Chronograph ('time-marker' or 'recorder'), an instrument to note, within a certain fraction of a second, the instant when a particular event occurs. The most recent are electrical, now indispensable to astronomers, since the transit of a star can, by touching a stud, be noted to within th of a second of time. The typical form of the chronograph is a cylinder which revolves once a minute, and carries on its surface a sheet of paper divided all round into equal parts, indicating fractions of a second. If, for example, the cylinder is 30 inches round, it is evident that very minute subdivisions of time are attainable.
A valuable application of the chronograph is for determining the longitude—e.g. in trigonometrical surveys. Thus two observers note simultaneously the transit of a star, say at Vienna and Paris (and that without any reference to right ascension or declination), and having telegraph as well as chronograph, determine very easily the difference of time with an accuracy never dreamt of formerly.
Benson's chronograph, by a lever movement, measures intervals of time down to tenths of a second for use at horse-races, &c. By a special contrivance the seconds hand on a horizontal dial carries ink, and marks the plate beneath at the instant when a string is pulled. Another mode of recording the exact instant of start or finish is by having a paper travel under a pencil, so that when the latter is pressed a mark is made on or near one of the cross lines showing tenths or twelfths of a second.
Other forms of the chronograph are used for determining the velocity of projectiles. The most general arrangement consists in causing the bullet to pass through a series of screens; the rupture of each screen breaks for a moment the continuity of an electric current, sets in action an electro-magnetic apparatus, and makes a permanent mark or record.