Coulter principle

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The Coulter principle states that particles pulled through an orifice, concurrent with an electrical current, produce a change in impedance that is proportional to the size of the particle traversing the orifice. The Coulter principle was named for its inventor, Wallace H. Coulter^[1].

Wallace was an electrical engineer by training with a passion for radio technology. During the second world war, Wallace joined the U.S. Navy. While working on a technique to detect submarines using sonar, he frequently detected large echos where no submarines were operating. In an attempt to determine the source, Wallace lowered a series of small bottles with remote trap doors to various depths. The bottles were constructed such that the remote door could be opened and shut at pre-determined depths, filling the bottle with seawater from that depth. The source of the false echoes turned out to be high concentrations of plankton. In order to count the number of plankton cells per millilitre of seawater accurately and reproducibly, Wallace created a device that would become the basis for the Coulter principle.

The device consisted of a dual chambered container whose two sides were separated by a thin membrane. A small hole in the membrane called an aperture was the only connection between the two chambers. Electrodes from a battery were placed in the chambers, positive on one side and negative on the other. An ohm meter was connected to the circuit so as to measure the resistance to the flow of current (impedance) from one electrode, through the orifice, and to the other electrode. Both chambers were filled with seawater from the trap bottles. Then one of the two chambers was partially drained, forcing seawater to flow from the opposite chamber, through the orifice to balance the level of liquid in the two sides. As the seawater passed through the orifice so did the plankton cells, which created momentary changes in impedance that were seen on the ohm meter. By counting the number of impedance pulses per unit of seawater, Wallace's device was able to count the number of plankton particles.

This technology found commercial success in the medical industry where it revolutionized the science of hematology. Red blood cells, white blood cells and platelets make up the majority of the formed elements in the blood. The average salinity of human blood is very close to that of seawater, and mixture of salt (NaCl) and water with the same salinity as seawater is said to be isotonic with whole blood. When whole anticoagulated human blood is diluted with isotonic saline, the Coulter principle can be applied to count and size the various cells that make up whole blood. The first commercial application of the Coulter principle to hematology came in 1954 with the release of the Coulter Counter Model A (developed by Wallace and brother Joseph R. Coulter^[2]). Within a decade, literally every hospital laboratory in the United States had a Coulter Counter, and today every modern hematology analyzer depends in some way on the Coulter principle.

Interesting trivia:

1. The first membrane used by Wallace Coulter came from the cellophane wrapper on a pack of cigarettes.

2. The first aperture was burned through the cellophane wrapper by a heated sewing needle.

[1] http://www.beckmancoulter.com/hr/ourcompany/oc_WHCoulter_bio.asp

[2] http://www.beckmancoulter.com/hr/ourcompany/oc_JRCoulter_bio.asp