Magnetic shields for cathode ray tubes (CRTs) made of mu metal for use in oscilloscopes, from an ad by James Millen Mfg. Co. in a 1945 electronics magazine. The electron beams used in cathode ray tubes to draw on the tube's screen could be deflected by magnetic fields from transformers and other electronic components in the oscilloscope's insides. So the sides of the long CRT tube were surrounded by a shield made of mu metal. Mu metal is a high permeability nickel-iron alloy which could shield against magnetic fields by conducting the magnetic field lines around the shielded space.