The circuit contains a primary coil on the core of a magnetic circuit with an air gap and switchable into a line carrying the measurement current. The topology, content and usage of a document collection or web site is determined. of plastics material having a pore size of 10 microns or greater, through which the applied liquid sample must pass en route to the porous carrier material, the labelled specific binding reagent being freely soluble or dispersible in any liquid sample which enters the macroporous body.