|Waltham 1908 model Balance|
Horologists please excuse some simplification and omissions here! Likewise I hope the non-specialist will not be too phased by a little bit of jargon.
What is the impulse jewel?
The impulse jewel is part of the balance wheel assembly, it is a small, often "D" shaped pin like jewel and is mounted on the "roller table".
The picture above right shows the balance assembly from an unrestored Waltham-1908 movement, the roller is the small disk in the centre of the picture with the balance staff (axel) going through its centre and the impulse jewel on the right - it is easier to see if you click on the image for a larger view.
By some clever engineering the impulse jewel also gives the balance a "kick" to keep it moving, it does this twice with every swing on a Swiss Lever escapement with a club tooth escape wheel, once on the unlock and once on the lock) and once every swing with a traditional English Lever escapement with a ratchet type escape wheel.
On an average size 16 or so pocket watch the roller is about 3.5mm / 0.14" in diameter so you can see that the impulse jewel it pretty small. And sometimes they break.