A tachymeter (pronounced ) is a scale sometimes inscribed around the rim of an analog watch with a chronograph. It can be used to conveniently compute the frequency in hours of an event of a known second-defined period, such as speed (distance over hours) based on travel time (distance over speed), or measure distance based on speed. The spacings between the marks on the tachymeter dial are therefore proportional to , where t is the elapsed time. The function performed by a tachymeter is independent of the unit of distance (e.g. statute miles, nautical miles, kilometres, metres etc.) as long as the same unit of length is used for all calculations. It can also be used to measure the frequency of any regular event in occurrences per hour, such as the units output by an industrial process. A tachymeter is simply a means of converting elapsed time (in seconds per unit) to rate (in units per hour). Measuring speed To use a tachymeter-equipped watch for measuring speed, the chronograph is started at a starting marker of a known distance.At the next marker, the point on the scale adjacent to the second hand indicates the speed (in distance between markers per hour) of travel between the two. The typical tachymeter scale on a watch converts between the number of seconds it takes for an event to happen and the number of times that event will occur in one hour. The formula used to create this type of tachymeter scale is where T is the tachymeter scale value; t is the time in seconds that it takes for the event to occur; and 3600 is the number of seconds in an hour. As a sample calculation, if it takes 35 seconds to travel one mile, then the average speed is 103 miles/hour. On the watch, 35 seconds gives scale value 103. Similarly, if one kilometre takes 35 seconds then the average speed would be 103 km/hour. Note that the tachymeter scale only calculates the average speed. As a second example, if it takes 20 seconds to travel one unit of distance, then the average speed on the watch used for the purpose of the picture only is 180 units of distance per hour (examine the picture of the watch which is here to simplify the idea, actual tachymeters may vary slightly). For events that happen either very quickly or slowly, one can