Abstract: A tomographic flow imaging system comprising three or more capacitance electrodes positioned around a pipe through which a flow to be monitored passes. The capacitance between each pair of the electrodes is measured, and an output is derived from the measured capacitances which is representative of the distribution of material within the pipe. A predetermined voltage signal is applied to one electrode at a time, and the electrodes other than said one electrode are connected to sources of equal fixed potential. The capacitance of each pair of electrodes is measured by measuring charge flowing between the said other electrode of the pair and the source of potential to which it is connected.

Abstract: A slippage sensor comprises a light-transmissive elastic component (14) defining an object-contactor face (15), input and output light guide (16a, 16b) defining a light transmission path through the component, and an indicator (19) connected with the output guide to detect transmission variations caused by elastic deformation of the component due, in turn, to relative slipping movement of an object (10) contacted thereby. In one form of sensor the input/output guides define a light path discontinous at the contactor face and to be completed by a surface (13) of the object as a reflector. In another form the component carries a reflector deformable therewith. In a further form the component is photoelastic and the input/output guides include polarisers. The sensor is useful to indicate when a minimum object gripping force is attained or it can indicate object characterisation.