Dipoles and monopolesA pair of magnetic poles is called a dipole. A lone pole, like the positive pole in a pro-ton, is called a monopole.Magnetic monopoles do not ordinarily exist in nature. If they could somehow beconjured up, all sorts of fascinating things might happen. Scientists are researching thisto see if they can create artificial magnetic monopoles.At first you might think that the magnetic field around a current-carrying wire iscaused by a monopole, or that there aren’t any poles at all, because the concentric cir-cles don’t actually converge anywhere. But in fact, you can think of any half-plane, withthe edge along the line of the wire, as a magnetic dipole, and the lines of flux as goingaround once from the “north face” of the half-plane to the “south face.”The lines of flux in a magnetic field always connect the two poles. Some flux linesare straight; some are curved. The greatest flux density, or field strength, around a barmagnet is near the poles, where the lines converge. Around a current-carrying wire, thegreatest field strength is near the wire.Magnetic field strengthThe overall magnitude of a magnetic field is measured in units called webers, abbrevi-ated Wb. A smaller unit, the maxwell (Mx), is sometimes used if a magnetic field is veryweak. One weber is equivalent to 100,000,000 maxwells. Scientists would use exponen-tial notation and say that one 1 Wb = 108 Mx. Conversely, 1 Mx = 0.00000001 Wb = 10-8 Wb.The tesla and the gaussIf you have a certain permanent magnet or electromagnet, you might see its “strength” ex-pressed in terms of webers or maxwells. But usually you’ll hear units called teslas orgauss. These units are expressions of the concentration, or intensity, of the magnetic fieldwithin a certain cross section. The flux density, or number of lines per square meter orper square centimeter, are more useful expressions for magnetic effects than the overallquantity of magnetism. A flux density of one tesla is equal to one weber per square meter.A flux density of one gauss is equal to one maxwell per square centimeter. It turns out thatthe gauss is 0.0001, or 10-4, tesla. Conversely, the tesla is 10,000, or 104, gauss.If you are confused by the distinctions between webers and teslas, or betweenmaxwells and gauss, think of a light bulb. A 100-watt lamp might emit a total of 20 wattsof visible-light power. If you enclose the bulb completely, then 20 W will fall on the inte-rior walls of the chamber, no matter how large or small the chamber might be. But thisis not a very useful notion of the brightness of the light. You know that a single 100-wattlight bulb gives plenty of light for a small walk-in closet, but it is nowhere near adequateto illuminate a gymnasium. The important consideration is the number of watts per unitarea. When we say the bulb gives off x watts or milliwatts of light, it’s like saying a mag-net has y webers or maxwells of magnetism. When we say that the bulb provides xwatts or milliwatts per square meter, it’s analogous to saying that a magnetic field has aflux density of y teslas or gauss.Magnetic field strength139