The distance between the field lines represent the strength of the field, the closer the field line, the stronger the field. First of all, we need to know that, in SPM, normally we use a circle with an arrow to represent compass. Second, we also need to know that the pointer of a compass is always in the direction of the magnetic field.
In figure (b) above, we can see that when a few compasses are put near to a bar magnet, the pointer of the compasses are all in the direction of the magnetic field.
If a compass is placed near to a current carrying wire, the pointer of the compass will point along the direction of the magnetic field generated by the current (as shown in the figure below). Earth’s magnetic field, a familiar directional indicator over long distances, is routinely probed in applications ranging from geology to archaeology. The work was conducted in the NMR laboratory of Alexander Pines, one of the world’s foremost NMR authorities, as part of a long-standing collaboration with physicist Dmitry Budker at the University of California, Berkeley, along with colleagues at the National Institute of Standards and Technology (NIST). The exquisite sensitivity of NMR for detecting chemical composition, and the spatial resolution which it can provide in medical applications, requires large and precise superconducting magnets. But chemical characterization of objects that cannot be placed inside a magnet requires a different approach.
Instead of a superconducting magnet, low-field NMR measurements may rely on Earth’s magnetic field, given a sufficiently sensitive magnetometer. Paul Ganssle is the corresponding author of a paper in Angewandte Chemie describing ultra-low-field NMR using an optical magnetometer. Earth’s magnetic field is indeed very weak, but optical magnetometers can serve as detectors for ultra-low-field NMR measurements in the ambient field alone without any permanent magnets. In high-field NMR, the chemical properties of a sample are determined from their resonance spectrum, but this is not possible without either extremely high fields or extremely long-lived coherent signals (neither of which are possible with permanent magnets). A key difference between this and conventional experiments is that the relaxation and diffusion properties are resolved through optically-detected NMR, which operates sensitively even in low magnetic fields.


The design was tested in the lab by measuring relaxation coefficients first for various hydrocarbons and water by themselves, then for a heterogeneous mixture, as well as in two-dimensional correlation experiments, using a magnetometer and an applied magnetic field representative of Earth’s. This technology could help the oil industry to characterize fluids in rocks, because water relaxes at a different rate from oil. The next step involves understanding the depth in a geological formation that could be imaged with this technology. Eventually, probes could be used to characterize entire borehole environments in this way, while current devices can only image inches deep.
Other authors on the Angewandte Chemie paper include Hyun Doug Shin, Micah Ledbetter, Dmitry Budker, Svenja Knappe, John Kitching, and Alexander Pines. For more information about the research in Alex Pines’ Nuclear Magnetic Resonance group, go here. Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. Check out our new site currently under development, combining the Biotechnology and Science Learning Hubs with a new look and new functionality. In the diagram, the magnetic field A is stronger than magnetic field B because the line in magnetic field A is closer. The arrow represents the pointer of a compass and it always points towards the North pole of a magnet.
Now it has provided the basis for a technique which might, one day, be used to characterize the chemical composition of fluid mixtures in their native environments. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) conducted a proof-of-concept NMR experiment in which a mixture of hydrocarbons and water was analyzed using a high-sensitivity magnetometer and a magnetic field comparable to that of the Earth.
The work will be featured on the cover of Angewandte Chemie and is published in a paper titled “Ultra-Low-Field NMR Relaxation and Diffusion Measurements Using an Optical Magnetometer.” The corresponding author is Paul Ganssle,  who was a PhD student in Pines’ lab at the time of the work.


In ex situ NMR measurements, the geometry of a typical high-field experiment is reversed such that the detector probes the sample surface, and the magnetic field is projected into the object.
This means that ex-situ measurements lose chemical sensitivity due to field strength alone. In contrast, relaxation and diffusion measurements in low-field NMR are more than sufficient to determine bulk materials properties. Relaxation refers to the rate at which polarized spin returns to equilibrium, based on chemical and physical characteristics of the system. There were attempts to perform oil well logging starting in the 1950s using the Earth’s ambient field, but insufficient detection sensitivity led to the introduction of magnets, which are now ubiquitous in logging tools. Other applications include measuring the content of water and oil flowing in a pipeline by measuring chemical composition with time, and inspecting the quality of foods and any kind of polymer curing process such as cement curing and drying.
The combination of terrestrial magnetism and versatile sensing technology again offers an elegant solution.
The current publication presents some of the work for which Berkeley Lab won an R&D 100 award earlierthis year on optically-detected oil well logging by MRI. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes.
Further, the sample of interest must be placed inside the magnet, such that the entire sample is exposed to a homogeneous magnetic field.
A main challenge with this situation is generating a homogeneous magnetic field over a sufficiently large sample area: it is not feasible to generate field strengths necessary to make conventional high-resolution NMR measurements.



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