The first automated counters to hit the market was the Coulter counter, named after its inventor, Wallace H. Coulter, an electrical engineer. These early counters used what is now known as the Coulter principle, which states that particles (for example a red blood cell) that pass through an orifice with an electric current, produce a change in impedance that is proportional to the volume of the particle. That change is registered and recorded. Coulter patented his method in 1953.