Abstract: A switch operating mechanism for use in a telephone instrument (for example, for operating a hook switch) has two arms (4 and 5) for projection from--and independent displacement with respect to--an outer surface of the instrument body (1). The arms (4 and 5) are arranged to co-operate with a switch actuating member (13) such that this member adopts a first position when both arms (4 and 5) are retracted with respect to the body (1), and a second position when one or both of the arms (4 and 5) is or are extended from the body (1). This mechanism is particularly advantageous for operating the hook switch in a one-piece telephone instrument because inadvertent depression of one of the arms (4 or 5) does not cause actuation of the hook switch.

Abstract: In previously proposed arrangements for charging an electret foil the electret charge implanted varies over the area of the foil. In order to promote more even charging of the foil, the method of the invention subjects the foil to a progressively increasing electric field. One form of apparatus used to carry out the method has rollers at progressively higher potentials and electric fields are applied across the foil between these rollers and a metallized layer on the rear face of the foil which is earthed by a roller.

Abstract: An electret transducer backplate is made of a thermo-softening plastics material rendered electrically conducting by electrically conducting particles distributed therethrough. An electret foil diaphragm is mounted on the backplate and the relative flexural rigidities of the diaphragm and the backplate are such that, in use, the deflection of the diaphragm is at least twenty times as great as the deflection of the backplate. A method of making the transducer involves heating an electret foil, bonding the electret foil to a retaining ring while the foil is hot and securing together the retaining ring and the backplate.