an instrument for measuring elapsed time intervals in hours, minutes, seconds, and fractions of a second. The first timers, built at the end of the 19th century, registered time in seconds with a single second hand. Small timing devices are manufactured as pocket and wrist instruments, usually with mechanical movements and hands to display elapsed time. Large timing devices are manufactured as table and panel-mounted units, including demonstrational timers. They are usually electrically powered, electromechanical, or electronic with a digital display. Like clocks and watches, timers have a basic mechanical, electrical, or electronic movement for measuring time. They have a special mechanism for starting, stopping, and resetting the indicator hands or digital display to zero, thus allowing measurement of elapsed time intervals.

The most common small timers use a balance wheel and a coiled spring with an oscillation period of 0.02 or 0.04 sec for measuring intervals up to several minutes and 0.2 or 0.4 sec for measuring intervals up to several hours. Starting, stopping, and resetting are controlled by pressing the winding stem and a push button. When timing is started, a lever-and-cam mechanism frees the balance wheel or engages gears that connect the movement with the hands on the dial; when timing is stopped, the balance wheel is blocked or the gears are disengaged. Small class–1 timers with mechanical movements are accurate to within ± 0.08 sec over a 60-sec interval and to within ± 0.3 sec over a 30-min interval.

Electric timers are equipped with synchronous electric motors and are usually powered by an external source of alternating current. Electromechanical and electronic timers usually have a quartz signal generator powered by an internal source of direct current. Large quartz timers have digital displays and a keyboard-operated control system. They are accurate to within ±0.001 sec.

The minute scales on timers are sometimes divided into 100 parts for the sake of convenience in certain timing operations in industry. Timer categories include cumulative timers, used, for example, to register the length of service of machine tools; splitsecond watches, which simultaneously register the duration of an entire process and the duration of one stage, for example, the total time of a race and the time of a single segment; multidial timers capable of registering the duration of several processes simultaneously; and timers with scales for determining the number of events in a unit of time, for example, the number of pulse beats in a minute.