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Resistivity is the term used when describing the specific electrical resistance of a material. Put into lay mans terms it is a measurement of the resistance of any given material to electric current; it is normally measured using a resistivity meter. A material that has a high resistivity will impede the flow of electricity greatly while those with low resistivity will freely allow the flow of electrons. Although materials all have a unique resistivity measurement, there are certain generalisations that can be made without having to use a meter. Metals and substances such as salt water tend to be great conductors of electricity where as materials like rubber, plastic and glass are extremely resistant. Because of the low resistance of metals, and especially copper, these materials are used for electrical wiring while the materials that cover them to prevent short circuits and shocks are made of plastic or rubber. This is where the resistivity meter is used most extensively, for finding the resistance of lengths of wire in electrical devices. The formula for resistivity of a wire that a meter automatically computes is a relatively simple equation based upon three factors. The first of these is the resistance, measured in Ohms and can be found using a table of resistances for a number of different materials. The second component of the equation is the area of the circumference of the wire. The final piece of information needed to find the resistivity of a wire is the length of the piece of wire. The equation simply multiplies the area and resistance and divides the resulting figure by the length; in terms of units of measurement the Greek symbol rho is used and will usually be present on the readout of the meter. It is not just wires however that are measured using a resistivity meter, these pieces of equipment are extensively used in measuring rock resistivities. Naturally rocks are not going to be used for electrical wiring but as the metallic minerals in rocks conduct electricity better than other non-metallic minerals, a meter is a useful tool in finding the metallic mineral content of rocks. The uses for this are vast but predominantly resistivity meters are used for geophysical exploration and more precisely the metal content of rock formations. Methods of measurement vary from surface testing to drilling holes in the ground. These methods of resistivity measurement do not always use a handheld meter but a far larger and more industrial technique. The two predominant forms of measurement are termed as active and passive. Active measurement involves introducing electricity to the ground and measuring the strength of the electrical field created between electrodes giving an indication of resistance. Passive methods on the other hand measure the natural flow of electricity caused by electrochemical reactions between minerals and fluids. Due to the benefits of this type of measurement the resistivity meter is extensively used in a variety of industries. The mining industries, especially those involved with the search of precious and semi-precious metals in rocks heavily use this equipment when prospecting. It is also extensively used in the oil industry. In terms of the resistivity of wires, electrical manufacturers use this type of meter to find the qualities of their components. Today it is hard to imagine a world without this type of meter, not only would there be less metals mined in the world but the electrical equipment we have in are homes would almost certainly be less well developed.
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