The analogue representation of the properties (in terms of complex frequency λ) of image-parameter transfer functions of the Zobel type by current-flow in an electrolytic tank provides, in the first place, a convenient means of designing such filters, and it has the important consequence that the properties of these current-flow representations are found to lead to a process for the solution of the approximation problem in the insertion-loss theory of filter design. The paper is in two parts, of which Part 1 is devoted to the representation of image-parameter transfer functions by the electrolytic tank. The electrolytic tank itself is first considered to be of the double-layer form described in an earlier paper;2for the accurate representation of functions of the filtering type, however, it is found more appropriate to work in a transformed version of the λ-plane in which the entire plane is represented in a finite rectangle—the electrolytic tank is then of particularly simple construction. In Part 2 of the paper, the use of the electrolytic tank as an aid to filter design on an insertion-loss basis is described. A transformation is developed, by means of which the approximation problem can be solved for a low-pass-filter specification of insertion loss, the poles and zeros of the insertion modulus function of the filter being accurately determined by simple measurements on the electrolytic tank. The transformation itself follows from the properties of image-parameter functions considered in Part 1. Detailed design procedures are given, including provision for the effects of inherent dissipation in the reactance elements of the filter. It is also found possible to extend the method of design to a general, unsymmetrical band-pass filter, and to apply the basic principles to certain root-finding operations involved in Darlington's method of network realization. Detailed examples and experimental results are given throughout the paper.