RESISTANCE AND OHM

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 (Ω).  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 vari- ous common sizes of wire.