Improvements in piezo-electric crystal vibrators

611,738. Compound sheet materials. SMITH, B. S., PRATT, E. W., and WRIGHT, C. S. Dec. 19, 1945, No. 34416. [Class 140] [Also in Groups XXXIX and XL(c)] In a piezo-electric crystal vibrator consisting of crystal plates s adhesively secured between metal plates, each adhesive joint embodies a grid of soft metal wires or other filaments so as to ensure a predetermined thickness of the adhesive layer when the parts are pressed together. The invention is described as applied to a sub-aqueous compressional wave sender and receiver as in Specification 611,739, [Group XL(c)], in which it is required to stick quartz plates to a steel plate b<1>. The parts are cleaned with methylated spirit and while heated are coated with a solution of a polyvinyl acetal resin in methylated spirit. A ring plate i carrying a grid j of fine copper wires or plastic filaments is laid on the adhesive coating, and the whole is heated to drive off the solvent and to bind the adhesive to the steel. When the plate has cooled the ring i is forced downwards to break off the wires at the outer edge of the plate b<1>. A desired pattern of quartz plates is arranged on the plate b<1> by means of a template which is then removed. Another face plate is similarly prepared, and an interplate is prepared by coating both faces with adhesive and a grid of wires. The two face plates are then applied to the opposite faces of the interplate and the whole is heated to about 160‹C with about 1 ton pressure and allowed to cool with full pressure maintained. This gives a constant thickness layer of adhesive in the finished unit, being slightly less than the original diameter of the wire as this is somewhat flattened in the process.