The present invention provides methods and apparatuses for determining at least one of a plurality of particle physical characteristics. The particle physical characteristics include particle size, shape, magnetic susceptibility, magnetic label density, charge separation, dielectric constant, and derivatives thereof. The method includes generating a region of space having a substantially constant force field, determining the velocity of at least one particle within the region by identifying and locating the particle and its coordinates in at least two temporally defined digital images, and determining the particle physical characteristics from the determined velocity and a predetermined force field magnitude and direction. A device for determining one or more particle physical characteristics is described which has a force field device for subjecting at least one particle to at least one force field, a substantially transparent flow channel, and a computer system for gathering and analyzing data associated with the at least one particle. A system for determining one or more particle physical characteristics is provided which has a force field device for generating at least one force field having a predetermined force field magnitude and direction and for subjecting at least one particle to the at least one force field, a flow system for regulating the introduction of the at least one particle into the force field device, and a computer system for gathering and analyzing data associated with the at least one particle. A pole piece assembly for producing a region of space having a substantially constant magnetic force field is also provided.