"The Coulter Principle"

Don Graham, PhD

The Wallace H. Coulter Department of Biomedical Engineering exists because of an elegant principle that is simply expressed: a cell or particle, suspended in an electrolyte and passed through a suitable constriction in which an electric current is also flowing, produces a detectable change in the current. But its apparent simplicity proves deceptive if this Coulter Principle is to be implemented in a practical instrument. The sensing structure is an electrochemical cell divided into two insulated compartments, and the sensing constriction is typically a short conduit, or Coulter aperture, connecting the two compartments through which the sample suspension and electrical current simultaneously flow. The hydrodynamic and electric fields in and near the aperture are complex, and their effects on electrical pulses generated by particles transiting the aperture must be recognized if the full potential of the Principle is to be realized. The history and practical implementation of the Coulter Principle will be outlined.