Stress-relieved amorphous metal toroid-shaped magnetic core

In an electrodeless fluorescent lamp means comprising a core;

• a mass of gaseous medium linking said core and adapted to sustain an electric discharge due to an electric field induced therein by said core and to emit radiation at a first wavelength when sustaining said discharge;
• a substantially spherical, evacuable light transmissive envelope containing said mass;
• a luminous phosphor on the surface of said envelope, said phosphor being adapted to emit visible light when excited by said first wavelength radiation;
• and means for energizing said core with a radio frequency magnetic field whereby said electric field is induced in said mass, the improvement in which said core comprises a stress-relieved toroid formed from a spirally wound unannealed ribbon of magnetic amorphous metal alloy of the formula:
• (Fe.sub.x Ni.sub.100-x).sub.100-y R.sub.y wherein R is at least one glass former, x is an integer in the range of from 20 to 35 and y is an integer in the range of from 15 to 25, said toroid being stress-annealed as wound.