Apparatus for sensing and characterizing particles (e.g., blood cells or
ceramic powders) suspended in a
liquid medium comprises a conduit through which the
particle suspension is caused to pass simultaneously with an
electrical current. According to the invention, the interior wall of the conduit effectively varies in resistivity along the length of the conduit to define a delimited
central region of high electrical resistivity which is smoothly contiguous on its opposing boundaries to uninsulated distal elements of lesser electrical resistivity. The delimited
central region of the conduit functions as a Coulter volumeter conduit. The uninsulated distal elements of the conduit are made to have a dimension along the conduit wall which is at least equal to the axial extent of the effective ambit electric fields of a traditional Coulter volumeter conduit having a cross-sectional geometry identical to that of the delimited
central region of
high resistivity in the improved volumeter conduit. According to a preferred embodiment of the invention, the delimited central region of the improved volumeter conduit is defined by a traditional Coulter conduit
wafer, i.e., a
dielectric wafer containing a central circular conduit, and the distal elements of lesser resistivity are defined by uninsulated,
electrically conductive, circular collars attached to opposite sides of the conduit
wafer. The conduit in the conduit wafer and the openings in the conductive collars collectively form a hydrodynamically smooth volumeter conduit, in which the electric and hydrodynamic fields of the traditional volumeter conduit are advantageously amended in the manner above noted.