During every minute of operation of a typical paper machine several square kilometers of interface between the solid and liquid phases of the pulp suspension pass down the wire. These interfaces possess a number of special properties. The formation of an electrical double layer on the surface of the solid phase, for instance, influences to a large extent the process of sheet forming, the retention of fibres, fillers and sizing agents and, in turn, the characteristics of the finished sheet. There is understandably, therefore, a strong interest in obtaining measurements of these interfacial characteristics with the intention of optimising the operation of the process and the performance of the products. This interest is demonstrated by the number of publications which have appeared on the subject over the last few years. Melzer,⁽¹⁾ for example, cited as many as 102 papers in 1972 and since then many more have appeared.