The watch outer ring with the stopwatch function will be engraved with 500/400/350 and so on in order to reduce the number of these representatives of the physical speed, in kilometers / hour; this bezel called TACHYMETER (tachometer).
How to use Take a vehicle traveling on an expressway as an example, because the expressway usually has a distance indicator rod standing on it. The stopwatch is started when the starting point passes. The stopwatch starts counting at 12 o'clock. When it moves 1 km, it stops counting. At this moment The number referred to by the needle is the parity rate of the previous kilometer;
For example, at 12 o'clock there are 60, because a minute walk 1 km, then the speed is 60 km / h.
Bezel on the number corresponding to the location is how to figure out it is actually very simple. To know that 500 should be carved in that position? Then need to be a simple division; that is, the calculation of 500 km / h speed of 1 km travel time, in seconds.
1 hour = 3600 seconds, then 500 kilometers need 3600 seconds to complete, according to the principle of equality, the time required for 1 km is 3600/500 = 7.2 seconds, so 500 must be engraved in the dial at 7.2 seconds.
Similarly, 400 should be engraved in 3600/400 = 9 seconds, while 100 is engraved in 36 seconds.
When the speed is lower than 60 km / h, the tachometer can not be used directly, unless you increase the speed of the scale value