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Questions And Answers

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Question What is resistance?  What is resistivity?
Answer

“The term ‘resistivity’ and ‘resistance’ are often confused and misused. Resistivity is an intrinsic electrical property of a material. Resistivity defines the type of material and determines its electrical classification. Resistivity is analogous to "density" if discussing a physical property. Measurements are expressed in ohm-cm for volume resistivity and in ohms/square for surface resistivity (although later on in this section you will see that ohms/square is technically incorrect). A resistivity measurement is almost always made on a precisely sized piece of material placed into a specific fixture with a controlled current and voltage applied. Specimen thickness must be known and the electrode configuration is defined by the specific measurement method. Industry standard test methods are defined for resistivity measurements.

Resistance is the measurement of retardation of electron flow through an item from one point to another. The measurement value is stated in ohms (Ω). A resistance value takes into account the resistivity of the material that makes up the item as well as the size and shape (geometry) of the item. Resistance is analogous to "mass" if discussing a physical property.

Resistance to a ‘groundable point or to ground' is a measurement that evaluates the performance of an item (in a practical application sense) if the item is suppose to be electrically bonded to ground.” [ESD Handbook ESD TR20.20-2008 section 2.5 Material Electrical Characteristics: Insulative, Conductive And Static Dissipative]

“Many materials-particularly those that are polymer based and used in the control of static electricity-have additives, coatings or treatments on their surface. Measurement of the electrical characteristics of such materials usually requires placing an electrode of appropriate configuration on the surface and applying a measuring current at some given potential. Depending on the electrode configuration, the results may be interpreted as resistance or resistivity. For surface treated materials, the conventional way to describe the electrical property has been ohms/square. As discussed in the literature, this terminology is incorrect. The actual measurement is ohms only. Details of this discussion may be found in the book, Electrostatics, by Professor Niels Jonassen, Technical University of Denmark.

Homogeneous, volume conductive materials may be measured for volume resistivity using test method ASTM D 991. A volume resistance measurement is described in ANSI/ESD STM11.12.

Care must be taken in selecting the appropriate resistance or resistivity test method for any given application or material/item. Choosing the correct test method also requires an understanding of the function of the material/item in an application. Many static control items are formed from laminations of dissimilar materials. Resistance or resistivity tests may not be completely appropriate for such items, since the actual function of the item may not be predicted. A simple surface resistance measurement may not be enough to understand a static control item's performance attributes. For example, installed static control items, such as worksurfaces and flooring materials, are properly evaluated for in-use performance by measuring resistance to ground from points on their surface. Measuring methods are described in ESD Association standards and test methods.” [ESD Handbook ESD TR20.20-2008 Section 2.6.1 Resistance and Resistivity]
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