Most storage devices that do not require vision and a brain to read data fall into this category. Electromagnetic data may be stored in either an analog data or digital data format on a variety of media.
Direct Tripwire Alarm - The wire placed across the path is directly connected with the cord, wire or string placed across the particular path. Laser Tripwire Alarm - An invisible laser, typically an infra-red, is used instead of a cord or string.
The ground wire is an alternative path to ground that is all. If there is a switch between the outlet and the box the path to ground may be severed(IE: Turned off) unless you have a ground wire and dont do something silly like using that adapter or breaking off the third prong.