AlphaLab Earth Magnetometer

Kr  7 800

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The AlphaLab Earth Magnetometer is used to map the earth's magnetic field. It shows the variation from place to place down to a resolution of .01 milligauss (= 1 nanotesla, or 1 gamma), which is about 1/500 percent of the earth's field strength. Because the earth's field fluctuates by typically 3-30 nanoteslas per hour, a resolution better than 1 nanotesla will not be of much additional benefit. Until now, resolution of 1 nanotesla or .01 milligauss could be achieved only with very expensive, heavy, and high-power-consuming magnetometers such as proton-, overhauser-, and cesium- types.


he AlphaLab Earth Magnetometer measures difference in field strength to accuracy of +/-0.5% of the difference, either for point-to-point or time-to-time measurements. In other words, if one location is 2.00 milligauss (200 nanotesla) stronger than another location, the Earth Magnetometer will indicate that difference to accuracy of +/- .01 milligauss or 1 nanotesla.This magnetometer has a magneto-resistive sensor, which uses the spins of electrons flowing through a circuit, rather than their charge, to measure magnetic field. This type of device most closely resembles a “flux gate” magnetometer, but the magneto-resistive technology is newer, smaller, less expensive and intrinsically more stable over temperature.Measuring the Earth's Magnetic FieldGeomagnetometers, such as this meter, are usually used to measure the strength of earth's field at several points on the land surface. These points are sometimes arranged in a square grid (with spacings such as 10 x 10 feet). In this case, a measurement is recorded at every point on the grid, and a map can be made from these numbers. The map might show for example, that the field is stronger in a circular area about 20 feet in diameter in the center of the grid than it is at the edges of the grid. This would suggest that a magnetized object is buried roughly 20 feet underground.Instead of measuring on a square grid, measurements are sometimes made at several points along a straight line. The relative strength of field is measured at each point (relative to the field strength at the starting point). If there is an area with a stronger (or weaker) field than average, then the place along that line where the field is strongest (or weakest) can be found with the meter. Then measurements are made along a straight line that is perpendicular to the original line, starting at that strongest (or weakest) point on the original line. If you're looking for the location of the strongest (or weakest) field, this method is faster than measuring every point of a grid.There is one complication involved with measuring the field strength: magnetic field is a vector, which means it has both a strength (“magnitude”) and a direction. At each point (or position) that a measurement is taken, both the strength of field and the direction may be different from that of the previous measured point. Most geomagnetic mapping is concerned with the strength only, and not with the direction. When using the more expensive meters such as proton magnetometers, the direction cannot be determined, because these types of meters measure the strength only. (In comparison, our Earth Magnetometer is capable of showing the field direction, but this additional information is difficult to quantify, because it would require that you measure the angle between the sensor rod and the vertical direction. A measurement like this could be done using a plumb-bob.)All types of geomagnetic meters require that you point them, at least approximately, in a certain direction (with respect to the earth's field direction). Otherwise the readings will be inaccurate. With a proton magnetometer, the sensor can be pointed as much as 30° off from the preferred direction before the readings start to become noisy. It's easy to keep the sensor pointed with +/- 30° of a certain direction when measuring in the field, so this is not much of a problem. However, the AlphaLab Earth Magnetometer is a vector magnetometer, and it must have its sensor pointed (at least momentarily) within one-third of a degree of the correct field direction in order to measure the actual field to .01 milligauss precision. This kind of precision required in pointing the sensor had been a problem when using vector magnetometers to measure the exact earth field strength. A “fluxgate” magnetometer (which is also a vector magnetometer) could have been available a long time ago as a less expensive alternative to the proton magnetometer, had it not been for the accuracy of pointing needed.

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