It is common industry practice to use specifically designed oil and water-based carriers for magnetic particles.

They contain microscopic magnetic particles, each about 6,200 times smaller than a human hair.

This makes the hysteresis dependent on the size of the magnetic particle.

Their ability to sense the magnetic field is because of magnetic particles.

From here, make the assumption that the moment of the magnetic particle is co-linear with the applied field.

This is a reasonable assumption given the small size and high susceptibility of the magnetic particles.

In this way magnetic particles are separated from liquid.

If the pipette tip P is moved or transported, the magnetic particles will not come off easily.

The position of the magnetic particles is then determined by a microscopic objective with a camera.

Mixed with the liquid are very fine black magnetic particles of iron oxide.