EICKEMEYER'S DIFFERENTIAL MAGNETOMETER

327

or 5 (horizontal abscissa). Hence, the spot of light, as the current increases or decreases, traces the desired curve of magnetisation or hysteresis loop. If the absolute measure of the coordinate scale is specially determined, the indications of this curve tracer may also be expressed in absolute measure; according to the inventor's statement, this simple instrument is chiefly adapted for purposes of demonstration.

n2

§ 213. Eickemeyer's Differential Magnetometer. This practical appliance, which is intended for an approximate comparison of the magnetic reluctances of two specimens of iron, has a magnetic circuit, a diagram of which is represented in fig. 76; s, F, n, and s2 F2 n2 are two equal, heavy, S-shaped blocks of Swedish iron: x and y are the two iron bars to be compared. The flux of induction is in the direction of the arrows F2n, xs, F, n, y s2 F2. The coil, a single horizontal turn of which is indicated by C, encircles the two middle cheeks of the iron blocks. A magnetometer needle, not represented in the figure, swings within the vertical field of the coil. This will set vertically, for reasons of symmetry, when the magnetic reluctances x and y are equal; but if x is greater, it will swing to one side, and if y is greater to the other, owing to leakage, which is now no longer symmetrically distributed, and which produces a horizontal component at the point where the needle is. By means of a set of standard bars of a definite kind of iron, arranged like a set of weights, the unknown reluctance x may be approximately equalised in a readily intelligible manner. Besides this zero method, several others, mostly imitations of Wheatstone's bridge, have been described by Edison and others, as to which hardly any information has been published (§ 215). In so far as magnetic shunts are concerned, we may again refer to the discussion § 123.

FIG. 76

'Steinmetz, Elektrotechn, Zeitschr. vol. 12, p. 381, 1891.

B. Electrodynamic Methods

§ 214. Ewing's Curve Tracer.-This instrument, which is represented in outline in fig. 77 and in perspective in fig. 78,' was invented at the same time as that of Searle, and, like it, has a mirror reading. The perpendicular to the mirror, or, in other words, the reflected ray of light, can again move in two planes at right angles to each other. The deflection in the horizontal plane is proportional to the intensity to be measured; that in the vertical to the induction B in the specimen bar. The deflections are, however, produced in quite

a different manner, which resembles the method of measurement described in § 192. The wires AA and BB conveying the current are stretched in the magnetic field by adjustable weights like strings, and when the latter is excited, they experience a sag proportional to the intensity, which is suitably transferred to the mirror E by means of cross wire.

Two specimen pieces DD of the material are required, which

Ewing, Elektrotechn. Zeitschr. vol. 13, pp. 516, 712, 1892; and vol. 14, p. 451, 1893. Also Ewing and Klaassen, Magnetic Qualities of Iron,' Phil. Trans. vol. 184, p. 985, 1893.

EWING'S CURVE TRACER

329

are either solid or are built up of wire, ribbon, or sheet. It is desirable that they should have the rectangular cross-section (about 2.4 x 1.2 cm.) of the two normal specimens, about 45 cm. in length, supplied with the instrument. The specimens are clamped in the two pole-pieces, and likewise in a yoke which connects them at the other end, as seen in fig. 78, where also the coils are represented which magnetise the specimens. The variable current, proportional to the abscissæ, passing through them also passes through the stretched wire BB. This moves in a constant vertical field, which is produced by the split iron tube C. The horizontal sag of that wire is therefore a measure for H. The constant current of a few amperes which, in order to magnetise the iron tube, is led through its coil, now also

traverses the wire AA, which is stretched in the horizontal field in the slit between the pole-pieces of the specimens. Its vertical elongation, therefore measures the intensity of this field-that is, the induction B in the specimen bars.

The amplitude of the deflection of the mirror may be regulated both by the strength of the constant current in C and in AA', as well as by the lever effect of the weights which stretch the wires. The motion is quite dead-beat. The succession of positions of the projected point of light on the screen is registered photographically or by hand. The scale of coordinates may be reduced to absolute measure-for the abscissæ 5 by calculation from the ampere-turns of the magnetising coils, and for the ordinate 2, if necessary, by means of exploring

coils round the specimen bars. Fig. 79 represents curves obtained with such an apparatus. Owing to the magnetic

FIG. 79

reluctance of the yoke, the pole-pieces, and the air-gap, these are to be read off from the slanting line zz, instead of from the axis of ordinates xx.1

If the variations of the magnetising current are sufficiently rapid-within about second-the projected spot of light

Ewing has, moreover, described an ingenious kinematic device by which the correction in question is made automatically. The motion of the mirror corresponding to the ordinates is so arranged that the perpendicular to the mirror

APPARATUS OF KOEPSEL AND KENNELLY

331

appears to trace a continuous line, which then directly represents the induction-curve. The instrument thus forms an apparatus for demonstration as ingenious as it is instructive. The condition above mentioned is satisfied by Ewing by means of a specially constructed rotating liquid rheostat, with or without a commutator. Fig. 80 shows another form of the curve tracer, with a fixed magnetic circuit KK instead of the test bars, as best suited for purposes of demonstration.

S

FIG. 81

§ 215. Apparatus of Koepsel and of Kennelly.-The measurement of the field by means of a coil directed by torsion springs is the principle on which is based an apparatus for investigating iron, described by Koepsel. It is represented in fig. 81 in section, and in fig. 82 in plan. The magnetising coils SS, traversed by a current up to five amperes, produce in the intermediate space a field parallel to their axis. Parallel also to the latter is the plane of the windings of a measuring coil, fastened above and below to a torsion spring, which, at the same time, conveys the current. The auxiliary current through this is adjusted to a constant value-of a deci-ampere, for instance. When the coils SS are excited, the measuring coil will tend to set parallel to them. The angle of torsion which brings it back to zero is a measure of the field of the coil. If a specimen bar is placed in each magnetising coil, the angle of torsion required is far greater; it is, approximately at least, proportional to the induction in the test bars. In the arrangement of the magnetic circuit described, the shape of the specimen is difficult to allow for; sweeps a plane inclined to the vertical; hence, when there is no current in the wire, the spot of light does not trace the axis of ordinates, but the line zz, from which then its horizontal deviations take place.

Electrician, vol. 30, p. 65, 1892.

FIG. 82

* Koepsel, Verhandl. phys. Gesellsch., Berlin, p. 115, 1890; Elektrotechn. Zeitschrift, vol. 13, p. 560, 1892.