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For the measurement of very small electrical signals (such as EEG or EMG) in environments with high levels of electrostatic noise it is sometimes desirable to make a whole room which is impervious to electrostatic fields. This can be achieved by including a metal mesh in the doors, walls, ceiling, floor and (if they must be present) the windows. All of the mesh must be connected together and connected to earth. Although this technique has been used widely in the past, modern equipment can be made with very high common mode rejection ratio (CMRR, e.g. 120 dB) which makes such arrangements unnecessary except in the noisiest environments such as next to an elevator shaft.
Unfortunately such cages have little effect on the penetration of magnetic fields.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details