Magnetic Measurements

(3) The A and B positions are gener ally used for the determination of the moments of permanent magnets. In a third arrangement which is sometimes conven ient, especially for studying the magnetiza tion of specimens in the form of long rods, the specimen is placed vertically with one of its poles at the level of the needle, the line joining the pole and the needle being at right angles to the magnetic meridian (see fig. 8).

The pole strength m is then determinable from the equation while M=m1, where 1 is the distance between the poles. In this "one-pole" arrangement, the position of the poles need not be known accurately, for the magnetometer deflection is not much altered by small upward or downward displacements. In the A and B positions, the magnet is at right angles to the earth's field. In the one-pole position, it is influenced by the earth's vertical field, which, however, may be eliminated by placing round the magnet a solenoid carrying a current to neutralize the effect.

Production of Magnetic Fields.

The most convenient ar rangement for the production of magnetic fields of moderate intensity is a coil of wire carrying a current. Fields of calculable intensity may be produced, which are uniform over considerable regions and which may be varied over wide ranges. Solenoids, consisting of long coils of one or more layers of wire wound on a tube as uniformly as possible, are much used. In a straight solenoid of n turns, of length 2/, and radius a, the field along the axis at a point x from the centre due to current (E.m.u.) is given by In the middle portion of the coil, the field is very nearly uni form; if i is the current in amperes, N the number of turns per cm., approximately, Toward the ends the field diminishes, and at the ends is reduced to one-half the maximum value. If the solenoid is in the form of a ring, of mean radius r, the field inside the coil is given by In this case the field is undisturbed by the influence of ends, but it varies inversely as the distance from the axis of the ring.

For fields greater than a few hundred gauss some form of electromagnet (q.v.) is usually employed, consisting essentially of a core of iron surrounded by a coil of wire. The core may be straight, but it is usually of such a form that the two poles are near each other. In a common form the magnet has two vertical parallel cores attached at their lower ends by a massive yoke. Pole pieces rest on the upper ends, and the width of the interpole space may be adjusted. A half-ring type due to du Bois is also much used. It is desirable that the coils should be placed so as to produce the maximum effect in magnetizing the iron of the pole pieces, and also to add to the field in the gap by the direct action of the current in them. Let N be the total number

of windings, L the effective length of the "magnetic circuit" (the region through which the magnetic induction "flows"), S the cross section of the material of the magnet ; 1 and s the length and cross section of the interspace between the poles.

For H to be .large, S must be large (massive cores and yokes must be used), and 1 and s small (the interpole distance must be short, and the cross section of the pole piece at the gap small). Conical pole pieces were introduced by Ewing, who showed that the maximum field was obtained for a semi-angle of the cones of 44'. For a magnet of constant cross section with plane parallel poles of radius r at a distance of 2a apart the interpole field is given by times greater than that with ordinary pole pieces. It will be noticed that the field due to the magnetic material is limited by the saturation intensity of magnetization. For this reason pole pieces of ferro-cobalt (Fe2Co) are sometimes used, for which the saturation intensity is io% higher than for iron.

With a large electromagnet, weighing 1,300 kg., and a winding of 3,36o turns capable of carrying a current of 6o amperes, a field of 46,00o gauss was produced in a 2 X3.6 mm. gap. With iron core electromagnets fields of the order of 50,000 gauss over a region of a few cu.mm. may be taken as a practical maximum. With uncored coils the field produced is proportional to the current, and is limited by the very powerful sources of electrical energy required and the necessity for avoiding overheating of the coils. Using powers of 340 kilowatts, Deslandres and Perot obtained fields of 49,00o gauss, with a current of 5,000 amperes in a water cooled spiral of silver ribbon. P. Kapitza (Proc. Roy. Soc. A, 1924) obtained very intense fields by the coil method, the overheating being eliminated by producing the fields only for a very short time (of the order of sec.). In the original method, large storage batteries were used which were discharged through special coils consisting of windings of copper band, the current being broken after a short time interval. With a coil I mm. in internal diameter, fields of 500,000 gauss could be produced for Ow sec. A specially designed electro-generator has been constructed which will be a more convenient source of power, and the great difficulties in designing coils to withstand the forces brought into play have been overcome, so that the possibility is opened up of investigations with fields some RD times greater than those that have previously been used. (See