Sample-Coil

Sample Coil Sets

Walker Scientific offers a range of sample coil sets for B, Bi and H field measurements. These coil sets are specifically designed for use with AMH series Automatic Magnetic Hysteresisgraphs and allow all measurements to be made using the two AMH fluxmeters or a Hall probe and AMH gaussmeter. The coil set is essentially a surrounding B coil that measures the B flux in the specimen by completely encompassing the specimen. The flux density within the specimen is determined by dividing the measured B field by the specimen area and subtracting the flux density of any air space inside the B coil. Coil sets are designed to accept samples which fill from 25% to 100% of the area within the B coil, allowing many different sample sizes to be tested with the same coil set without regard to specimen geometry (providing that the sample has flat parallel faces). The specimen aperture is easily positioned in the center of the pole face by means of an indexing plate on the bottom of the coil set which is designed to fit the profile of the lower pole tip of the electromagnet. A label on the coil set indicates the area turns of the B and H coils and the "effective" B coil turns. Entry of this information into the configuration menu of the AMH operating program ensures accurate sample measurement.

Coil sets contain five coils inside the paddle section of the coil set. These coil sets are designed with a switch to change the electrical connection of the coils in order to directly measure either B vs. H or B minus H vs. H. Coil sets utilize an elaborate five coil configuration for accurate measurement of all hysteresis parameters: the B coil, inner H and minus-H coils on the inner circumference of the aperture, then outer H and minus H coils on the outer circumference of the aperture. The H component measurement is equal to the flux in the inner H coil subtracted from the flux inside the outer H coil. The H reading is measured as close to the specimen as possible, and is not affected by the flux in the specimen. The minus-H component is measured in the same fashion and is electrically subtracted from the B coil reading. Regardless of the type of surrounding coil used, this method yields a B reading averaged over the entire specimen area for excellent repeatability.

The multiple concentric coil design has major benefits over other designs with fewer coils. Measurement of the H component is performed as close as possible to the sample for the most accurate H and minus-H readings. Competitive designs make this measurement with only two coils (one B and one H coil), requiring the H coil to be placed further from the test specimen that can cause measurement errors due to differences in field homogeneity. With the five-coil design, the true intrinsic (B minus-H vs. H) curve is generated directly by electrically subtracting H from B, rather than through software calculation.

Coil sets are available in sizes to accommodate a range of sample configurations: circular, rectangular or hemispheric. Coil sets are designed to be thinner than the specimen in order that the specimen always contact both the upper and lower electromagnet pole faces for a closed magnetic circuit condition. Standard coil set thickness is 5 mm; other thicknesses are available on request.

Advantages of the multiple concentric coil set design:
Measurement of the H component is done as close to the sample as possible for the most accurate H and minus H readings. Other manufacturers make this measurement with only two coils (one B and one H coil), resulting in the H coil being positioned further away from the test specimen that can possibly cause errors due to differences in field homogeneity.

Using five coils, the true intrinsic and extrinsic curves can be generated directly rather than relying on software interpolation. The operator has only to toggle a switch to select the direct measurement method.

Important Coil Set Considerations:
A coil set must always be thinner than the specimen so that the specimen contacts both the upper and lower pole faces for a closed magnetic circuit condition. Standard coil set thickness is 5 mm, but thinner coil sets can be manufactured (consult factory). The coil set must also be designed so that the aperture center is close to the center of the electromagnet pole face for optimum accuracy. In cases where the specimen is much thicker than the coil set, spacer(s) should be used to ensure that the coil set is parallel to the pole faces and fairly close to the vertical center of the specimen.

Coil sets are typically less durable than an embedded coil pole cap. Since they must be removed from the electromagnet gap each time a test is run, there is a chance of dropping them or even crushing them in the electromagnet. With proper care, however, they should last many years.

The sample area must be accurately measured in order for the results to be valid. The B readings are based on an accurate measurement of the area of the specimen. Therefore, odd shaped samples may be cumbersome as their area may be difficult to calculate.

The specimen diameter is limited by the aperture size, which must be a certain percentage smaller than the pole face diameter for maximum accuracy.

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