# What is Electrical Resistance? #### Electrical Resistance is a measure of the opposition that a circuit offers.

Electrical Resistance is a measure of the opposition that a circuit offers to the flow of electric current. You might compare it to the diameter of a hose. In fact, for metal wire, this is an excellent analogy: small-diameter wire has high resistance (a lot of opposition to current flow), and large-diameter wire has low resistance (not much opposition to electric currents). Of course, the type of metal makes a difference too. Iron wire has higher resistance for a given diameter than copper wire. Nichrome wire has still more resistance.

The standard unit of resistance is the ohm. This is sometimes abbreviated by the upper-case Greek letter omega, resembling an upside-down capital U (Q). In this book, we'll just write it out as "ohm" or "ohms."

You'll sometimes hear about kilohms where 1 kilohm = 1,000 ohms, or about megohms, where 1 megohm = 1,000 kilohms = 1,000,000 ohms.

Electric wire is sometimes rated for resistivity. The standard unit for this purpose is the ohm per foot (ohm/ft) or the ohm per meter (ohm/m). You might also come across the unit ohm per kilometer (ohm/km). Table 2-1 shows the resistivity for vari­ous common sizes of wire.

When IV is placed across 1 ohm of resistance, assuming that the power supply can deliver an unlimited number of charge carriers, there will be a current of 1A. If the re­sistance is doubled, the current is cut in half. If the resistance is cut in half, the current doubles. Therefore, the current flow, for a constant voltage, is inversely proportional to the resistance. Figure 2-3 is a graph that shows various currents, through various re­sistances, given a constant voltage of IV across the whole resistance.

Usually, four-terminal resistors used to measure current by measuring the voltage drop between the voltage terminals with current introduced at the current terminals. These standards, designed for use with potentiometers for precision current measurement, correspond in structure to the shunts used with millivoltmeters for current measurement with indicating instruments. Current standards must be designed to dissipate the heat they develop at rated current, with only a small temperature rise. They may be oil- or air-cooled, the latter design having much greater surface, since heat transfer to still air is much less efficient than to oil. An air-cooled current standard of 20 uQ resistance and 2000-A capacity, has an accuracy of 0.04%. Very low resistance oil-cooled standards are mounted in individual oil-filled containers provided with copper coils through which cooling water is circulated and with propellers to provide continuous oil motion.

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