Particle counting and measurements of particle size distributions in liquids can be based on pulse height analysis of signals obtained from a changing gate impedance, upon particle transition. Signal-to-noise ratio is markedly improved, making use of a four-electrode detector. An instrumentation amplifier with a high-pass filter is used as a preamplifier. Cylindrical electrodes and a coaxial aperture location result in a decreased detector capacitance, lowering the signal detection level and contributing to improved pulse shape. Volume sampling is flexible and accurate, making use of a stepping motor actuated digital sampling system. A newly developed baseline restorer, combined with a log-antilog amplifier circuit, allows for linearisation of the pulse height/particle size relationship.