Electron Shadows Map Force Fields
December 1949 Radio-Electronics

December 1949 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

When I saw the images in this "Electron Shadows Map Force Fields" article from a 1949 issue of Radio-Electronics magazine, the first thing I though of was how as kids back in the 1960s we would hold magnets against the front of the television cathode ray tube (CRT) to see how they distorted the picture. If I still had a CRT TV or computer monitor around, I'd take some photos of it for the sake of those who have never seen what happens. The difference between that and the images formed here is that the professionals inserted the object of interest directly in the electron beam, between the cathode and the fluorescent glass grid. As with the images in the article, magnets of various shapes created unique responses. If you drag the magnet across the face of the CRT, the pattern would follow it and, depending on the strength of the magnet, would leave a fluorescent persistence trail behind it. The danger, of course, was that in doing this you were causing a much greater intensity of electrons hitting the fluorescent dots on the mask so they glowed much more intensely than designed for. My parents had a conniption fit when I pulled that trick on our new color television and the afterglow remained for many minutes. It's a darn good thing the rare earth supermagnets we have now were not available back then, or I would definitely have ruined the tube.

Electron Shadows Map Force Fields

New technique makes magnetic and electrostatic fields visible to the eye.

Next time you see a light shining against a wall and decide to improve the opportunity by making hand shadows resembling ducks and rabbits, you may feel a little kinship to serious-minded scientists of the National Bureau of Standards. These men are using shadow pictures too, though for a purpose far removed from innocent enjoyment. Their shadow-casting rays are not light, but electron streams projected on a fluorescent screen or photographic plate. And the shadows, cast by electric and magnetic fields in a technique developed by the Bureau's Dr. L. L. Marton, are yielding important information on hitherto invisible phenomena.

Figs. 1 and 2 show how one typical experiment was conducted. In Fig. 1 a stream of electrons emitted at the left is focused by a magnetic lens, much as a glass lens would focus light. The beam then converges at the focal point: the electrons deflected downward by the lens continue to travel downward and those deflected upward continue upward. The focus at this "crossover" point is very sharp. Thus the crossover is a virtual electron source.

Between the crossover and the photographic plate or fluorescent screen is a mesh of fine wires, so that the plate shows an enlarged gridiron pattern.

The field to be analyzed is placed between the point electron source and the magnetic lens. Fig. 2 illustrates what happens when a piece of recording wire which has been magnetized by a series of evenly spaced short pulses is examined. The magnetic field around the wire distorts the rays from the source before they reach the lens, the amount of distortion at any point depending on the intensity of the magnetic field at that point.

Because electron rays have been displaced, the focus at the crossover is disturbed. It becomes larger, and the virtual source of electrons which it forms is no longer simple. When the diverging beam reaches the wire mesh and casts its shadow on the screen, the even pattern of the mesh no longer appears. The disturbed rays of the source make curves and aberrations in the gridiron, vary the sharpness of focus, and make strange-looking whorls.

An enlarged photograph of the screen of Fig. 2 is shown in Fig. 3. Another picture, the pattern distorted this time by an ordinary horseshoe magnet (you can see the magnet's shadow), is Fig. 4. With the aid of complex mathematical formulas, researchers can measure and evaluate the distortion of the mesh pattern and determine exactly the strength and nature of the magnetic field set up. Even without a scientific background, however, it is easy to see the bulges above and below the shadow of the wire in Fig. 3, indicating the alternating magnetic field induced by the recorded pulses.

The electronic "shadowgraph" is expected to allow exploration of complex electric and magnetic fields of very small size, its special value being that field strength at any point can be measured. Many of these fields could never before be evaluated because a probe any larger than an electron disturbs the field. Investigation of the fundamental nature of ferromagnetism, for instance, is now under way at the Bureau of Standards. Space - charge fields, fields produced by contact potentials, charge distribution in gaseous plasma, waveguide problems - all these and more will yield their secrets to the probing electron ray.

One of the important fields the technique may be applied to is waveguides. Often the shapes of waveguides are too complex to allow mathematical analysis and the engineer depends on experiment. By measuring the fields in waveguides of various sizes and shapes with electron shadows, it may be possible to set up formulas to predict performance accurately and eliminate much guess work.

Posted September 22, 2020