A metal detector is an electronic instrument which detects the presence of metal nearby. Metal detectors are
useful for finding metal inclusions hidden within objects, or metal objects buried underground. They often consist
of a handheld unit with a sensor probe which can be swept over the ground or other objects. If the sensor comes
near a piece of metal this is indicated by a changing tone in earphones, or a needle moving on an indicator. Usually
the device gives some indication of distance; the closer the metal is, the higher the tone in the earphone or the
higher the needle goes.
A boom barrier, also known as a boom gate, is a bar, or pole pivoted to allow the boom to block vehicular or
pedestrian access through a controlled point. Typically the tip of a boom gate rises in a vertical arc to a near
vertical position. Boom gates are often counterweighted, so the pole is easily tipped. Boom gates are often paired
either end to end, or offset appropriately to block traffic in both directions. Some boom gates also have a second
arm which hangs 300 to 400 mm below the upper arm when lowered, to increase approach visibility, and which
hangs on links so it lies flat with the main boom as the barrier is raised. Some barriers also feature a pivot roughly
half way, where
as the barrier is raised, the outermost half remains horizontal, with the barrier resembling an
upside-down L when raised.
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