The term stopwatch
is used as a noun in modern English to refer to a device used to measure time intervals through the start and stop of a clock mechanism
(to 'stop the watch' hence is to indicate the end of the timing interval). As a verb, it typically refers to the activity of timing an event or interval with sub-second accuracy
. More recently, a stopwatch may not involve a physical device at all; in computer programming, a stopwatch function
is a piece of code
designed to measure or maintain either system or wall-clock time
between or during particular activities of the computer.
The first true stopwatches were purely mechanical designs. Balance wheel clockwork mechanisms were used to run the chronograph, and the works could be interrupted (freezing the indicator) using mechanical switches. Later designs, beginning in the latter half of the twentieth century, used quartz movements as electronics became more affordable and small enough to be fit to handheld units. Although there are other ways of carrying out the base function (for example, one could simply interrupt the readout of an atomic clock) the term 'stopwatch' means an independent timing mechanism, not simply the interrupted output of a chronometer. Further, stopwatches are more concerned with accurate measurement of smaller and smaller fractions of a second, rather than long-term accuracy.
The stopwatch differs from the chronometer in that it is designed not to maintain an indication of an external standard time, but rather to measure finite intervals with great accuracy. The Swiss company Tag Heuer claims the first stopwatch patent, registered in 1869. This device was accurate to 1/5 of a second. Later, in 1916, the same firm introduced the Micrograph, a handheld mechanical timer that indicated 1/100 of a second intervals.
While Heuer was busily upping the run accuracy of the stopwatch, in 1960 Seiko bid for the contract to supply timing to the 1964 Olympic Games (held in Japan). This was naturally a publicity move; however, in order to wrest the contract from Tag Heuer and other traditional European manufacturers, Seiko's engineers concentrated on a different problem: that of accurate starting and stopping of the mechanism. As the accuracy of timing increased, the error introduced by the starting and stopping of the watch began to interfere to a greater degree. There was little point in having a stopwatch accurate to 1/100 of a second if the act of starting and stopping it introduced error into its readings of 1/4 second or more.
This had been handled in early Olympic and similar large timing events by requiring each timing to be performed by teams of stopwatch operators; the Olympics used teams of 25 observers at each event, and their results were averaged in order to determine the 'official' time. Seiko isolated much of the stop error to the method by which the oscillating balance was stopped - the action would 'kick' the balance wheel off slightly. They compensated by introducing a cam-based mechanism which isolated the balance from the start/stop mechanism, and hence allowed for smoother starts and stops. In addition, they concentrated on improving longer-term accuracy; most stopwatches of the time kept very precise time over short intervals but rapidly degraded as longer periods were measured due to the greater forces bound up in the works for the longer run. The story goes that the head of the 1964 Olympic timing committee was impressed by Seiko stopwatches undergoing his personal testing, which involved starting/stopping pairs of watches (one in each hand) over designated intervals and examining the timing. The one-hour interval produced measurements that differed by only 1/10 of a second - considerably better than the other entrants. With this, Japanese timekeeping officially entered the 'varsity' in the post-World War II era.
As electronic timing became more common, the error-prone mechanical start/stop controls were replaced with photoelectric sensors and other electronic links (depending on the application) which allowed a further increase in accuracy.
While the mechanical stopwatch continues to improve in accuracy (Tag Heuer recently introduced a wristwatch with a mechanism that vibrates at 600 Hertz in order to allow for 1/100 second accuracy in a long-term chronograph) most official timing is done with electronic devices.
The term split second originally meant not a 'fraction of a second' but was a marketing term for stopwatches, and indicated that the watch in question had multiple (sub)second hands which could be stopped independently - hence, when the first interval was stopped, the second hand would 'split' with the lower one continuing to run. It first was used to indicate fractions of a second in 1946.