Magnetic units

The size of a magnetic field is measured in units called webers, abbreviated Wb. One weber is mathematically equivalent to one volt-second. For weaker magnetic fields, a smaller unit, called the maxwell, is sometimes used. One maxwell is equal to 0.00000001 (one hundred-millionth) of a weber, or 0.01 microvolt-second.

The flux density of a magnetic field is given in terms of webers or maxwells per square meter or per square centimeter. A flux density of one weber per square meter (1 Wb/m2) is called one tesla. One gauss is equal to 0.0001 weber, or one maxwell per square centimeter.

In general, the greater the electric current through a wire, the greater the flux density near the wire. A coiled wire will produce a greater flux density than a single, straight wire. And, the more turns in the coil, the stronger the magnetic field will be.

Sometimes, magnetic field strength is specified in terms of ampere-turns (At). This is actually a unit of magnetomotive force. A one-turn wire loop, carrying 1 A of current, produces a field of 1 At. Doubling the number of turns, or the current, will double the number of ampere-turns. Therefore, if you have 10 A flowing in a 10-turn coil, the magnetomotive force is 10 x 10, or 100 At. Or, if you have 100 mA flowing in a 100-turn coil, the magnetomotive force is 0.1 x 100, or, again, 10 At. (Remember that 100 mA = 0.1 A.) Another unit of magnetomotive force is the gilbert. This unit is equal to 0.796 At.