Sensors which measure the strength of a magnetic field are referred to as magnetometers. Magnetometers are used in many scientific disciplines which rely upon the application of magnetometry including: biology, physiology, geology, and oceanography. Magnetometers are used commercially for the construction of dipping needles, pipeline locators, buried mine locators, bore hole controls, mineral lode indicators, and similar devices. Further, magnetometers are used in medical devices such as MRI scanners which chart the interior of the body. As potential applications of magnetometry have increased, so have the requirements for increased sensitivity magnetometers.
Many different types of magnetometers are well known to those skilled in the art. Examples of these devices include Hall effect devices, fluxgate devices and classical Gauss and sine galvanometers. Once such device is disclosed in U.S. Pat. No. 4,402,142 to Dinsmore. This patent teaches the use of a Hall effect sensor in a compass.
Standard magnetometers are not adequate for the measurement of low level magnetic fields in the range of one nano Tesla (1 nT) or smaller. Currently, there are only a few specially designed magnetometers which are capable of measuring magnetic fields at this low level.
One of these magnetometers, the proton precession or Larmor magnetometer has a sensitivity in the 1 nano Tesla range. This device depends on the random magnetic resonance measurements in a fluid or gas plasma. These devices have been used by the military to detect magnetic anomalies and other magnetic signatures caused by submarines and ships. This device, even if the circuits used are solid state, is relatively large, requires a considerable amount of power, and is non-directional, measuring total magnetic field only.
A second device, which is most closely related to the present invention, was first described by Dr. Victor Vacquier U.S. Pat. No. 2,406,870. This phase shift magnetometer depends on the peculiar behavior of certain high permeability magnetic materials in a magnetic field.
Alloys having high permeability exhibit a unique hysteresis in their B-H curve. If two nearly identical coils containing this peculiar high permeability material are driven into saturation by an appropriate AC power supply, then voltages from the sensing coils produce unusual shaped wave forms. If these coils are wound in opposition and if an ambient magnetic field is applied to these coils, then the field and flux developed are increased in one core and decreased in another. Therefore, one coil will reach saturation slightly before the other yielding a net time shift in the distorted voltage signals with respect to one-another. This time shift, which is proportional to the level of ambient magnetic flux, is measured by a pair of secondary coils which subtract these voltages. This device had a sensitivity in the range of 0.5 nT.
A third device, also based upon the phase shift effect was designed in the 1950's by the Dinsmore Instrument Company. The system as designed by Dinsmore Instrument Company was used for anomaly measurement on auto and aircraft assembly lines. The compasses produced by the Dinsmore Instrument Company could be "adjusted" against the vehicle caused anomalies while on the assembly line.
The earlier Dinsmore Instrument device, and a later Dinsmore Instrument device which was the subject of an National Science Foundation grant, differed from the patent of Vacquier in that, these subsequent devices require only primary and not secondary coils for measurement of the magnetic field. Further these devices supply an additional electrical phase shift between the currents in the two coils, yielding a sensitivity as low as 0.01 nT.
A single magnetometer has a sensitivity of less than 1 pico Tesla. This device, referred to as the "Josephson Junction" device, predicts the passage of paired electrons, so-called Cooper Pairs, through a weak connection sandwiched between superconducting materials. A Josephson Junction in the presence of a magnetic field induces a current and subsequently draws voltage across a parallel pair of Josephson Junctions. The voltage drop across the paired Josephson Junctions is utilized as a measure of a magnetic anomaly or ambient magnetic field. Although this device is extremely sensitive, it requires a cryogenic environment. A recent advance in this field is a Superconducting Quantum Interference Device (or so-called "SQUID"), a Josephson Junction produced with relatively high temperature superconducting compounds. Resolution in the range of 0.00001 nano Teslas (or 0.01 picoTesla) have been reported. However, these so-called "high temperature superconducting compounds" must still be maintained at in a bath of liquid helium at near absolute zero temperatures.
Hence, at present, there is only one magnetometer which is capable of measuring magnetic field strengths at a level of one pico Tesla or below. This device is large and expensive since, among other reasons, it relies upon a superconducting material which must be maintained at cryogenic conditions. This is a serious drawback which makes the device unsuitable for many applications.