So the math will look like: The rule of thumb for a Faraday cage is that the openings should be no larger than 1/10th of the wavelength, which in our case is 12.5 mm (approximately 1/2 inch).

However, a compass does work inside a Faraday cage because the cage does not block the earth's magnetic field. Constructed of a metal mesh, current is conducted on the cage itself. It was invented ...