HOW VARIOUS SUBSTANCES RESPOND TO A MAGNETIC FIELD

Imagine doing some experiments with a very intense magnetic field. To be definite, suppose we have built a solenoid of 10-cm inside diameter, 40 cm long, like the one shown in Fig. 11.1. Its outer diameter is 40 cm, most of the space being filled with copper windings. This coil will provide a steady field of 30,000 gauss, or 3.0 teslas, at its center if supplied with 400 kilowatts of electric power—and something like 30 gallons of water per minute, to carry off the heat. We mention these practical details to show that our device, though nothing extraordinary, is a pretty respectable laboratory magnet. The field strength at the center is nearly 105 times the earth's field, and probably 5 or 10 times stronger than the field near any iron bar magnet or horseshoe magnet you may have experimented with. The field will be fairly uniform near the center of the solenoid, falling, on the axis at either end, to roughly half its central value. It will be rather less uniform than the field of the solenoid in Fig. 6.18, since our coil is equivalent to a “nested” superposition of solenoids with length-diameter ratio varying from 4:1 to 1:1. In fact, if we analyze our coil in that way and use the formula (Eq. 44 of Chapter 6) which we derived for the field on the axis of a solenoid with a single-layer winding, it is not hard to calculate the axial field exactly. A graph of the field strength on the axis, with the central field taken as 30 kilogauss, is included in Fig. 11.1.