Michael Faraday’s studies and experiments regarding charge, magnetism found that charge on a conductor resided only on the outer surface. He also discovered that nothing inside that conductor was affected by any change in electrical charge on the outside. Faraday believed that an electric field extended into space beyond a charge. We now know that the electrostatic repulsion of like charges causes a redistribution of charge to the outside of a conductor resulting in a net electrostatic field within the conductor of zero.
This occurrence produces the result that any and all noise with an electronic component that exists outside the cage is completely cancelled within that space being the same mechanism we electrochemists use to justify disregarding electrostatic fields in highly conductive electrolyte solutions. Also, any noise created inside the cage is prevented from escaping to the outside world. This is what keeps us safe near a microwave oven.
There are 3 things to keep in mind:
1. Breaks in the cage cause gaps that allow for penetration by outside electromagnetic (EM) fields. For a hole drilled into a solid box, the penetration of EM radiation is limited to oscillations that have wavelengths shorter than twice the diameter of the opening. So a 1 cm opening allows 2 cm and shorter wavelengths, which correspond to 150+ GHz noise.
2. A long or complete break in continuity of the conductive material. Access to a Faraday Cage via lid or door creates the real possibility for such a break in continuity. If one side is discontinuous, even if it is conducting, then charge may not redistribute properly. This cancelling effect will not exist, and a non-zero field will exist within the cage. A wire to connect discontinuous edges can help with low-frequency work, but is often insufficient for experiments that have higher operating frequencies and speeds.
3. The conductivity of the cage. This is seldom much of an issue, but as the size of the cage increases it can become a larger concern. The more resistive the conducting layer is the slower charge redistributes, resulting in a non-cancelling field.
When You Should Use a Faraday Cage
Use a Faraday cage whenever possible because it will always reduce noise, particularly power-line (mains) which every laboratory in every building in every country with an AC power-grid has. Some experiments see a larger effect than others and those are experiments that deal with low currents or high frequencies, and experiments where very accurate measurements are required.
Just about anyone doing physical electrochemistry (cyclic voltammetry, pulse voltammetry, chronoamp, etc.) falls into the latter category. Corrosion may not often require as much precision and accuracy, but corrosion-resistant alloys easily can lead to measured currents in (and below) the nA range, where a Faraday cage is definitely needed. Electrochemical Impedance Spectroscopy involves higher frequencies so any reduction in noise is welcome. In short, use a Faraday cage whenever it is physically possible to do so.
For more information on the faraday cage, visit Gamry Instruments Application Note.
Gamry Instruments also sells the Vista Shield/Faraday Cage on the online store.