1. They are gradually replacing iron cored transformers in neon signs. 2. This resonant current directly drives a switching transistor through a ring core transformer . 3. During the 1950s he invented the insulating-core transformer , producing high-voltage direct current. 4. Tight coupling is when the coupling coefficient is around 1 as with conventional iron-core transformers . 5. It has a circular cross-section due to a poloidal graphite limiter and an iron core transformer . 6. Ferrite-core transformers are widely used in ( intermediate frequency ) ( IF ) stages in superheterodyne radio receivers. 7. In the closed-core transformer the iron core is a closed ring around which the two coils are wound. 8. This Joule heating reduces efficiency of iron-core transformers and electric motors and other devices that use changing magnetic fields. 9. The 3-core transformers are oil-cooled, to which the control unit with its 28 running steps is connected. 10. Early superheterodyne receivers used IFs as low as 20 kHz, often based on the self-resonance of iron-cored transformers .