• Science Of Gadgets


Updated: Mar 26

high voltage power lines

An electromagnetic field meter or EMF meter is a gadget that helps us detect electromagnetic fields. Electromagnetic fields are combinations of magnetic fields and electric fields. These invisible fields are surrounding us everywhere, and their sources can be both natural and man-made. The term electromagnetic field is often used very loosely, in reality, there are both static electric and magnetic fields, as well as dynamic electric and magnetic fields. In the first case, the electric or magnetic field is constant over time, in the second case the strength of the electric or magnetic field changes over time. Everyone is familiar with static electricity, the static electric field is generated by stationary charges. A static magnetic field is, for example, a magnetic field around a permanent magnet, similarly, a magnetic field around the Earth can be considered static. In the same way, when a charge moves at a constant speed, as in a direct current (DC) circuit, it generates a constant magnetic field around it. Since these fields are constant over time, we can say that they have a frequency of 0 Hz.

Static magnetic fields

However, when the charges do not move at the constant speed, but they accelerate, like in the alternating current (AC) network, operating at 50 Hz in Europe (60 Hz in the United States), the current constantly changes direction, moving back and forth 50 or 60 times a second, changing electric and magnetic fields are created. These accelerating charges cause disturbances in the electromagnetic field and generate electromagnetic waves. The changing electric field induces a magnetic field and changing magnetic field induces an electric field. Such a combination of changing electric and magnetic fields forms an electromagnetic wave, that propagates in space at the speed of light, approximately 300 000 km/s. The electric current in the alternating current network generates electromagnetic waves, that oscillate at the same frequency as the current in the network - 50 or 60 Hz, depending on the frequency of the network.

Electromagnetic wave

In an electromagnetic wave, the electric field, and the magnetic field oscillate perpendicular to each other and perpendicular to the direction of wave propagation. Electromagnetic waves also do not need any medium to propagate, they can propagate even in a vacuum.

There are many sources of electromagnetic radiation, which generate these invisible electromagnetic waves of different frequencies. Visible light makes up only a small fraction of the full scale of electromagnetic waves. Many household appliances such as vacuum cleaners, microwave ovens, cell phones, washing machines, etc. are sources of electromagnetic radiation. Man-made sources of electromagnetic waves are also high voltage power lines and antennas that transmit radio waves.

Electromagnetic field meters have a very wide selection. Depending on the price range, they are also able to measure electromagnetic radiation in different frequency ranges, some of them are also able to measure static magnetic fields, or designed to measure only magnetic fields in lower frequencies, etc. Typically, higher-priced devices are more accurate and designed to measure a specific frequency range or a specific field. Although they all carry the same name, you should first look at what exactly can be measured with this particular device.

The image below shows the electromagnetic field meter GQ EMF - 390 and its display with the measurement results.

For each device, it is always specified in which frequency range it can measure electromagnetic radiation. From 0 to 300 Hz are low frequencies, emitted by power lines and many home appliances, From 300 Hz to 100 kHz (1 kHz is 1000 Hz) are intermediate frequencies, and from 100 kHz to 300 GHz (1 GHz is 1 000 000 000 Hz) are radio frequencies. This particular device in the picture above can measure radio frequencies between 0,01 GHz and 10 GHz, and also electric and magnetic fields in lower frequencies. Radio frequencies up to 10 GHz are usually the upper limit, that most electromagnetic field meters can measure. This means, for example, that they are not able to measure the high-frequency electromagnetic radiation used in 5G networks, with a frequency between 29 and 39 GHz but can measure radio waves at medium frequencies in the 5G frequency bandwidth.

Electromagnetic waves always carry energy. You probably realized this, when you were warming up food in the microwave oven or sunbathing somewhere on the beach. The intensity of electromagnetic radiation is measured usually in watts per square meter (W/m2). It shows, how much electromagnetic energy is absorbed in a unit of time, per square meter. Our EMF meter in the picture displays 1.48 milliwatts per square meter (milliwatt is 1/1000 watts) in the RF (radio frequency) field, which means that in every second 0.00148 watts of energy is absorbed per square meter.

GQ EMF - 390 radio frequency field

The peak value in the upper right corner shows a maximum reading that has been measured, which is 22.02 mW/m2 (0.022 W/m2).

At the time I write this post, it's September 27, and it's 5 p.m. In my home country Estonia, which is located about 60 degrees north latitude, the weather is sunny and the intensity of electromagnetic radiation from the Sun is 191 W/m2.

Now we move down the frequency band and look at what electromagnetic field meters measure at low frequencies.

The sector in the display in which the EMF (electromagnetic field) is written, shows the strength of the magnetic field.

GQ EMF - 390 Magnetic field

Magnetic flux can be defined as how many of these invisible magnetic field lines pass through a given area. The stronger the magnetic field, the more of these magnetic field lines we have. As can be seen from the picture above, the magnetic flux through a surface also depends on the angle between the magnetic field lines and the surface. The magnetic flux through a surface is maximal when the magnetic field lines are perpendicular to the surface and zero when the magnetic field lines are parallel to the surface. This is important to know because many electromagnetic field meters do not have multiple sensors that measure the magnetic field and then the measuring device must be moved or rotated to get the maximal magnetic field reading.

The unit tesla (T) is the unit of magnetic field strength (B), which is simply magnetic flux density through a given area. The second unit that is used, is gauss (G) which equals 0,0001 teslas. From the display of the EMF meter in the previous picture, we can see that the strength of the magnetic field is 0.05 microteslas (1 microtesla is 1/1000000 teslas). The measurement results are usually given in uT (microtesla) or in mG (milligauss) because even 1 tesla is a very strong magnetic field. Near the high voltage transmission lines, magnetic field strength could be up to 80 mG and around some appliances, it can be up to 300 mG. Currently, the world's strongest magnetic field is 45.5 teslas, and it is produced by a partially superconducting electromagnet.

The third sector on our electromagnetic field meter display, where EF (electric field) is displayed, shows the strength of the electric field. The unit of electric field strength is volts per meter (V/m). Many household appliances generate low-frequency electric fields. Household plugs and sockets have an alternating voltage, that changes direction 50 or 60 times per second, this means that the electric field changes direction 100 or 120 times per second. Electrical wiring is producing electric fields constantly, even when any appliances are not used.

Let's take now a brief look at what voltage and electric field strength mean.

Every charge or charged body creates an electric field around it. The electric field can be compared in some way to the gravitational field, only if the gravitational field attracts bodies, then the electric field can be repulsive or attractive because unlike charges attract and like charges repel each other.

Electric field and gravitational field

If you raise a body of mass m in a gravitational field to some height h from the ground, the gravitational potential energy of that body increases. The body naturally wants to fall back to the ground, towards a lower potential. In the same way, the electrical potential energy of the charged particle increases, when it is moved in an electric field. Charged particle wants naturally to move towards a lower potential. The voltage (V) simply shows, how big is the electric potential energy per unit charge in some particular point in an electric field.

Electric and gravitational potential energy

The strength of the gravitational field depends on the mass. Similarly, the strength of the electric field depends on the charge. The more charge we have, the stronger the electric field around that charge. The strength of an electric field indicates how much force acts on a charge in that field, just as the strength of gravitational field indicates, how much force acts on a mass in that field. The top figure shows two plates, which are two meters apart. The voltage between the plates is 100 volts. This means that the electric field strength between the plates is 50 V/m.

Electromagnetic field meters are quite popular among researchers of paranormal phenomena and ghost hunters. These so-called Ghost meters generally measure magnetic field strength at very low frequencies, from 30 Hz up to 20 kHz. Since most of these Ghost meters are not quite high quality and also cheap in terms of price, no excellent measurement accuracy can be expected. However, they still give some idea of the strength of the electromagnetic fields around you and can be useful tools for a low price.


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