is increased many times for a given length of wire compared with the flux produced bya single-turn loop. This is how inductors are made in practical electrical and electronicdevices.For any coil, the magnetic flux density is multiplied when a ferromagnetic core isplaced within the coil of wire. Remember this from the study of magnetism. The in-crease in flux density has the effect of multiplying the inductance of a coil, so that it ismany times greater with a ferromagnetic core than with an air core.The current that an inductor can handle depends on the size of the wire. The in-ductance does not; it is a function of the number of turns in the coil, the diameter of thecoil, and the overall shape of the coil.In general, inductance of a coil is directly proportional to the number of turns ofwire. Inductance is also directly proportional to the diameter of the coil. The length ofa coil, given a certain number of turns and a certain diameter, has an effect also: thelonger the coil, the less the inductance.The unit of inductanceWhen a battery is connected across a wire-coil inductor (or any kind of inductor), ittakes a while for the current flow to establish itself throughout the inductor. The cur-rent changes at a rate that depends on the inductance: the greater the inductance, theslower the rate of change of current for a given battery voltage. The unit of inductance is an expression of the ratio between the rate of currentchange and the voltage across an inductor. An inductance of one henry, abbreviated H,represents a potential difference of one volt across an inductor within which the currentis increasing or decreasing at one ampere per second.The henry is an extremely large unit of inductance. Rarely will you see an inductoranywhere near this large, although some power-supply filter chokes have inductancesup to several henrys. Usually, inductances are expressed in millihenrys (mH), micro-henrys (µH), or even in nanohenrys (nH). You should know your prefix multipliersfairly well by now, but in case you’ve forgotten, 1 mH0.001 H10-3 H, 1 µH 0.001mH0.000001 H10-6 H, and 1 nH0.001µH 10 -9 H.Very small coils, with few turns of wire, produce small inductances, in which thecurrent changes quickly and the voltages are small. Huge coils with ferromagneticcores, and having many turns of wire, have large inductances, in which the currentchanges slowly and the voltages are large.Inductors in seriesAs long as the magnetic fields around inductors do not interact, inductances in seriesadd like resistances in series. The total value is the sum of the individual values. It’s im-portant to be sure that you are using the same size units for all the inductors when youadd their values.Problem 10-1Three 40-µH inductors are connected in series, and there is no interaction, or mutualinductances, among them (Fig. 10-3). What is the total inductance?Inductors in series185