MAGNETIC FIELD INDICATING SHEET GREEN PAPER BASIC INFORMATION AND TUTORIALS



This plastic sheet (usually, but not always, green) is often used to inspect magnetic parts, especially for the transitions between magnetic poles (north-south transitioning to south-north, etc.). Unfortunately, a lack of understanding of the way the sheet is constructed often leads to misinterpretation of the results.

Microscopic flakes of nickel are first coated with an oil, in which a plastic material has been dissolved. The plastic then separates out, forming a skin around the oil drop. The flake is then free to rotate within the shell of plastic, which is invisibly small in diameter.

Many layers of these spheres (perhaps 30) are deposited onto a plastic sheet, which forms a support. The support sheet may be on the order of 0.005 in thick, and the layers of spheres may add on the order of 0.002 in to the total thickness.

When no magnetic field is present, the flakes lie flat in the bottom of their spheres, reflecting upward the color of the plastic sheet. When a magnetic field is present in the plane of the sheet, the brightness is intensified.

On the other hand, if a magnetic field is present which is normal to the sheet—that is, at approximately right angles into or out of the sheet—the flakes stand on end, aligning with the field. When this occurs, the light reflected off them bounces back and forth until it is absorbed, in a manner similar to the light in a metal tube, and no light is reflected (it is black).

It can be seen, then, that the green color means either that no field is present, or that there is a field, in the plane of the sheet. For example, a region in which flux is leaving the sheet at 45° from left to right as it rises will appear to be black when viewed from the right side.

The same region viewed from the left side, however, will appear to be green! In order to have a consistent result from the indications of this material, it must be viewed from directly overhead, not from an angle.

The nickel flakes saturate at a relatively small field. This observer noted a change in color at about 10 G and full transition to black at about 100 G for one type of sheet.

The transition width for two neodymium-iron magnets side by side in air may be from on the order of +4000 G to −4000 G, but the part of this transition which is indicated by the plastic sheet is much narrower—on the order of 1⁄40 as wide.

Based on the indications of the plastic sheet, some have thought they were seeing a very narrow transition between magnet poles, to a degree which is physically impossible.

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