Editing Rotary dial (section)

===Principal dial mechanisms in the United States===
In the United States, two principal dial mechanisms arose in the engineering laboratories of the largest manufacturers, that of the [[Western Electric Company]] for the Bell System, and that of the [[Automatic Electric Company]].

The Western Electric dial had [[spur gears]] to power the governor, so the axis of the governor was parallel to the dial shaft. The Automatic Electric governor shaft was parallel to the plane of the dial at a right angle to the dial shaft. The governor shaft had [[worm gear|worm gearing]] in which, very atypically, the gear drove the worm. The worm, highly polished, had extreme pitch, with teeth at about 45° to its axis. This was the same as the gearing for the speed-limiting fan in traditional music boxes. The Western Electric governor was a cup surrounding spring-loaded pivoted weights with friction pads. The Automatic Electric governor had weights on the middle of curved springs made from strip stock. When it sped up after the dial was released, the weights moved outward, pulling the ends of their springs together. Springs were fixed to a collar on the shaft at one end and to the hub of a sliding brake disc at the other end. At speed, the brake disc contacted a friction pad. This governor was similar to that in spring-driven windup phonograph turntables of the early 20th century.

Both types had [[Clutch#Other designs|wrap-spring clutches]] for driving their governors. When winding the dial-return spring, these clutches disconnected to let the dial turn quickly. When the dial was released, the clutch spring wrapped tightly to drive the governor.

While winding the dial, a spring-centered pawl in the Western Electric dial wiggled off-center when driven by the cam on the dial shaft. Teeth on that cam were spaced apart by the same angle as dial hole spacing. During winding, the pawl moved off-center away from the normally-closed pulsing contacts. When the dial was released, the cam teeth moved the pawl the other way to open and release the dial contacts.  In the Automatic Electric dial, the pulsing cam and governor were driven by a wrap-spring clutch as the dial returned. When winding, that clutch disconnected both cam and governor.