Tachymeter scale
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For example, if one were driving on the highway and timed how long it took to travel 1 mile, the tachymeter scale allows one to determine at a glance the speed in miles per hour. The typical tachymeter scale on a watch converts between the number of seconds it takes for an event to happen and the number of that event that will occur in one hour. The formula used to create this type of tachymeter scale is:
,
where T is the time in seconds that it takes for the event to occur.
As a sample calculation, if it takes 45 seconds to travel 1 mile, then the speed is 80 mph. Note that the tachymeter scale only calculates the average speed.
For events that happen either very quickly or slowly, one needs to adjust the sixty-second tachymeter scale commonly found on watches. The scale on a watch is only valid for things that happen in 60 seconds or fewer, and the scale is also difficult to resolve for events that take fewer than 10 seconds or so to occur. As an example, if it takes 100 seconds to eat an apple, cutting that number in half allows one to say that it takes 50 seconds to eat half an apple. Using the tachymeter scale one can calculate that 72 half apples (36 whole apples) could be eaten in one hour.
