Windings are held in slots in the rotor or armature covered by stator magnets. The exciter has stationary field coils and a rotating armature (power coils). The field coils are connected in series or parallel with the armature winding. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. As soon as the clutch armature and rotor are engaged, lock up is 100%. In these systems, the projectile is accelerated by a moving coil "armature". A brushed motor can have its speed controlled by varying the voltage on its armature. The platter functions as a motor armature. The revolving part of alternators can be the armature or the magnetic field. Sutton designed an electric continuous current dynamo with a practical ring armature. An induction machine requires an externally-supplied armature current. As the motor accelerates, the armature (and hence field) current reduces. This causes a demagnetizing effect due to armature reaction. The entire puddled iron armature designed by Gustave Eiffel was replaced. Puppets, models or clay figures built around an armature are used in model animation.