Hertz is the unit that indicates the number of vibrations or cycles per hour (vph), often abbreviated to hertz (Hz) and named after the German physicist Heinrich Rudolf Hertz. The vibration frequency of watches is often indicated in vibrations (referred to as A/h), but also in Hertz.
A mechanical watch has a balance wheel that swings and one swing equals one vibration. One vibration is one-half of an oscillation. If the balance spring swings to one side, clockwise or counter-clockwise, then it is called a vibration. If the balance spring swings two times, clockwise then counter-clockwise, then it is called an oscillation.
Most wrist watches nowadays have a frequency of 2.5 Hz (21,600 vibrations per hour), 4Hz (28,800 vibrations) or 5Hz (36,000 vibrations per hour). The frequency in Hertz is the number of oscillations per second. So most mechanical wrist watches have 4 oscillations per second. And if you double the number of oscillations per second, you will get the number of semi-oscillations. When you multiply that number by 3600 (number of seconds in one hour) you will get the vph or A/h.