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An Evans balance, also known as a Johnson-Matthey balance(after the most prolific producer of the Evans balance) is a device for measuring magnetic susceptibility. Magnetic susceptibility is related to the force experienced by a substance in a magnetic field. Various practical devices are available for the measurement of susceptibility, which differ in the shape of the magnetic field and the way the force is measured. In the Gouy balance there is a homogeneous field in the central region between two (flat) poles of a permanent magnet, or an electromagnet. The sample, in the form of a powder in a cylindrical tube, is suspended in such a way the one end lies in the centre of the field and the other is effectively outside the magnetic field. The force is measured by an analytical balance The Evans balance employs a similar sample configuration, but measures the force on the magnet.〔(Illustration of commercial Evans balance )〕 ==Mechanism== Two pairs of magnets are placed back to back on a suspension strip making a balanced system with a magnetic field at each end. When a sample, which is fixed in a glass tube holder, is introduced into the field of one magnet, that magnet experiences a force which deflects the beam. The deflection is detected by an optical transducer. A magnetic field is generated at the second magnet which, by negative feedback, restores the beam to its original position. The magnetic field required to do that is generated by passing a current through a coil of wire. One end of that wire is between the poles of the second magnet. The current required to do this is proportional to the force exerted on the first magnet. There is a second coil of wire that creates an electrical zero and a copper sheet that can critically dampen the system.〔http://iopscience.iop.org/0022-3735/7/4/007/pdf/0022-3735_7_4_007.pdf〕 The original Evans balance was described by Dennis F. Evans in 1973 based on a torsional balance developed in 1937 by Alexander Rankine. Evans used Ticonal bars with cadmium plated mild steel yokes as the magnets, a Johnson-Matthey gold alloy (hence the other name of the balance)for the suspension strip, all glued together with epoxy resin onto a phosphor brown spacer. The tubes were made from NMR tubes and the current came from CdS photocells.〔http://iopscience.iop.org/0022-3735/7/4/007/pdf/0022-3735_7_4_007.pdf〕 This original was modified with help from the Johnson-Matthey company Two pairs of magnets were glued between the arms of an H-frame. The sample was placed into the gap between one pair of magnets and a small coil in the gap between the second pair of magnets. This entire construction pivoted horizontally around a torsion strip. When a sample tube was placed between the first pair of magnets, the torsional force was restored by the current passed through the coil between the second pair of magnets, giving a reading on a display instead of a Helipot (as was used in the original).〔http://www.rsc.org/chemistryworld/Issues/2011/July/TheEvansBalance.asp〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evans balance」の詳細全文を読む スポンサード リンク
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