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Ohms per Square What?by: Steve FowlerThe answer is: "It does not Matter." 
                Square anything!This term has been used for years by many who deal 
              in electrical measurements but it is still not clearly understood. 
               ASTM D-254 test method is used to measure the surface resistivity 
              of materials. The units of measurement are Ohms per Square. Even 
              though the ESD Associations Test Method S11.11 uses resistance measured 
              in Ohms, the resistivity of many materials is still required for 
              characterization and contract requirements. Most people do not ask 
              what it means for fear of showing their ignorance. Let 's try to 
              clear it up a bit.Ohms per square is the unit of an electrical measurement 
              of surface resistivity across any given square area of a material. 
              It is the measurement of the opposition to the movement of electrons 
              across an area of a material's surface and normalized to a unit 
              square. This measurement is intended to be a basic material parameter 
              and not dependent on absolute area, length or thickness. Unlike 
              resistance, resistivity is not exactly a point-to-point measurement. 
              It is a measurement where the electrons can take multiple paths 
              across a uniform surface. However, the electrons are considered 
              to generally flow only on the surface.The measurement of resistance is not the same as 
              resistivity. Resistance is the opposition to electron flow across 
              or through a material, and is measured from point-to-point . It 
              is not normalized. This measurement is very dependent on size, length, 
              cross-section, etc.The above figure shows the measurement of a material with the same 
              dimensions on all sides. It is a square area of the material.  It 
              can be 1 inch or 1 mile on a side.  When electrodes are attached 
              as shown, and the Ohm meter measures 1 Ohm, we can state that this 
              material has a surface resisitivty of 1 Ohm per square.If two squares of the material (the same size as above) are laid 
              end to end and electrodes attached as shown below, the resistance 
              is the addition of these two squares or 2 Ohms.  This measurement 
              is of two resisters in series.Or if two of these squares are stacked as shown below, the resistance 
              is that of two resisters in parallel or 1/2 Ohm.Now if we combine these two examples we have four 
              squares of the materials in an arrangement of two parallel resisters 
              in series.  Or the resulting four resisters which now form 
              a square again read 1 Ohm on the Ohm meter.If we take the examples to a higher level where 
              we have 16 squares of the material arranged as shown below,  the 
              result is again 1 Ohm.  It is again in this example a square. 
               If the first example is 1 square inch then this example is 
              4 square inches and the results are the same - 1 Ohm.So you see that it really doen't matter.  A 
              square inch or a square mile of a material with a surface resistivity 
              of 1 Ohm per square will read just 1 Ohm on an Ohm meter.Strange isn't it?