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By: Vincent A. Ardizzone, EE
Executive President Product Development
Magne-Rite, Inc.

Abstract: The ability to visually see dc magnetic field patterns provides certain advantages to manufacturers in the area of quality control. A process employing the micro-encapsulation of colloidal nickel has made possible the development of a magnetic viewing medium that is both paper-thin and highly flexible. This new viewing medium renders the typical bulky, rigid dc magnetic field viewing devices obsolete.

Key Words: Quality control, Micro-encapsulation, Colloidal Nickel, DC Magnetic Field

Various attempts have been made over the years to provide a viewing medium for dc magnetic fields. The majority has used magnetically sensitive particles suspended in a fluid medium contained in a flat rigid enclosure. These devices provided a fairly effective way to view dc magnetic field orientations emanating from flat magnetic surfaces.

As electromechanical devices have advanced in design and complexity, however, the need for more elaborate magnetic components has risen proportionately. To fill this need, developments in permanent magnet manufacturing have evolved utilizing new methods of producing high-energy magnets through bonding and injection molding processes. These new methods of manufacture have made possible the mass production of permanent magnets which possess geometric configurations and tolerances never before attainable. As a result, permanent magnet designs today often assume amorphous shapes, which render typical rigid viewing devices useless.

With the exception of highly expensive electronic scanning devices which map points of magnetic intensity and present a visual depiction of flux lines onto a computer monitor, only a highly flexible magnetic viewing device can offer the ability of viewing a magnetic field emanating from a contoured magnetic surface in a simple and inexpensive format.

A unique process of micro-encapsulation was developed which allows a very thin 2-mil layer of magnetically sensitive slurry to be bonded to a 5-mil sheet of plastic film. In the encapsulation process, colloidal nickel particles suspended in the slurry become engulfed within gelatinous membranes. The slurry is then coated onto the plastic film and allowed to dry. After complete drying, the particles maintain freedom of movement within the gelatinous membranes.

When a dc magnetic field is applied to the bonded film, the nickel particles congregate in alignment with the flux lines emanating from the dc magnetic source. This mass grouping together of nickel particles causes a darkened appearance to the film directly where the magnetic field is impinging on the film. As a result, an exact two-dimensional image or impression of the magnetic pole or pole pattern (if there are more than one) is produced. This image is easily erased once the film is removed from the magnetic source and the nickel particles are allowed to freely re-disperse within their gelatinous cells. A common bar magnet can be swiped across the film to ensure complete erasure of the previous image, or by reapplying the film to another magnetic source the new image produced will completely replace the old one.

It should also be pointed out that, due to the small size of the colloidal nickel particles, the film can detect fields as low as only a few gauss and still produce and clear image.

A particularly useful application of this viewing film is in the identification of changes in magnetic polarity. As an example, if two magnets identical in size and shape were placed adjacent to each other, as shown in figure 1, and one wished to quickly determine if they were identical or opposite in polarity, for instance, placing the film on the magnet's surface would immediately reveal the answer. If the magnets were opposite in polarity, for instance, the film would show the typically darkened areas where each magnetic pole is located but would also clearly reveal a much lighter line where the magnetic fields change polarity. See figure 2. The lighter line is produced because no flux lines are present at the border where magnetic fields change over in polarity and hence the nickel particles in the film do not congregate along that line. If the magnets were of the same polarity the film would show a solid darkened region over the entire area of both magnetic poles with no line of separation. See figure 3.
Figure 1:
Two Adjacent Magnets
Figure 2:
Film Revealing Change Over in Polarity
Figure 3:
Film Revealing Identical Polarities
Needless to say, for a multi-polar arrangement of magnets, the film would reveal a series of light lines depicting each change in magnetic polarity.

Clearly, magnetic viewing film offers distinct advantages over the bulky and rigid devices available. Most notably, due to the film's high flexibility, it easily conforms to magnetic surfaces with high degrees of concavity and convexity. Also, because the film is so thin and can be cut down to practically any size without losing its properties, it can be used in tighter places inaccessible to the bulkier, unalterable devices. Finally, since there are no moving or electronic parts, the film never needs maintenance or repair. Obviously, the same cannot be said for magnetic scanners, not to mention their prohibitive cost. Simplicity of function, ease of use, reliability, and economy of cost all make dc magnetic viewing film a logical choice for manufacturers who need to check the magnetic polarity of their dc magnetic array components or merely to detect the presence of dc filed components that are obscured from view.

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