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term accounting for the volumetric magnetic susceptibility of air and an A term for the cross-sectional area of the sample, but these terms can be ignored for solid samples. To calculate the volume magnetic susceptibility (χ) for a liquid sample, the equation would include the V term in the numerator and divide by the density (d) of the solution instead of the mass (m). The accuracy of the measurement using these simple formulae can be influenced by the
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57:. It can be measured using various devices that modify the shape of the magnetic field and measure resulting forces. The Evans balance operates by measuring the force exerted on a magnet within a magnetic field shared with a sample as an indirect proxy for the magnetization of a sample, rather than by measuring the force exerted on the sample directly.
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The original design was later modified, with assistance from
Johnson Matthey, to place two pairs of magnets within an H-frame. The sample would be inserted between one pair of magnets while a small coil was positioned between the second pair. This entire assembly pivoted horizontally around a torsion
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near to the magnets. As the electric currents vary to maintain the equilibria, those variations in the electrical current provide a measure of variations of the force required to maintain the permanent magnets in suspension. The balance consists of magnets positioned on one end of a beam that shift
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is the length of the sample, C is the calibration constant (usually 1 if the device has been calibrated), and m is the mass in grams. The reading for the empty tube accounts for the diamagnetism of the tube glass; in more complete forms of the susceptibility equation there is also an additional V
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that alters the electrical current in a coil interacting with the magnets to counteract the force of interaction between the permanent magnets and the sample, thereby restoring the beam to its initial position. The current flowing through the coil is easily measured, for instance by a
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153:, albeit with reduced accuracy and sensitivity. The Evans balance is capable of measuring within a range of 0.001 x 10 to 1.99 x 10 CGS volume susceptibility units. The original model demonstrated an accuracy within 1% of literature values for
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solids. The device facilitates measurements across solid, liquid, and gaseous forms of a wide spectrum of paramagnetic and diamagnetic materials, typically requiring approximately 250 mg of sample for each measurement.
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strip. When a sample tube would be introduced between the magnets, the torsional force would be counterbalanced by the current passing through the coil, providing a reading on the display instead of using a
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The Evans balance determines susceptibility by referencing a calibration standard with a known susceptibility. A commonly used calibration compound is mercury cobalt thiocyanate, HgCo(NCS)
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measurement. The first two readings provide a calibration constant (C). A solid sample's mass susceptibility (χg) in grams is calculated using the simplified formula:
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In contrast to other magnetic balances, the Evans balance does not require a precision weighing component. It offers faster measurements compared to a
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as their magnetic fields interact with a sample. These forces to suspend the permanent magnets are provided by electric currents from a
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gold alloy, hence the alternate name "Johnson
Matthey balance". These components were bonded together using epoxy resin on a
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580:
Figgis, B.N.; Lewis, J. (1960). "The
Magnetochemistry of Complex Compounds". In Lewis. J. and Wilkins. R.G. (ed.).
182:, with a susceptibility of 1.104 x 10 erg G cm. Calibration involves taking three readings: one with an empty tube
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one with a tube filled with the calibration reference material, and one with the tube filled with the sample
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http://alpha.chem.umb.edu/chemistry/ch371/documents/MicroscaleDeterminationofMagneticSusceptibility_001.pdf
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The Evans balance operates by measuring the forces necessary to maintain the positional equilibria of
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located opposite the equilibrium point of the beam. These photodiodes transmit signals to an
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354:"Magnetic Susceptibility Balances (MSB) : Johnson Matthey | PDF | Magnet | Physics"
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bars with cadmium-plated mild steel yokes as magnets and a suspension strip made from a
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in position upon interaction with the sample. This displacement is detected by
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quantifies the extent to which a material becomes magnetized in an applied
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381:. Progress in Inorganic Chemistry. Vol. 29. Wiley. p. 203.
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285:{\displaystyle \chi _{g}={\frac {CL(R_{s}-R_{0})}{10^{9}m}}}
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Evans, D.F. (1974). "A new type of magnetic balance".
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325:"Magnetic Susceptibility of Coordination Compounds"
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118:bronze spacer. The sample tubes were crafted from
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35:Johnson Matthey magnetic susceptibility balance
436:Journal of Physics E: Scientific Instruments
377:O'Connor, C.J. (1982). Lippard, S.J. (ed.).
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141:Comparison to alternative magnetic balances
403:"Illustration of commercial Evans balance"
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94:The original Evans balance, devised by
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379:Magnetic susceptibility measurements
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87:with leads attached to a precision
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470:"Classic Kit: The Evans balance"
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67:suspended permanent magnets
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456:10.1088/0022-3735/7/4/007
91:in series with the coil.
106:in 1937. Evans utilized
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51:Magnetic susceptibility
39:magnetic susceptibility
332:www2.chemistry.msu.edu
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102:developed earlier by
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448:1974JPhE....7..247E
584:. New York: Wiley.
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159:paramagnetic
147:Gouy balance
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475:5 September
302:homogeneity
166:Calibration
155:diamagnetic
76:photodiodes
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308:References
16:See also:
338:April 26,
249:−
215:χ
120:NMR tubes
85:voltmeter
80:amplifier
61:Mechanism
656:Category
636:cite web
561:cite web
516:cite web
116:phosphor
89:resistor
444:Bibcode
108:Ticonal
47:liquids
586:p. 415
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358:Scribd
295:where
43:solids
621:(PDF)
614:(PDF)
546:(PDF)
539:(PDF)
501:(PDF)
494:(PDF)
328:(PDF)
149:or a
642:link
567:link
522:link
477:2023
383:ISBN
340:2024
71:coil
45:and
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