EMF meters are used by Ghosthunters, EMF is an Electro Motive Force and is in reality the force that causes electrons to flow through an electrical circuit and thus produces an electrical current.
EMF is also an abbreviation for electromagnetic field. This is a wave, with both electrical and magnetic components, operating 180° out of phase, usually generated by electrical activity such as in the oscillator section of radio transmitters, alternating current through a conductor, movement of neurotransmitters within brains, and ostensibly, ghosts and such.
Investigators today use EMF meters to locate unusual or unexpected electromagnetic fields at the investigation site so that they may be a little closer in determining a probable cause for some of the activity being reported.
Here is a great web link for more EMF information and for products to help lower EMF in your home. http://www.safespaceprotection.com/
Some Ghost Hunters believe that Ghosts emit EMF's or distort the Earth's natural electromagnetism. Ghosts use the electricity supply to obtain energy that allows them to manifest, this cause the EMF to change and this can be measured. There are many variations on these themes and many books have earnestly declared that using an EMF meter will allow the investigator to detect the energy or presence of a ghost or spirit.
Research has been done that suggest that when some people are exposed to EMF near their head and brain, they may report sensations they believe are paranormal in origin. These effects include a sense of presence, a sense of touch, anxiety, unease, fear, nausea and even an 'Out of Body' sensation.
EMF's are actually made up from two parts, a static electric field that is always present whenever any electrical charge is present or when an electric cable or appliance is connected to the supply even if it is turned off, and a magnetic field that is created when the electricity is flowing such as in a lightning bolt or when an electrical appliance is turned on. The two components are the Electrostatic component and the Magnetic component make up the Electromagnetic field that is found around every electrical appliance or cable and is universally present within the Earth itself.
Natural sources of electromagnetism are often referred to as Extremely Low Frequency EMF's. These 'Geomagnetic Fields' and are normally only measured by expensive and specialist equipment that are rarely to be found in a paranormal investigators kit. Geomagnetic fields are typically very low in frequency 0 - 10Hz.. In certain situations these low frequency Geomagnetic fields may impinge themselves upon and directly affect the man made electricity supply generated EMF.
Higher frequency sources typically those above 1kHz (1,000Hz) are to found in fluorescent lights and visual displays. These are capable of being measured by many readily available EMF meters although the actual value will normally be very different from the displayed value as the meter is calibrated or weighted for the much lower electricity supply frequency of 50 or 60Hz.
Most paranormal investigators use an AC EMF meter of a type that is designed to measure frequencies around 50Hz or 60 Hz, which are the domestic electricity supply frequencies for the USA. Typically they will also measure EMF at frequencies above and below this calibration point although often with a greatly a reduced accuracy and sensitivity as the frequency moves further from the 50/60Hz optimum.
Most EMF meters are designed for measuring EMF's caused by the domestic electricity supply. This is either 50Hz in the UK or 60Hz in the USA. That means that the meter is designed to be most accurate when measuring EMF's within that range of frequencies. Just about every building has an electricity supply or is located close to the electricity supply network of over ground and underground cables.
Electrical appliances generate EMF and depending upon a number of factors such as the distance from the appliance or the wiring, the power being consumed by the appliance and the type of appliance you may expect to see EMF readings from 0.1 milli Gauss (mG) to more than 100mG. For example a TV set on standby may give a reading of less than 1mG at 30cm but over 50mG when it is turned on. A bedside clock radio can give you a surprisingly high reading of around 100mG at 20-30cm and it may be wise to keep it several feet away from your head.
Many types of meter are described as 'Frequency Weighted' or Frequency Calibrated. These do not give a simple measure of the field strength but instead their measurements are proportional to the frequency of the field. Instead of the actual strength of the EMF being shown the meter actually indicates the amount of energy the field is carrying - as the frequency of the EM emission increases so does the energy it carries.
The Alpha Labs TriField is just such a meter. An actual EMF of 3mG at a frequency of 60Hz will be shown as 3mG which is the true value. However if the EMF frequency is then increased to 120Hz the same strength field will now be shown as 6mG on the scale. Much of the existing research into EMF's and reported paranormal phenomena is concentrated on it's effects on the human brain and body so a measurement indicating the 'power' from the EMF is probably more useful to the investigator than would a measurement of the actual field strength.