I was shuffling through the previous years question papers & I got this question !! '' A sensitive instrument is influenced by the strong electric field. Write a possible way to prevent this effect. Why is the electric field normal to the surface? '' I am not sure what the answer it. But this is what i figured out. " It is obvious that the instrument is a conductor. Therefore it has got excess of free electrons. These free electrons will arrange them in such a way that develops an inner electric field which will cancel the electric field of the outside. There will be accumulation of negative charges on one side & positive on the other. Hence the effect of the outside electric field is neutralized. Electric field lines are always perpendicular to the conducting surface; because it they form any other angle with respect to the surface they would cancel each other out because of symmetry. "