Use a Multimeter to Detect Electromagnetic Fields
Click graphic to visit Wikipedia entry: Electromagnetic Radiation
Electromagnetic (EM) radiation is produced by energy, including life, and is classified and categorized by the number of peaks in its waveform per second (frequency in Hertz, or Hz.
The shape and range of the EM radiation from its source comprises a 'field' of effect which is strongest near its source and weakens with distance. We can see this in the way light radiation is emitted from a bulb.
Our world has become a huge, interrelated tangle of electromagnetic fields oscillating (pulsing) at rates ranging from 50/60 Hz (which we can sometimes hear as a low buzzing or 'hum' in audio equipment) running through wiring in homes and cabling along streets, through the hundreds, thousands and millions of kilohertz produced by all kinds of radio waves.
It has been claimed that strong EM fields affect and interact with the weak fields produced by life processes in our bodies, causing long-term maladies like headaches and sinus troubles. These sources recommend using EM detection to locate and avoid placing cribs, beds, couches, easy chairs, etc. within powerful EM fields radiating from electric and electronic devices and sometimes even from faulty wiring.
Purpose-built gear exists (discussed further below) for EM field detection and precision measurement. However, most multimeters are well able to detect EM fields also, even allowing EM field size and shape to be perceived, as well as relative field strength.
That said, I would be remiss if I did not mention that EM field detectors are often employed by investigators of paranormal activity. I can only presume it is accepted that the phenomena under investigation generate EM and perhaps other fields. Presuming one is so inclined, this device may also be viewed as a rudimentary 'paranormal detector' of sorts.
Below, I have selected some of the more popular purpose-built devices for EM field detection sold at Amazon.com. The prices of some of these products are prohibitive, especially if the intended use is likely to be limited to occasional, curious poking-around, whether in search of ghosts or the best spot for a bed.
How to Get It Goin' Oan
AKA All That and a Bag for EM Sickness
Of course, you'll need a DMM (digital multimeter). I haven't tried using an analog multimeter (displays measurements using a needle over printed scales), but feel free to try to report your results.
Practically any DMM should work, since we'll be reading AC millivolts, a range even the lowest-cost meters ought to be able to measure. For detecting weaker fields, though, the meter should be able to measure down to 1 mV. But again, if your meter is limited somehow, give it a try anyway and let me know how it works for you.
You'll also need a length of wire stiff enough to stay vertical on its own, about the same length as the DMM, with an inch or two of insulation stripped from one end and folded over so it fits into and makes contact with the positive (red) socket of the DMM.
I use a retractable antenna taken from a very old Nokia CDMA cell phone. It's fairly short (4 inches or 10 centimeters) with a threaded metal base. The base is a loose press-fit into the banana-plug DMM socket and works well.
Purpose-Built EM Field Detection Gear - Everything You Wanted To Know About Finding EM Fields, But Didn't
Indeed, one can purchase EMF detection items which are purpose-built and more than capable of alerting their users to EM fields of divers type.
l shall now attempt to direct your attention to some of those products with the warning that, should you be eager to indulge your Cat's Man Ray, a hefty dent in your chequebooque is the likely result.
So What Can I Do About These EM Fields I've Found?
Experimentation is the source of most discovery, so don't be afraid to follow ideas.
Try using an 'antenna' in both positive and negative sockets extending away from each other.
Try using a longer 'antenna' in the positive DMM socket, then making a coil of several turns at the end, varying the size.
Anyone who has made an antenna can vouch for the fact that a wire coil of a certain number of turns and of a certain length and width will 'resonate' at a certain frequency, making EM fields at that frequency MUCH easier to detect at a distance.