Abstract: An alarm system for surveillance of a container located within an electric or electromagnetic field surrounding the container includes a detector device which comprises generating elements for generating an electric or electromagnetic field, balancing elements for building up a balanced electric or electromagnetic field, filtering elements to prevent the detector device from being affected by chances in temperature and humidity. The container includes conductive material and connected to the detector device so as to constitute an antenna for sensing small capacitive changes in the electric or electromagnetic field. The generating and balancing elements respectively generate and build up an electric or electromagnetic field around the container.

Abstract: A capacitive detector is provided which includes an antenna for detecting small capacitive changes in an electric or electromagnetic field surrounding the antenna, and particularly to an alarm system which uses the capacitive detector to generate an alarm signal upon detection of capacitive changes in the generated electric or electromagnetic field. The detector is realized with an antenna connected to an electronic circuitry which generates an electric field around the antenna. The electronic circuitry generates an electric or electromagnetic field around the antenna, builds up a balanced electric field around the antenna, maintains the generated electric or electromagnetic field around the antenna balanced, prevents the detector device from being affected by changes in temperature and humidity, detects small changes in the generated electric field around the antenna, and indicates that a change in the generated electric field has occurred.