Evans balance
Encyclopedia
An 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. Two pairs of magnets are placed at opposite ends of a beam making a balanced system with a magnetic field at each end. When a sample, which is fixed in a holder, is introduced into the field of one magnet, that magnet experiences a force which would deflect the beam. A magnetic field is generated at the second magnet which, by negative feedback
, restores the beam to its original position. The current required to do this is proportional to the force exerted on the first magnet. The advantage of this system is that it is cheap to construct as it does not require a precision weighting device.
where L is the length of the sample and m is its mass in grams. The reading for the empty tube is needed because the tube glass is diamagnetic.
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
. Magnetic susceptibility is related to the force experienced by a substance in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. 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. Two pairs of magnets are placed at opposite ends of a beam making a balanced system with a magnetic field at each end. When a sample, which is fixed in a holder, is introduced into the field of one magnet, that magnet experiences a force which would deflect the beam. A magnetic field is generated at the second magnet which, by negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
, restores the beam to its original position. The current required to do this is proportional to the force exerted on the first magnet. The advantage of this system is that it is cheap to construct as it does not require a precision weighting device.
Calibration
The Evans balance measures susceptibility indirectly by referring to a calibration standard of known susceptibility. The most convenient compound for this purpose is mercury cobalt thiocyanate, HgCo(NCS)4, which has a susceptibilty of 16.44×10−6 (±0.5%) CGI at 20°C. Three readings of the meter are needed, of an empty tube, R0 of the tube filled with calibrant and of the tube filled with the sample, Rs. The first two provide a calibration constant, C. The mass susceptibility in grams is calculated aswhere L is the length of the sample and m is its mass in grams. The reading for the empty tube is needed because the tube glass is diamagnetic.