Stopwatch can be divided into single hand and double hand. The single hand stopwatch can only measure the time period of a process, while the double hand stopwatch can measure the time of two processes that start and end at the same time. The stopwatch shown in Figure 2-9 is a single hand stopwatch. Stopwatch is composed of mechanical oscillation system with low frequency, anchor escapement governor, control mechanism (including button), spring, gear and other mechanical parts for starting, braking and returning to zero of the second hand.

If the pointer in the small disk on the stopwatch rotates, it means that the measuring time has exceeded half a minute. When reading, first read the time in the small disk, and then add the time indicated by the long pointer in the large disk. The sum of the two is the stopwatch reading. If the pointer in the small disc does not rotate, it means that the measurement time is less than half a minute, then the time of directly reading the long pointer is the reading of the stopwatch.

Source: @lucasjtattoo