Front, Face, Back Case Mark Cuvette, Cuvette Mark,  Movement, Movement in Case, Stratton Letdown, Vibrator Closeup, Under Dial, Patent Mark, Vibrator bottom pivot 

American Watch Co. KW16 with Fogg's Patent Vibrating Hairspring Stud. The vibrating hairspring stud was fitted to a small number of  KW16 and KW20 watches. The combination of American Watch Co grade and the vibrating hairspring is quite scarce. Even the Appleton Tracy grade vibrator is hard to find. The upper pivot of the vibrator is jeweled, but the bottom pivot is not. The watch has a total of 20 active jewels. The serial number of this example is 125,466.

The idea behind the vibrating hairspring stud was to give isochronous performance to the hairspring and balance without the need for a Breguet overcoil. When these watches were made, the overcoil was just beginning to be understood and it required considerable skill to make one correctly. By providing a long arc for the attachment point, the form of the hairspring could be maintained during the entire arc of the balance and the balance would be immune to the amount of force driving it on each impulse. Once Eduoard Phillips' work was widely distributed, producing an overcoil became much less of a mystery and the need for devices such as the vibrating hairspring stud disappeared.

Several English makers also experimented with similar mechanisms. In most cases they used a long extension of the hairspring and pinned it to the base of the balance cock. Dent used this on quite a few watches and some chronometers.

Although the balance appears to be free sprung, it actually is not. The arc of the vibrator is controlled by a screw and a cam fitted to the balance cock. When the cam is turned the arc of the vibrator is increased and decreased. Changing the arc of the arm effectively changes the active length of the hairspring just as curb pins normally do.