Van Siclen (1988 Am. J. Phys. 56 1142) reported a curious property of a dielectric sphere in the field of an external point charge: the field outside the sphere generated by the combination of the original charge exterior and the Kelvin image charge interior to the sphere is independent of the permittivity of the sphere. In this paper, we simplify and correct the original derivation and give a detailed analysis of the sources of the field. We also present various checks for the theory, providing instructive exercises for advanced undergraduates.

A new synthesis route for obtaining a single-phase hexagonal YMnO3 was developed, based on the annealing of the amorphous

precursor powder obtained by the glycine–nitrate combustion method. The process was monitored by XRD as well as by magnetic and

heat capacity measurements. The analysis of these data shows that precursor powder undergoes a gradual phase transformation that

depends on the annealing temperature. The metastable orthorhombic YMnO3 phase is the first to appear at temperatures below 700 1C,

while a mixture of ortho- and hexa-phases was found to exist within the 700–900 1C range. An almost complete conversion to hexaphase occurs at 900–1000 1C, but to obtain the pure, well crystallized h-YMnO3 phase an annealing temperature of 1300 1C was

necessary. The synthesis method applied in this work has been proved to be more convenient for obtaining single-phase h-YMnO3 than

by a solid state reaction. In addition, it is capable of producing sample free of parasitic phases that are often present in specimens

obtained by other synthesis methods.

The Kelvin image theory for a conducting sphere is extended to the case of a conducting oblate spheroid in uniform motion along its axis of revolution (a Heaviside ellipsoid) using the well-known method provided by Special Relativity. The results derived are checked in various ways.