11. Electric motors, field coils , transformers and capacitors may also be assembled with ultrasonic welding. 12. Some older or simpler arrangements use a single field coil with a pole at each end. 13. Field coils were once widely used in loudspeakers before the general availability of lightweight permanent magnets.14. Unusually, both the field coils and the armature winding are carried on the fixed stator. 15. Design challenges include the toroidal and poloidal field coils , vacuum vessels and plasma-facing components. 16. Two antennae and two field coils are used, both arranged at right angles to each other. 17. In this instrument a stationary field coil produces a rotating magnetic field, just like a polyphase motor. 18. This flows through the field coils , creating a larger magnetic field which generates a larger armature current. 19. This increases the magnetic field around the field coils which induces a greater voltage in the armature coils. 20. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.
