Measurements from inside conductors We have known for some time now that when electric field is applied across any conducting shell, then electric field inside it would be zero. It also has some fantastic applications such as electrostatic shielding.  
However, is it possible to know for sure that the field inside a conductor becomes zero? For example, if we place a transmitter inside a conducting shell to resolve whether the field inside shell is non-zero or otherwise, won't it disturb the field setup while transmitting the message?
In other words, is it possible to be sure about this effect for a uniform conducting shell, and similar closed-body structures? 
 A: I did my own research (more like web search and thinking) and thought that maybe the best possible way would be to place an electret inside the conductor and then apply an electric field. If there would be any electrical field going inside the conductor then the electret would get charged. Now if the conductor could be carefully broken down to take out the electret, we would know once and for all whether electric field even of negligible magnitude penetrates the conductor or not. 
A: Well, you can always imagine a means of transmitting information which is not electrical in nature (say, with sound waves from the measurement apparatus to the surface of the metal).
However, the problem would rather be finding a means of measuring the electric field without this assumption (so: no wire, to begin with...). For example : you could enter a faraday cage and wave a multimeter all around while being in a strong electric field, and check that the multimeter shows 0 consistently. However, you would implicitly assume that the wires of the multimeter are at a constant potential, which is what you want to check in the first place.
I guess you could imagine a system making use of the stark effect, for example, to measure locally the electric field. However in any appratus that I can think of, you'd be making more assumptions than you'd want...
Ultimately, the idea of free-moving charges (that one can be checked!) implies quite directly that $E=0$.
A: well, you could build a faraday cage and go inside it to look at your instruments, no need to transfer information outside - or you could place a recording device inside.
if it's a problem for you in your mind that there is a void inside the conductor, that problem would apply on smaller scale as well, your instrument for measuring would still take some space, so you might just as well scale it up in principle.
