A NEW METHOD FOR OBSERVING ELECTRIC AND MAGNETIC FIELDS

Recently a number of notes have appeared,\(^{1—3}\) in which two electron-optical methods for observing electric and magnetic fields are described, methods which apparently have a great future.

1) The schlieren method. For monitoring small changes in the refractive index in optics, the schlieren method has long been used; its essence is as follows.

Fig. 1.

Let a parallel beam of rays fall on a converging lens. Then, if an opaque disk that blocks the light beam is placed in the plane of the principal focus of the lens, there will be no illumination on a screen located beyond the focal plane.

But if slight local changes in the refractive index occur in the object space, then part of the light beam will pass the focus and illumination will appear in the corresponding region of the screen, from the distribution of which one can qualitatively judge the distribution of changes in the refractive index.

In the electron-optical case the role of the light beam is played by an electron beam; changes in the refractive index can be produced by electric or magnetic fields.

In the first experiments, a thin wire (diameter \(0.117\ \mathrm{mm}\)), used for magnetic sound recording, was placed in the object space; the axis of the wire was perpendicular to the optical axis of the electron lens of the electron microscope; the wire was magnetized along its length by short pulses of current of alternating sign; the sign of the pulse changed after every \(0.43\ \mathrm{mm}\) of the wire length. The distance

from the wire to the center of the lens was 20–50 mm. The shielding of the electron beam at the focus of the lens was accomplished by a copper disk (diameter—1 mm); the fluorescent screen was placed at a distance of 300 mm from the lens; the accelerating voltage reached 40 kilovolts.

Fig. 2.

Fig. 3.

In Fig. 1 a photograph of the screen is shown (overall magnification—50×); it clearly shows the distribution of the external magnetic fields of alternating signs existing around the conductor.

2) Shadow method. A more refined method, allowing one to make not only a qualitative but also a quantitative estimate of the fields, is the shadow method, the essence of which is explained by Fig. 2.

A parallel beam of electrons falls on the lens; to the right of the lens there is a small opaque object \(GG_+\). On the fluorescent screen \(S\), illuminated by the electrons, a shadow \(NN_+\) will appear, since part of the electron beam \(UU_+\) is stopped by the object.

If now a source of an electric or magnetic field is placed at the point \(S'\), the conditions of motion of the electrons change; it will no longer be the beam \(UU_+\) that strikes the obstacle, but some other beam \(DD_+\); as a result, the shadow will move to the region \(AA_+\) of the screen.

By moving the obstacle, one can judge the distribution of the field that distorts the rectilinear trajectories of the electrons.

But it is still simpler, instead of a single obstacle \(GG_+\), to place in a plane perpendicular to the optical axis a wire mesh; the distortions of the shadow cast by it immediately characterize the configuration of the distorting field.

In this way it proved possible to obtain a picture of the external fields produced by the domains of a ferromagnetic crystal.

Fig. 3 shows a picture obtained when the trajectories of the electrons were distorted by the electric field of a small barium titanate crystal fastened to a tungsten wire \(0.25\) mm in diameter. The mesh that gave the shadow on the screen had 40 cells per 1 mm. In this case as well, the field pattern obtained is very clear.

The authors give a theoretical calculation of their method, indicating the broad prospects for its application.

N. N. Malov

CITED LITERATURE

1. L. Marton, J. Appl. Phys., 19, 687, 863 (1948).
2. L. Marton and S. Lachenbruch, J. Appl. Phys., 20, 1171, 1258 (1949).
3. L. Marton, Phys. Rev., 73, 1475 (1948).