A/h or ALTERATIONS (or ALTERNANCE) PER HOUR
In a watch movement with a balance wheel, one vibration is a single swing of the balance wheel. A vibration is one-half of an oscillation or a semi-oscillation. So, one swing of the balance wheel in either direction, for example clockwise, is one vibration or semi-oscillation, and two swings, for example clockwise then counter-clockwise, is an oscillation. The frequency of a watch movement can be expressed in Hertz (Hz) or in vibrations per hour (vph), which is also sometimes referred to as A/h. The frequency in Hertz is the number of oscillations per second.
Double the hertz figure to get the number of vibrations or semi-oscillations per second. Multiply that number by 360 (the number of seconds in an hour) to get the vph. The most common frequency for modern mechanical wristwatch movements is 4 Hz, or 28,800 vph. A watch ticking at 4Hz makes 4 oscillations per second or 8 semi-oscillations (or vibrations) per second. There are 60 seconds in a minute, so this watch would tick at 480 semi-oscillations (or vibrations) per minute.
Multiplying the 480 vibrations per minute by the 60 minutes in an hour yields the 28,800 vph figure. Since this watch ticks 8 times per second, a chronograph in a 28,800 vph watch can time events to the nearest 1/8 of a second. Following this formula, a watch with a rate of 2.5 Hz or 18,000 vph can time events to the nearest 1/5 of a second. A rate of 3 Hz or 21,600 vph yields an accuracy of 1/6 of a second. A watch with a rate of 36,000 vph or 5 Hz can time to the nearest 1/10 of a second.