Many people must have experienced the embarrassment of their mobile phone ringing in the least appropriate moment. It can't easily be prevented unless you are in the possession of so called Faraday cage.
Faraday cage is a kind of enclosure with is a screen made of metal gauze. It is used to shield the inside of the enclosure from the outside electric fields. The strength of the electric field inside the Faraday cage is zero. A sieve covered with aluminium foil could be such a cage.
All this can be proved. Place an electroscope inside the cage. The one from the next experiment could be used. Even if we strongly electrify the cage using, for example, the piezo-electric switch or the electrostatic generator (we can do it by connecting another electroscope to the outer surface of the wire gauze), the lobes of the inside electroscope will not move.
Similarly, the mobile phone placed inside the cage doesn't work./p>
Shielding properties of a metallic, closed box result from so-called Gauss's law. It says that the flux of the electrical field E across a closed surface S determined by the sum of the electrical charges inside q:
&Fi; = ∫E·ds = 4 π/ε0 Σqin
where ε0 is the dielectric permeability of vacuum.
In other words, if inside a closed Gauss surface no electrical charges are present, the net electrical field flux across this surface is zero.
Let us invert the reasoning: the electrical charges outside the closed surface create a zero flux across this surface. As a result, the fields which have sources outside the surface can not induce charges inside Faraday's cage - the radio receiver or a cell phone are mute inside the box.
In real life, alternate electric fields, in particular at high frequencies, can partially penetrate inside non-perfectly closed surfaces. So, it can happen that the cell phone inside a metal sieve would ring, but in a perfect metal box - it should not.
