Abstract: Gypsum and magnetite which are both coarse and of good quality can simultaneously be produced by introducing calcium carbonate into an aqueous solution containing ferrous sulfate while an oxidizing gas is blown, and then carrying out a neutralizing and oxidizing operation at a pH of 5-6 and a temperature of 60-80.degree. C, and can be separately recovered by magnetic separation.

Abstract: A tone signal detecting circuit for use in a paging receiver or the like employs an N-path filter in place of an active filter to facilitate circuit integration. The circuit includes a clock signal generating circuit responsive to a first control signal for selecting and reading out one of plural clock signals of a frequency equal to N (N being an integer equal to or greater than 2) times the frequency of the plural tone signals. The N-path filter has a prescribed rise and fall time and selects and responds to a tone signal in accordance with a clock signal from the clock signal generating circuit. A low-pass filter having prescribed frequency characteristics removes harmonic signals of the tone signals contained in the output of the N-path filter. A tone designating circuit responsive to a second control signal is connected to the low-pass filter and changes its frequency characteristics.

Abstract: A miniature radio antenna formed integral with a molded plastic housing comprising a body 1 and a cover 2. The antenna 1', 1", 2', 2" includes sheets, foils or films of electrically conductive metal or coats of conductive paint laid on the inside surfaces of the opposing housing sections, or conductors embedded therein. The antenna is free from adverse acoustic effects due to resonant vibrations in sympathy with a speaker mounted in the housing, and the housing may be molded into any desirable external shape, including fully rounded corners, without increasing the size.