Amplitude is how far a watch’s balance wheel turns on each swing, given as an angle in degrees. It matters because it affects the rate. When amplitude is low, the watch tends to run fast. A low reading usually means the watch needs a service.
How is amplitude measured?
Watchmakers read amplitude with a timing machine. The same machine also measures other running values, so one test shows several things about how the movement is behaving. Amplitude is taken per swing. Each single swing of the balance, clockwise or anticlockwise, is called a beat. The figure describes the angle the balance wheel covers between the two ends of its rotation, given in degrees. A larger number means a wider swing.
Why does amplitude rise and fall?
Amplitude is not fixed. It is higher when the mainspring is fully wound, because the spring then sends more energy to the balance. When the swings get shorter, they finish sooner. That is why a watch with low amplitude tends to gain time. A low reading on the timing machine usually points to a service. Cleaning, fresh oil and a new mainspring often raise amplitude by about 40 to 80 degrees.
Not to be confused with
Amplitude is one figure among several. Beat error and rate are separate measurements, and both also affect how accurate a watch is. Amplitude does influence rate, since low amplitude tends to make a watch run fast. Even so, the two are not the same thing. Beat error is a different measurement again. When looking at the results from a timing machine, it helps to read each value on its own terms.
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