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The aperture is a hollow glass tube with a hole in it through which cells are pulled in the Coulter counter. This tube is filled with salt solution and an electrode is placed on the inside and outside of the tube and electrical current flows through the hole in the tube. When a sample is measured, a vacuum is placed on this inside of the tube that pulls cells or particles through the hole. Every time a cell enters the hole of the aperture it displaces salt solution which results in a visible electrical pulse. The size of the pulse tells you how big the cell (or any particle) was when it went through the hole. The number of pulses tells you how many things went through the hole.