Amplitude is the angle covered by a pendulum or a balance during one vibration. It is a very important measure to know the good functioning of a watch. When the amplitude is too low it is often a sign that the oils are dry and the watch needs to be serviced. The amplitude is often one of the first things a watchmaker checks on a watch, much like a doctor checks the blood pressure on a patient. In a way, we can also say that the amplitude is the blood pressure of a watch.
On a timepiece equipped with a balance-spring the amplitude in vertical position must be about 3/4 of a turn, i.e. 270 degrees. When the same watch is placed in a horizontal position, this amplitude will be about 30 degrees greater because the balance wheel has less friction in the horizontal position than in the vertical position. In fact, in the vertical position, these two pivots are as if they were lying with friction on the two rubies, the one on the plate side and the one on the bridge side. When the watch is in horizontal position, there is only one friction on the tip of one of the two pivots, a bit like a dancer who turns on the tip of one foot. This creates a difference in amplitude of about 30 degrees.
Also, on almost all chronographs the engagement of the chronograph will create a drop in amplitude, again of about 30 degrees. Indeed, for a large part of the chronographs, the fact of engaging it is a bit like hooking a trailer to a car, this adds an additional gear and makes the amplitude drop. This phenomenon is normal. There is a chronograph movement where the opposite is true. This is the FP 1185 caliber where the movement is in constant friction when the chronograph is stopped and the movement is released when the chronograph is engaged, so it will have an increase in amplitude of about 30 degrees when engaged.
A watch can also have too much amplitude. That’s why as a general rule we try to make sure that it does not exceed 300 degrees. If it reaches 330 degrees a defect will appear which is called knock and will create strong disturbance in the precision of the watch.