| 摘要 |
1,257,175. Microphones. AMERICAN DISTRICT TELEGRAPH CO. 20 Dec., 1968 [22 Dec., 1967], No. 60845/68. Heading H4J. [Also in Division G4] A burglar alarm system (see Division G4) uses a contact microphone having a frequency response of 7 KHz to 50 KHz and which comprises an aluminium disc 17 to which is fixed, by means of an electrically insulating glue e.g. an epoxy glue, a crystal e.g. of barium titanate. An aluminium housing 19 is similarly fixed to the disc, and has a slot 23 through which a shielded cable 20, having conductors 21, 22, passes. These conductors are connected to respective contact plates at each side of the crystal or to respective silvered surfaces of the crystal lead out. |
