Measurement of Magnetization and Induction

Bar and Yoke Tests.

The ballistic method can be applied to long straight rods, or wires, corrections being applied as in the magnetometer method. Cylindrical bars are, however, more easily prepared than rings or wires. By embedding the ends of such a test bar in a massive yoke, so that the whole forms a "magnetic circuit," a condition of approximate endlessness may be secured. The original "bar and yoke" apparatus of Hopkinson is shown in rig. 15.

The bar, which slides through holes bored in the yoke, is in two parts abutting against each other near the middle of the yoke. Magnetizing coils surround each portion of the test bar. The search coil is arranged with a spring so that when one of the test rods is suddenly drawn back, the coil jumps out of the field. The induction corresponding to any field can thus be measured. Strong fields can be used, and the actual induction at any moment measured; but accurate results are not possible owing to the large leakage which may occur at the joint.

In Ewing's double bar two-length test, two similar test bars are placed side by side, each pair of ends being connected by a short massive block of soft iron in which the bars are clamped. There are two pairs of magnetizing coils, the shorter pair being half the length of the longer, and containing half the number of turns. Induction coils are wound on the middle parts of both bars, and connected in series. Two sets of observations are taken, with the longer magnetizing coils and a length L between the yokes, and with the shorter and a length L/2. Let N be the number of turns in the longer coil ; let and be the values of the current for the same value of B in the two tests, and and H2 the corre sponding "apparent" values of the field. It may be shown that the true value of H corresponding to this value of B is given by This method is somewhat tedious, but is very accurate, and may be used in standardizing bars with which others may be compared. This may be done by means of the Ewing permeability bridge, represented in section in fig. 17. The standard bar, and that being tested (the two being of the same dimensions) are placed side by side within magnetizing coils which are joined in series. The number of turns on one of the coils may be varied un til the induction through the two rods is the same. The B, H curve of the standard

rod being known, and also corresponding values of H for the same induction through the standard rod and that under test (from the ratio of the number of turns in the magnetizing coil), the B, H curve of the specimen may be constructed. The equality of B in the two rods is determined by means of a small compass needle placed between two long curved horns which project upwards from the middle of the yokes (fig. i 7). This indicates when there is no induction from one yoke to the other through the hoins, which can only occur when the induction through the two bars is the same. In practice comparison tests may be made quickly and easily by this method. The general principle of making the induction constant round a magnetic circuit (which can also be tested by search coils) has been employed extensively in the magnetic testing of bars with most satisfactory results.

Traction Methods.

The force required to draw apart two magnetized surfaces has been made use of in the measurement of induction, a method first employed by S. Bidwell. Various forces may come into play according to the conditions. If, for example, a transverse cut is made in a bar and the two ends are placed in ability is S. P. Thomson's permeameter, represented in fig. 18. It consists of a slotted rectangular block of iron containing a mag netizing coil. The sample to be tested has the form of a rod one end of which is faced true. The force required to detach the rod is registered by a spring balance. If P is the pull in grams weight and S the sectional area of the specimen, then There is some uncertainty due to the presence of the joint. This is avoided in the magnetic balance of Dubois (fig. 19), in which the force exerted between two pieces of iron separated by a narrow air gap of definite width is measured. The test piece A, surrounded by a magnetizing coil, is clamped between two soft iron blocks BB. The yoke YY of soft iron constitutes the beam of the balance. The distance through which the weight W must be moved from its zero position to tilt the yoke over to the stop S gives a measure of the induction through the specimen. The instrument must be calibrated by means of a standardized bar.