician.