an instrument for precise registration of the time at which an event occurs. Chronographs are classified by the method of registration used as pen recorders, printers, and photographic instruments.

In a pen recorder chronograph, the instant at which the event occurs is recorded by several specially constructed pens on a uniformly moving paper tape. Each pen has an electromagnetic system that holds the pen in one of two stable positions. The changeover of positions occurs when a current is fed to or disconnected from the electromagnet; at this instant the trace of the pen on the tape will show a break. One pen is usually controlled by the reference clock, and the other pens are controlled by the test instruments, such as other clocks, contact micrometers of transit instruments, and relays that are controlled by accurate radio time signals. Measurements of the coordinates of the break points in the trace are used to calculate the instants of time according to the reference clock system. Several other designs of pen recorder chronographs have been used in astronomy, in which plotting or perforating needles or electric sparks are substituted for pens. Pen recorder chronographs have an accuracy of approximately 0.01 second. They became obsolete by the mid-20th century.

In a printing chronograph, numerals are printed on a paper tape at the instant when a current is fed to or disconnected from the control circuit of the electromagnet. The numerals correspond to the instant of time with respect to some conventional time scale that the chronograph produces with its own quartz oscillator. A printing chronograph has three cylindrical disks of equal diameters, with raised graduation marks on the outside surfaces. The first and the second disks have 60 graduation marks each, and the third disk has 100 marks; the graduation marks are numbered from zero to 59 and from zero to 99, respectively. The first disk makes one revolution per hour and registers minutes; the second disk makes one revolution per minute and registers seconds; and the third disk makes one revolution per second and registers tenths, hundredths, and thousandths of a second. At the instant when the electromagnet is activated, a paper tape and an inked tape are both very briefly pressed against the rotating disks, and impressions of the numerals, graduated marks, and reference index are printed on the paper. The disks are driven at a uniform speed by a synchronous motor fed by a quartz oscillator. Modern printing chronographs register time with a precision of approximately ±0.005 second.

Photochronographs use disks of practically the same type as those in printing chronographs. The disks and the reference index are photographed at the instant a flash lamp is fired. The reference signal and the test signal are fed to the control circuit of the flash lamp. The difference of the two readings can be seen on the developed photographic film and is used to define the instant of the appearance of the test signal with respect to the time system of the reference clock. Photochronographs are practically inertia-free instruments because there are no mechanical components in the recording units. They are therefore used where the precision of printing chronographs proves inadequate. Photochronographs register time with an error no greater than ±0.001 second. Chronographs are used primarily in astronomy.