Abstract: A tape cassette has at least one opening in a surface thereof closed by a film monolithically formed with the cassette body and which is broken by a protuberance on a tape recorder with which the cassette is used following first use of the cassette. An unbroken film is indicative of a fresh, unused cassette.

Abstract: Ferromagnetic metal particles of high quality are produced by a process which comprises heat-treating starting material particles selected from the group consisting of the particles of ferromagnetic metals, ferromagnetic alloys, and oxides and hydroxides of ferromagnetic metals, in a reducing atmosphere at a temperature required for converting the material particles into ferromagnetic metal particles of improved magnetic properties. Electrostatic charges are imparted to the material particles during the heat treatment, whereby aggregation of material particles is hindered during the treatment.

Abstract: In a process for producing a magnetic metal powder, a magnetic metal compound is reduced by wet reduction in a state mixed with a clay or is heat treated in the presence of a clay powder. Alternatively, a magnetic metal powder is prepared by reducing a magnetic metal powder by wet reduction or by reducing a magnetic metal compound in a state mixed with a clay by dry reduction, and then the resulting powder is heat treated in the presence of the clay powder. The clay is typically kaolin.