It is 1 second for the second hand to walk a small grid. From 3 to 6, there are 3 large grids, each with 5 small grids, and a * * * with 5×3= 15 small grids, that is, 15 seconds.
Time unit second is the basic unit of time in the international system of units, with the symbol S, sometimes marked with the English abbreviation sec.
Definition of second: When cesium-133 atom is in undisturbed ground state at sea level, the radiation frequency Δ ν Cs corresponding to two ultra-fine energy level transitions is expressed in Hz (that is, it is equal to s- 1), and a fixed value of 9 19263 1770 times is taken to define second. According to the new definition, a day will reach 24 hours, 56 minutes and 24 seconds.
Seconds is the basic unit of time in the international system of units.
The prefix of the international system of units is often combined with seconds to make a finer division, such as ms (millisecond, thousandth of a second), μs (microsecond, millionth of a second) and ns (nanosecond, billionth of a second). Although the international unit prefix system can also be used to extend time, such as Ks (thousands of seconds), Ms (millions of seconds) and Gs (billions of seconds), it is rarely used in this way, and people are still used to using 60-decimal minutes, hours and 24-decimal days as the extension of seconds.
The origin of the second hand:
"Seconds" was originally a unit of length, and began to become a unit of time after Matteo Ricci's time reform in the late Ming Dynasty. The time scale of "second" was really used for measurement in the middle of Qing Dynasty after the three-pointer with the second hand entered China. However, for a long time, China people only regarded the three-watch as a high-grade ornament. Therefore, China people's time concept of "second" has not been awakened by the popularity of three-needle watches.
With the overwhelming influx of western scientific knowledge and cultural ideas into China in the late Qing Dynasty, the more accurate "time" symbolized by the "second hand" finally began to be linked with science and civilization like the steam engine and electricity.