EMF Detectors are devices used to measure Electro-Magnetic Fields (EMF). Electromagnetic fields can be generated in either the AC or DC current range. Most EMF Detectors will detect AC currents which are generally produced by man-made and other electronic devices. Keep this in mind while conducting paranormal investigations. Your investigative gear and any electronics in the household or location all emit an electromagnetic field which may account for some of the measurements.
Currents produced in the DC range are generally produced by the Earth are more naturally occurring. DC electromagnetic fields are typically measured with a Gauss Detector. Many of the more expensive ghost detection units will focus primarily on DC currents and ignore AC altogether as more often than not AC currents have a man-made explanation.
A typical EMF device will consist of a probe that acts like an invisible flight light which measures the area in front of the device. There are two main types of EMF Detectors, single axis detectors and tri axis detectors. Single axis devices are often more common simply because they are cheaper. However they are often misused, as to get an accurate reading these single-axis EMF detectors must be turned and tilted on all three points to get a reliable measurement. Tri-axis devices already have this built-in and do not require the operator to tilt, turn, and gather measurements from three different points. These devices are usually much more fool-proof, reliable, and accurate, but also more expensive.
Because electromagnetic fields do occur both naturally and as a result of man-made electronics, it is important to get an idea of what and where the magnetic fields already exist at the location you are investigating. This means usually conducting readings with the EMF equipment once prior to the actual investigation to pinpoint explainable sources. Typically, EMF activity attributed to ghost activity or something paranormal and unexplainable will be brief, occur in an area where there is no explainable source. Electromagnetic fields don't just occur without an energy source behind them. If there is not a fixed man-made or natural device producing the field and it comes and goes, then this could be identified as possibly being associated with a spirit or some other unexplainable energy.
Ghost Hunting Store - EMF Detectors
TheGhostHunterStore.com carries a large selection of EMF equipment. Some of the cheapest, smallest, and most affordable detectors start around $10-$25, while some of the more professional devices can approach upwards of $200.