Abstract: There is provided a method for preparing a large and perfect oxide crystal useful for oxide superconductors and laser transmitting elements. In the present method for preparing a large oxide single crystalline material such as superconductive crystals of RE123, a crystal precursor material is supercooled prior to the solidification thereof in the course of crystal growth of the oxide by a supercooling solidification process, followed by subjecting said precursor material to continuous slow heating while keeping the supercooled condition to promote crystal growth, as shown in FIG. 7. Seed crystals may be added to the crystal precursor material prior to solidification, if necessary, as also shown in FIG. 7.

Abstract: A protector circuit comprises series connection of a bidirectional voltage limiting element and a bidirectional thyristor between a signal line connected to a communication equipment and ground. The breakdown voltage V.sub.B of the bidirectional voltage limiting element is set to be slightly higher than the battery feed voltage V.sub.Bf on the signal line and lower than a predetermined non-operation voltage V.sub.A of the protector circuit. The sum of the breakdown voltage V.sub.B and the break-over voltage V.sub.BO of the bidirectional thyristor is set to be lower than breakdown voltage V.sub.R of the equipment and higher than the non-operation voltage V.sub.A.