Abstract: A surface elastic wave filter has a pair of electrodes on a surface of a piezoelectric body, and at least one of the electrodes is constructed as electrode fingers in the form of comb teeth which are progressively spread away from each other along a direction normal to the direction in which a surface elastic wave is propagated on the surface of the piezoelectric body. A grating or dot structure as a scattering reflector for scattering and reflecting a portion of the surface elastic wave is disposed in a desired position between the electrodes. Alternatively, some of electrode fingers of a surface elastic wave filter are dropped or removed, or divided voltages are applied to electrode fingers, or a scattering reflector is added to a filter with some of its electrode fingers removed or with divided voltages applied to electrode fingers. With such an arrangement, a filter having good selectivity and desired frequency characteristics can be achieved.

Abstract: A ball mill comprising a horizontal mill casing provided with means of magnetizing-force generation which are identically constructed and equidistantly spaced electromagnets forming a plurality of electromagnet rows, fixed in space, equally spaced apart and extending from one end to the other of the casing within two angular zones fixed in space, first from 135.degree. to 180.degree. and second from 225.degree. to 360.degree., thus presenting a plurality of magnet columns in circular direction, and which is provided with a switching circuit network for keeping the electromagnets of the lowermost row in the second zone in energized state and energizing and de-energizing all other electromagnets alternately and cyclically in two groups, such that every electromagnet of one group is sided, in crisscross directions through row and column, by electromagnets of the other group.

Abstract: A plurality of parallel-connected surface acoustic wave (SAW) resonators, having different oscillation frequencies and different turnover temperatures, are connected to an active element containing circuit to form a SAW oscillator. The oscillator frequency deviation due to temperature is kept within a small allowable range .DELTA. fa by satisfying the following formulas:f.sub.H -f.sub.L .ltoreq..DELTA.faT.sub.p1 <T.sub.p2. . .<T.sub.pnwheref.sub.H : a maximum frequency among those frequencies of a plurality of SAW resonators connected to the active element-containing circuit which are produced at turnover temperatures on a temperature curve;f.sub.L : a minimum frequency among those of the plurality of SAW resonators; andT.sub.p1 to T.sub.p2 : the turnover temperatures corresponding to the individual SAW resonators.