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BELL SYSTEM TECHNICAL JOURNAL 



entire polar areas. The damping winding consists of insulated closed 

 turns of heavy copper wire distributed over the pole faces in the slots 

 as shown in Fig. 3. It will be noted that this damping winding has 

 no effect upon the flux through the poles as long as the flux density 

 over the polar surface does not shift. In other words, the damping 

 winding permits the total flux of the motor to increase or decrease as 

 required by the regulating circuit but will oppose any tendency of the 

 flux to shift back and forth across the pole face. As will be explained 



SCANNING DISK 



Fig. 2 — Schematic diagram of control circuit. 



later on, this feature is essential in order to prevent hunting or insta- 

 bility of the image. 



The hydraulically damped coupling between the motor shaft and 

 the scanning disc is also essential in order to avoid hunting. It employs 

 flexible metal bellows filled with oil and connected by a small pipe 

 equipped with a needle valve for adjusting the amount of damping. 

 Figure 4 shows its construction. The scanning disc itself is centered 

 on a ball bearing which allows the disc to rotate with respect to the 

 shaft within approximately ± 5 degrees mechanical movement. 



