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 . 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

by calculation from the ampere-turns of the magnetising coils, and for the ordinate B, 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.

§ 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

四 S

FIG. 81

S

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

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

Koepsel has therefore recently modified the apparatus in the following manner :

A magnetising spiral contains a single test-piece, which is enclosed in a yoke (§ 218). The measuring coil is wound on an iron cylinder, which rotates in a suitable cavity in the yoke like an armature. Its deflection is not compensated by torsion, but is read off on a scale by an index. This gives directly the flux of induction in absolute measure; whereby, however, a definite value of the auxiliary current is assumed, and must be separately adjusted or measured.

F

F

D

FIG. 83

An arrangement by Kennelly may, in conclusion, be mentioned, by which an approximate comparison of the magnetic reluctances of two specimens of iron is possible, as in the differential magnetometer described in § 213. Both specimens are placed in AF or FC respectively (fig. 83). If the reluctances are equalised, there will, from symmetry, be no induction in the central iron cross-piece FD. The criterion of this is the fixity of a copper disc D, traversed by a current in a radial direction, which can rotate in a suitable slit about the unifilar suspension OP, which conveys a current.

C. Induction Methods

§ 216. The Ballistic Method, notwithstanding its many disadvantages, is still undoubtedly the most important for experimental physics--chiefly on account of its universal applicability, and of the unlimited range within which it can be utilised by raising or lowering the sensitiveness. It is less suitable for practical measurements, owing to the considerable disturbance to which it is liable from external influences.

Its application to the measurement of the intensity of a field has been discussed in detail (§§ 195-197), and the way in which

1 Koepsel, Elektrotechn. Zeitschr. vol. 15, p. 214, 1894.

it is used for determining magnetic distributions has also been pointed out. In Chapter V we have elucidated by an example its application to the investigation of toroids, so that we may here dispense with further general discussion. The present method is the only one which can be used with completely closed magnetic circuits. But as in this case it is not possible to pull away the exploring coil, the flux of induction actually existing at any time can never be ascertained, let alone measured, but its sudden variation can be determined. The ballistic method fails when the time required for such a variation exceeds about a second; in that case it is not possible to make the period of the galvanometer sufficiently long in comparison. With large electromagnets, and with high selfinduction, this order of magnitude is very soon reached (§ 170).2 This forms a chief objection to the ballistic method; in other respects it leaves little to be desired, especially for laboratory purposes.

§ 217. Isthmus Method. It only remains to mention the particular modification in which the ballistic method may be employed for special purposes. For measuring magnetisation at high intensities up to $25,000 C.G.S., Ewing and Low3 introduced what is known as the Isthmus method. The ferromagnetic substance to be examined-for example, a piece of iron-is turned on a lathe into the shape of an ordinary bobbin; its ends are either plane, or cylindrical about an axis at right angles to the principal axis. In the former case the iron can be suddenly withdrawn from the magnetic circuit of the electromagnet used in this method; a diagram of the latter form is represented in fig. 84. By means of a handle the whole iron bobbin can be turned about an axis at right angles to the plane 1 Further details are found in Ewing, Magnetic Induction, chapters iii. and iv.

In cores of soft iron thicker than say 1 cm., of high magnetic permeability and electrical conductivity, eddy currents play a considerable part (§ 187). They seem, indeed, to diminish the self-induction, and therefore accelerate variations of current in the magnetising spiral (note, p. 282); but apparently, by their screening action, they produce a considerable time-lag of the vector B in respect of H, the amount of which is difficult to check or bring into calculation.

3 Ewing and Low, Proc. Roy. Soc. vol. 42, p. 200, 1887; Phil. Trans. vol. 180 A, p. 221, 1889.