An EMF field is a component of the structure of a device or object which is designed to provide some shielding from electromagnetic fields. Such fields can interfere or severely disrupt the normal operation of a device, making shielding very important, especially with delicate equipment like sensitive scientific instruments. Most electronic devices are required by law to include EMF shielding, and citizens of some nations may note that such devices are often stamped with information indicating that they comply with laws requiring EMF shielding of electronics.
Typically, an EMF shield is designed both to prevent electromagnetic fields from interrupting a device, and to limit the electromagnetic field generated by a device so that it cannot interfere with other devices. While such shielding cannot be perfect, it can be very high quality, making it difficult for EMF disruptions to occur.
A simple example of an EMF shield is a component in an electrical cord which covers the conductive area of the cord and lies underneath the insulation, designed to prevent the field generated by the passage of electricity through the cord from spreading. EMF shields are also used in the casing and housing of electrical devices in the same way. These fields can protect from electromagnetic radiation, but they will not necessarily block radio waves.
Even a good EMF shield, such fields can still cause disruptions. For example, leaving a cell phone next to a computer monitor can distort the display, even though both devices are theoretically shielded. Concerns about electromagnetic fields have also led to requirements to turn off devices such as cell phones and computers in certain environments. In a hospital, for example, cell phones are not allowed in some areas because they could interrupt medical equipment, and on aircraft, the use of electronic devices is restricted due to concerns that they could interfere with the operation of the aircraft.
A more sophisticated variation on the basic EMF shield known as a Faraday shield or Faraday cage provides additional protection, and may be used around sensitive equipment or when very high volumes of electricity are being used. Whatever the type, an EMF field must be regularly evaluated and tested to confirm that it is working properly, as degradation of the shield can cause it to malfunction. Consumers may also have noticed that older devices can have imperfect shielding, caused by different manufacturing standards or natural wear and tear which damaged the EMF shield.