Definitions


                 Cosmic Rays
                 DC Field, Current or Voltage
                 Electric Field Strength
                 Electromagnetic Field   
                 Electromagnetic Spectrum
                 EMF
                 Epidemiology
                 Free Radical
                 Frequency
                 Gamma Rays
                 Ion
                 Ionisation
                 Ionising Radiation

                 Light      
                 Magnetic Flux Density
                 Microwave Radiation
                 Non-Ionising Radiation
                 Non-Thermal Effects
                 Power Density
                 Radiation
                 Radio-Frequency Radiation (RF)
                 Resonant Effects
                 Specific Absorption Rate (SAR)
                 Thermal Effects
                 Wavelength






Cosmic Rays

High-energy ionising radiation from Outer Space.


DC Field, Current or Voltage

DC historically meant Direct Current, that is a current that has a value that always remains either positive or negative, like that produced by a battery.  Likewise a DC voltage is one that always remains either positive or negative.  A magnetic or electric field produced by such a current or voltage then possesses a field that does not vary in direction.  The field can then also be called Static.

For example: The Earth's magnetic field is called a DC field as the magnetic north pole attracts the north point of a compass.  Lightning is a Static electrical field that builds up within clouds.  Static electricity is also that electric shock you receive as you try to get into or out of a car during very dry weather.


Electric Field Strength

The intensity of an electric field measured in units of volts per metre (Vm-1 or V/m).  Higher values are sometimes quoted as kilovolts per metre (kVm-1 or kV/m).

1 kilovolt per metre = 1000 volts per metre


Electromagnetic Field

A location where the electromagnetic energy from a source exerts an influence on an object.  Examples are; High frequency fields are used for radio communication, Radar, etc.  Low frequency magnetic fields force round the rotating part of electric motors.


Electromagnetic Spectrum

A description or diagram of all electromagnetic frequencies displayed in order of increasing or decreasing frequency.


EMF

This means Electric and Magnetic Fields, or, Electromagnetic fields.  The difference in the use of these two terms usually depends upon the frequency involved.  At low frequencies it is necessary to consider the electric and magnetic fields separately.  At high frequencies the two fields are linked together and the term electromagnetic or EM field is commonly used.  However, even at high frequencies, the electric and magnetic fields are not linked at very small distances from the transmitting antenna and need to be measured separately.


Epidemiology

The statistical study of disease within large groups of people or populations


Free Radical

A group of atoms that normally cluster with other atoms.  They can exist on their own, but usually only for a fraction of a second.  They are thought capable of damaging other cells.


Frequency

The number of complete cycles of an electromagnetic wave in one second.  The unit is the Hertz, symbol Hz. Frequency used to be simply called cycles per second.  High frequencies are also usually given a prefix, ie:

                     1000Hz = 1KiloHertz (1KHz)
                     1000000Hz = 1MegaHertz (1MHz)
                     1000000000Hz = 1GigaHertz (1GHz)
                     1000000000000Hz = 1TeraHertz (1THz)


Gamma Rays

High-energy ionising radiation emitted by radioactive material.


Ion

Electrically charged atom or group of atoms.


Ionisation

The production of ions when an atom or group of atoms acquires or loses an electric charge.  Making them positive or negative ions.  High voltage conductors produce ionisation of the air surrounding the conductor and produce ions.  There is some speculation that this mechanism could be capable of transferring toxic elements.


Ionising Radiation

A type of electromagnetic radiation that causes the production of ions in matter.  This type of radiation has sufficient energy to damage DNA and is present in high-energy ultraviolet light, X-rays, Gamma rays and Cosmic rays.


Light

The electromagnetic radiation that our eyes are designed to detect at frequencies between those of infrared and ultraviolet.  Between approximately 385 and 750 TeraHertz, ie at frequencies between approximately 385,000,000,000,000Hz and 750,000,000,000,000Hz. These frequencies have wavelengths of approximately 0.0077millimetre and 0.00390millimetre respectively.


Magnetic Flux Density

A measure of the magnetic field strength over an area.  The unit of measurement used to be the Weber per square metre (Wb/m2).  This is now known as a tesla, symbol T.  America uses the unit Gauss.  The relationship between these units is: 

1 Weber per square metre = 1 Tesla  = 10000 Gauss

For environmental measurements of magnetic flux density it is usual to use units of microtesla (
m T).  That is a unit that is 1/1000000 of a Tesla.  There is seldom use of the nanotesla, which is 1/1000 of a microtesla.


Microwave Radiation

The name given to non-ionising electromagnetic radiation within the Radio Frequency part of the electromagnetic spectrum.  In the frequency range, 300MHz to 300GigaHertz.  Between 300,000,000Hz and 300,000,000,000Hz.  These frequencies have wavelengths of 1metre and 1millimetre respectively.


Non-Ionising Radiation

A type of electromagnetic radiation that does not causes the production of ions in matter.  This will have a frequency that is lower than that of the high-energy ultraviolet radiation.  A frequency that is less than approximately 3000THz and a wavelength greater than 0.0001millimetre.  When this type of radiation passes through our bodies it does not damage our DNA unless there is enough energy to cause a thermal (heating) effect.


Non-Thermal Effects

Sometimes also called Athermal effects.  These are biological effects that a small number of reports claim to be caused by electric, magnetic or electromagnetic fields that are very much smaller than those set for guidelines to avoid harmful thermal effects.


Power Density

The unit of power measured over one unit of cross sectional area.  The unit is in watts per square metre, symbol Wm-2 or W/m2.  Small levels of power density are sometimes quoted as watts per square centimetre (Wcm-2 or W/cm2) or microwatts per square metre (mWm-2or  mW/m2).

1 watt per square metre = 1000000 microwatts per square metre.


Radiation

Radiation simply means the process of emitting energy in the form of waves or particles.  For clarity it should be identified by its frequency to be either ionising or non-ionising.  A hot water radiator emits electromagnetic radiation in the infrared part of the electromagnetic spectrum and we detect this as heat, sometimes called black heat for it is at a frequency just below visible light.


Radio-Frequency Radiation (RF)

Electromagnetic radiation at frequencies used for Radio, TV, mobile phones and similar devices.  Also used in industry to weld plastics, dry materials, etc.  This term is usually applied to frequencies between 300Hz and 300GHz and includes Microwave frequencies.


Resonant Effects

These occur under certain conditions and are dependent upon the size of the object or person.  The effect of resonance increases the transfer of energy from a radio frequency radiation into the body.  For an adult stood on an earthed surface, the resonant frequency effects would be greatest at approximately 40MHz.  This does depend on how tall the person is.  For a rat the resonant frequency would be about 650MHz. For a mouse the resonant frequency would be approximately that used in a Microwave Oven, 2.45GHz.


Specific Absorption Rate (SAR)

The rate at which energy is absorbed by a unit mass of tissue, due to the presence of an electromagnetic field.  The unit is in watts per kilogram, symbol W kg-1.


Thermal Effects

These are known biological effects that occur as our body temperatures rise.  The cause of the temperature rise that is of interest here is that due to Radio Frequency or Microwave electromagnetic radiations.  Other causes of temperature rise ie, exercise, hot weather, etc, are taken into account when setting thermal guidelines.  It is considered that we can tolerate a bodily increase of 1 degree Celsius.  The guidelines are set to protect us from body temperature increases that are only a fraction of this level.  Very high body temperatures produce 'heat strokes' that are life threatening.


Wavelength

The distance between successive crests of a cyclic waveform.  The symbol Lambda (l) is commonly used and it is measured in metres (m).



Home

© 2005 Electromagnetic Surveys Limited