There are several types of volumetric IDSs. What type of IDS emits a measurable magnetic field that it monitors for disruptions?

A.
Capacitance detector

B.
Passive infrared system

C.
Wave-pattern motion detectors

D.
Photoelectric system

Explanation:
A: A capacitance detector, or proximity detector, emits a measurable magnetic field. The detector monitors this magnetic field, and an alarm sounds if the field is
disrupted. Capacitance change in an electrostatic field can be used to catch a bad guy, but first you need to understand what capacitance change means. An
electrostatic IDS creates an electrostatic magnetic field, which is just an electric field associated with static electric charges. All objects have a static electric
charge. They are all made up of many subatomic particles, and when everything is stable and static, these particles constitute one holistic electric charge. This
means there is a balance between the electric capacitance and inductance. Now if an intruder enters the area his subatomic particles will mess up this lovely
balance in the electrostatic field, causing capacitance change, and an alarm will sound.
B is incorrect because a passive infrared system (PIR) identifies the changes of heat waves in an area it is configured to monitor. If the particles’ temperature
within the air rises, it could be an indication of the presence of an intruder, so an alarm is sounded.
C is incorrect because wave-pattern motion detectors generate a wave pattern that is one of several frequencies: microwave, ultrasonic, or low frequency. The
IDS generates a wave pattern that is sent over a sensitive area and reflected back to a receiver. If the pattern is returned undisturbed, the device does nothing. If
the pattern returns altered because something in the room is moving, an alarm sounds.
D is incorrect because a photoelectric system (or photometric system) detects the change in a light beam and thus can be used only in windowless rooms.
These systems work like photoelectric smoke detectors, which emit a beam that hits the receiver. If this beam of light is interrupted, an alarm sounds. The beams
emitted can be cross-sectional and can be invisible or visible beams. Cross-sectional means that one area can have several different light beams extending across
it, which is usually carried out by using hidden mirrors to bounce the beam from one place to another until it hits the light receiver.