A comparison of ferromagnetic properties in microwave, low frequency and direct current fields

Hall, Alexander M (1955) A comparison of ferromagnetic properties in microwave, low frequency and direct current fields. PhD thesis, University of Glasgow.

Abstract

The thesis describes research work carried out at the Royal Technical College, on the measurement of microwave permeability, and of comparison measurements made with direct and 50c/s fields. Permeability may be defined both as a real and a complex quantity. The reasons for considering it complex are discussed and the relationships between the permeabilities obtained from measurements by different methods and the real and imaginary parts of the complex permeability is derived in a novel manner. This is followed by a historical survey of published work which is considered important to the development of the technique used by the author. In this the Q of a coaxial resonator with a 16 S.W.G. magnetic wire as inner conductor was compared with the Q of the same resonator with a copper inner conductor. From those measurements, the product of resistivity and permeability of the magnetic specimen was calculated. The design of the resonator was such as to allow a current to be passed through the specimen, lengthwise. This produced a magnetic field parallel to the microwave magnetic field, thus reducing the measured permeability. By using very strong fields, it was possible to estimate the value of the product of resistivity and permeability as the permeability tended to unity. This value is the resistivity and from it the permeability with no polarising field was calculated. When the polarising field was varied in a cyclic manner, a hysteresis curve of permeability was drawn out. This curve has been called a ''butterfly" curve by previous workers. The author's apparatus is somewhat simpler than that previously described and, as a new extension to this technique the author produced a similar type of curve using a field at right angles to the microwave field. This caused an increase in the resistivity-permeability product as the field increased, presumably due to an increase in resistivity caused by the perpendicular field. Curves were also produced with combined parallel and perpendicular fields. For comparison purposes the resistivity and permeability were measured using d.c. The permeability was measured ballistically with a field applied lengthwise to the wire specimen by a long solenoid. It was realised that this was not the same field configuration as that used in the microwave measurements, therefore a new technique was developed* In this, a 50c/s alternating current was passed through the wire specimen and a reference specimen (eureka) in series. The voltage drop along each of these wires was compared by means of an a.c. potentiometer and the results of this comparison were used to calculate the complex permeability of the specimen. As the magnetic field was circular, it was of the same form as that used in the microwave measurements. The calculation of the permeability from these results required the extension of an existing proof of the voltage drop along a magnetic conductor to the case of one with a complex permeability. This extension introduced Bessel Functions of Complex Argument, which made the computation difficult. A family of curves was therefore drawn to ease this problem. The rationalised M.K.S. system of units have been used throughout the thesis, except for some historical equations which have been quoted in their original units, and a suitable note added.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: F M Bruce
• Keywords: Electromagnetics
• Date of Award: 1955
• Depositing User: Enlighten Team
• Unique ID: glathesis:1955-73504
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jun 2019 08:56
• Last Modified: 14 Jun 2019 08:56
• URI: https://theses.gla.ac.uk/id/eprint/73504