Magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, one that measures the direction of an ambient magnetic field, in this case, the Earth's magnetic field. Other magnetometers measure the magnetic dipole moment of a magnetic material such as a ferromagnet, for example by recording the effect of this magnetic dipole on the induced current in a coil.
The invention of the magnetometer is usually credited to Carl Friedrich Gauss in 1832. Earlier, more primitive instruments were developed by Christopher Hansteen in 1819, and by William Scoresby by 1823.
Magnetometers are widely used for measuring the Earth's magnetic field, in geophysical surveys, to detect magnetic anomalies of various types, and to determine the dipole moment of magnetic materials. In an aircraft's attitude and heading reference system, they are commonly used as a heading reference. Magnetometers are also used by the military as a triggering mechanism in magnetic mines to detect submarines. Consequently, some countries, such as the United States, Canada and Australia, classify the more sensitive magnetometers as military technology, and control their distribution.
Magnetometers can be used as metal detectors: they can detect only ferromagnetic metals, but can detect such metals at a much greater distance than conventional metal detectors, which rely on conductivity. Magnetometers are capable of detecting large objects, such as cars, at over, while a conventional metal detector's range is rarely more than.
In recent years, magnetometers have been miniaturized to the extent that they can be incorporated in integrated circuits at very low cost and are finding increasing use as miniaturized compasses.
Introduction
Magnetic fields
Magnetic fields are vector quantities characterized by both strength and direction. The strength of a magnetic field is measured with the unit tesla in the SI units, and in gauss in the cgs system of units. 10,000 gauss are equal to one tesla. Measurements of the Earth's magnetic field are often quoted in the unit nanotesla, also called a gamma. The Earth's magnetic field can vary from depending on location, fluctuations in the Earth's magnetic field are on the order of, and magnetic field variations due to magnetic anomalies can be in the picotesla range. Gaussmeters and teslameters are magnetometers that measure in the unit gauss or tesla, respectively. In some contexts, magnetometer is the term used for an instrument that measures fields of less than 1 millitesla and gaussmeter is used for those measuring greater than 1 mT.Types of magnetometer
There are two basic types of magnetometer measurement. Vector magnetometers measure the vector components of a magnetic field. Total field magnetometers or scalar magnetometers measure the magnitude of the vector magnetic field. Magnetometers used to study the Earth's magnetic field may express the vector components of the field in terms of declination and the inclination.Absolute magnetometers measure the absolute magnitude or vector magnetic field, using an internal calibration or known physical constants of the magnetic sensor. Relative magnetometers measure magnitude or vector magnetic field relative to a fixed but uncalibrated baseline. Also called variometers, relative magnetometers are used to measure variations in magnetic field.
Magnetometers may also be classified by their situation or intended use. Stationary magnetometers are installed to a fixed position and measurements are taken while the magnetometer is stationary. Portable or mobile magnetometers are meant to be used while in motion and may be manually carried or transported in a moving vehicle. Laboratory magnetometers are used to measure the magnetic field of materials placed within them and are typically stationary. Survey magnetometers are used to measure magnetic fields in geomagnetic surveys; they may be fixed base stations, as in the INTERMAGNET network, or mobile magnetometers used to scan a geographic region. An early adoption of airborne magnetometry by Inco prompted the discovery of nickel ore deposits that led to the founding of Thompson, Manitoba.
Performance and capabilities
The performance and capabilities of magnetometers are described through their technical specifications. Major specifications include- Sample rate is the number of readings given per second. The inverse is the cycle time in seconds per reading. Sample rate is important in mobile magnetometers; the sample rate and the vehicle speed determine the distance between measurements.
- Bandwidth or bandpass characterizes how well a magnetometer tracks rapid changes in magnetic field. For magnetometers with no onboard signal processing, bandwidth is determined by the Nyquist limit set by sample rate. Modern magnetometers may perform smoothing or averaging over sequential samples, achieving a lower noise in exchange for lower bandwidth.
- Resolution is the smallest change in a magnetic field that the magnetometer can resolve. This includes quantization error which is caused by recording roundoff and truncation of digital expressions of the data.
- Absolute error is the difference between the readings of a magnetometer true magnetic field.
- Drift is the change in absolute error over time.
- Thermal stability is the dependence of the measurement on temperature. It is given as a temperature coefficient in the unit nT per degree Celsius.
- Noise is the random fluctuations generated by the magnetometer sensor or electronics. Noise is given in the unit nT/Hz, where frequency component refers to the bandwidth.
- Sensitivity is the larger of the noise or the resolution.
- Heading error is the change in the measurement due to a change in orientation of the instrument in a constant magnetic field.
- The dead zone is the angular region of magnetometer orientation in which the instrument produces poor or no measurements. All optically pumped, proton-free precession, and Overhauser magnetometers experience some dead zone effects.
- Gradient tolerance is the ability of a magnetometer to obtain a reliable measurement in the presence of a magnetic field gradient. In surveys of unexploded ordnance or landfills, gradients can be large.
Early magnetometers
In 1823 William Scoresby, an English explorer, scientist and clergyman, was deeply involved in magnetic science, particularly in improving ships' compasses. In 1823, he published a paper in the Transactions of the Royal Society of Edinburgh titled "Description of Magnetimenter, being a new instrument for measuring magnetic attractions and finding the dip of the needle; with an accont of experiments made with it."
In 1833, Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field. It described a new instrument that consisted of a permanent bar magnet suspended horizontally from a gold fibre. The difference in the oscillations when the bar was magnetised and when it was demagnetised allowed Gauss to calculate an absolute value for the strength of the Earth's magnetic field.
The gauss, the CGS unit of magnetic flux density was named in his honour, defined as one maxwell per square centimeter, which corresponds to 10−4 tesla.
Francis Ronalds and Charles Brooke independently invented magnetographs in 1846 that continuously recorded the magnet's movements using photography, thus easing the load on observers. They were quickly utilised by Edward Sabine and others in a global magnetic survey and updated machines were in use well into the 20th century.