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<p>NMR studies magnetic nuclei by aligning them with an applied constant magnetic field and perturbing this alignment using an alternating magnetic field, those fields being orthogonal. The resulting response to the perturbing magnetic field is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance imaging, which use very powerful applied magnetic fields in order to achieve high resolution spectra, details of which are described by the chemical shift and the Zeeman effect.</p>

<p>NMR phenomena are also utilised in low field NMR and Earth's field NMR spectrometers, and some kinds of magnetometer.</p>

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<p>Nuclear magnetic resonance was first described and measured in molecular beams by Isidor Rabi in 1938.<sup class="reference" id="_ref-0">[1]</sup> Eight years later, in 1946, Felix Bloch and Edward Mills Purcell refined the technique for use on liquids and solids, for which they shared the Nobel Prize in physics in 1952.</p>

<p>Purcell had worked on the development and application of RADAR during World War II at Massachusetts Institute of Technology's Radiation Laboratory. His work during that project on the production and detection of radiofrequency energy, and on the absorption of such energy by matter, preceded his discovery of NMR.</p>

<p>The development of nuclear magnetic resonance as a technique of analytical chemistry and biochemistry parallels the development of electromagnetic technology and its introduction into civilian use.</p>

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<h3><span class="mw-headline">Nuclear spin and magnets</span></h3>

<p>The precessing nuclei can also fall out of alignment with each other (returning the net magnetization vector to a nonprecessing field) and stop producing a signal. This is called <em>T</em>₂ relaxation. It is possible to be in this state and not have the population difference required to give a net magnetization vector at its thermodynamic state. Because of this, <em>T</em>₁ is always larger (slower) than <em>T</em>₂. This happens because some of the spins were flipped by the pulse and will remain so until they have undergone population relaxation. In practice, the <em>T</em>₂ time is the life time of the observed NMR signal, the free induction decay. In the NMR spectrum, meaning the Fourier transform of the free induction decay, the <em>T</em>₂ time defines the width of the NMR signal. Thus, a nucleus having a large <em>T</em>₂ time gives rise to a sharp signal, whereas nuclei with shorter <em>T</em>₂ times give rise to more broad signals. The length of <em>T</em>₁ and <em>T</em>₂ is closely related to molecular motion.</p>

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<p>NMR spectroscopy is one of the principal techniques used to obtain physical, chemical, electronic and structural information about molecules due to the chemical shift and Zeeman effect on the resonant frequencies of the nuclei. It is a powerful technique that can provide detailed information on the topology, dynamics and three-dimensional structure of molecules in solution and the solid state. Also, nuclear magnetic resonance is one of the techniques that has been used to build elementary quantum computers.</p>

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<h3><span class="mw-headline">Medicine</span></h3>

<p>Various magnetometers use NMR effects to measure magnetic fields, including proton magnetometers or proton precession magnetometers (PPM), and Overhauser magnetometers. See also Earth's field NMR.</p>

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<p>Major NMR instrument makers include Bruker, General Electric, JEOL, Kimble Chase, Philips, Siemens AG, Varian, Inc. and SpinCore Technologies, Inc.</p>

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<li><a class="external text" title="http://www.vega.org.uk/video/programme/21" rel="nofollow" href="http://www.vega.org.uk/video/programme/21">Richard Ernst, NL - Developer of Multdimensional NMR techniques</a> Freeview video provided by the Vega Science Trust. </li>