What is EMF?
Electromagnetic fields (EMF) are produced by anything that is passing electricity, and is present all around us in our day-to-day lives: from Wifi routers to poorly-insulated wiring, to our own phones and computers. Even our own bodies emit EMF from the electrical impulses in our brains, granted it is too low to measure on any ghost hunting device. It is believed among most paranormal investigators that spirits maintain a level of EMF which can be detected by us, via EMF meters.
EMF Meters
Meters used to measure EMF are often used by electricians to look for electrical issues, but ghost hunters adopted the use of these meters when it was theorized that spirits not only emit their own EMF, but can also interact with it, alerting us to their presence. The most common EMF meter used by ghost hunters today is the K-II EMF meter, which features a row of different colored lights that tell you the strength of EMF that may be present. This is important because studies have shown that certain levels of EMF can have an effect on our brain, causing haunting-like symptoms such as paranoia/that feeling of being watched, headaches, and more. This makes the K-II Meter great not only for debunking high EMF as a source of a "haunting", but it can also be used to ask spirits to interact with and trigger the lights on the device. You can do this by looking for intelligent responses by repeatedly getting the same answers to questions, or look for other signs of intelligence. For example, you might ask a spirit to light up a KII meter to the color yellow. If it does, I can ask for them to light it up to red and so forth, looking for precise responses to specific questions.
One theory as to how spirits can do this is based on the idea that our soul or spirit maintains some level of the EMF that our living brain already has. In spirit form, they could "feed" on other electronic sources, increasing the spirits EMF and their ability to interact with us. This could also explain batteries draining quickly, lights flickering, and other issues with electronics while ghost hunting.
Be mindful when using an EMF meter that it is nearly impossible to rule out any/all naturally occurring EMF. (As in EMF being emitted by non-spirits.) Our cell phones are always emitting EMF, so it is crucial that all people present on a ghost hunt put their phones on airplane mode or turn them off completely. In cities, it is difficult to rule out other stray radio-waves or interference. If there is a storm nearby, lightening can also trigger some devices, including an EMF meter.
REM Pods
REM pods work by emitting their own electromagnetic field around the device via an antenna, and if anything disrupts this field they will light up and make noise to alert you that something might be present. They even come with different lights and pitches that will sound to tell you how close something was to the device. If something is farther away, the sound will be a lower pitch with only one or two lights going off; if it is closer, the pitch goes up and more lights will turn on.
The "REM" stands for "radiating electromagneticity" while "pod" comes from the general round, pod-like shape of the device. Like most devices covered so far, a lot of things can trigger this device that are hard to completely rule out. Two-way radios, which are used by many larger ghost hunting groups, can trigger a REM pod that is in the path of the radio waves. Issues with the antenna can also effect their accuracy (I accidentally bent the antenna on my own REM pod and began having issues with constant false positives.) I have also found that when they are low on battery, they will sometimes make sounds periodically until new batteries are put in.
It is said, however, that some REM pods on the market are built using Theremin instrument kits which are designed to be used by children who want to learn basic electronic and soldering skills, such as the MadLab Junior Theremin kit which will be slightly altered and fitted into a hard-plastid pod/case. This isn't necessarily a bad thing, but it is something buyers should be aware of when investing in these devices, and how reliable and durable they might be.
Other interesting variations of these devices have been hitting the market in recent years, one such favorite being the BooBuddy from GhostStop. This device acts similar to a REM pod, in that it alerts you to movements and changes in the environment around it, including fluctuations in EMF. The perk to the design of this device is that no electronics can be seen, giving it a more inviting and approachable appearance. EDI (Environmental Detection Instrument) and Mel Meters also measure and detect a wide variety of things in the surrounding environment, including EMF.
Some of these items do come with hefty price tags, so keep in mind that they are not necessary to be a good ghost hunter. Basic EMF meters can suffice, and apps have been slowly popping up that can be downloaded on your own cell phone, turning it into your own EMF meter or REM pod. As with all apps, be wary of gimmicks and apps that are desgined purely for entertainment.