Velocimeter, an instrument for measuring the velocity of projectiles. Prior to 1840 such measurements were made by suspending a gun in a pendulum, and observing the arc described in its recoil. This gave the means of computing the velocity imparted to the projectile, with a probable error of a very few feet per second. In 1840 Wheatstone suggested the use of electricity for obtaining the data required for the computation, and for a quarter of a century it has been used exclusively. The velocity of a moving body becomes known when we know the time it takes to pass through a measured portion of its path. The space is determined by merely choosing certain points (usually about 100 ft. apart) in the path of the projectile, and measuring the distance between them. The time required to pass over this distance therefore becomes the sole object of inquiry; and a velocimeter ig merely an instrument for measuring with extreme accuracy small intervals of time. To accomplish this, a screen of fine wire carrying an electric circuit is placed at each end of the measured interval, in such manner that the passing projectile shall rupture the two circuits, and the two ruptures instantly telegraph themselves to a machine, which records them in such a way that the interval of time between them can be immediately read.
The recording instrument is called a chronograph. The number and variety of chronographs are very great, but they all involve one mechanical principle, viz.: the records must be made by means of parts of the machine moving at rates which are known with great exactitude, during intervals which begin and terminate with the two ruptures respectively. This may be illustrated by the following contrivance, which is a modification of the Navez chronograph by Col. J. G. Benton of the United States ordnance department. It consists of a vertical metallic semicircle, 5, graduated with the ordinary circular units and supported upon a bed plate, a. Two pendulums, p p, are swung upon the axis of the arc, and have their mass so distributed that their times of oscillation are equal. When deflected in opposite directions from a vertical position to 90°, they touch the magnets m m', which hold them in the new or horizontal position. The magnets are excited by circuits, c c, c' c', which pass through the wire screens in front of the gun.
If they were both ruptured at the same instant (the instrument being perfectly adjusted), the pendulums would be simultaneously released, and would pass each other opposite the zero mark at the lowest point of the graduated are; but being ruptured successively, they pass at some point more or less distant from it, this distance being dependent upon the length of the interval of time between the two ruptures. To mark the point where this passage occurs, a stud, attached to the pendulum p, strikes the oblique head of a pin attached to a lever in the other pendulum, and causes it to make an indentation or an ink mark upon a piece of paper clamped to, the graduated arc, leaving a record of the angle of deflection of the two pendulums at the in-' stant of passage. By a simple formula the interval between ruptures can be computed from this angle. - A very simple form of velocimeter, probably used more extensively than any other, is that invented by Capt. Le Boulengé of Belgium. A metallic standard, S, fig. 2, sustains two electro-magnets, A B. The magnet A, when excited, holds a bar 0 as its armature, called the " chronometer." A zinc tube, D, removable at pleasure, tits over the latter.
The magnet B holds a smaller armature, F, called a "registrar;" and immediately beneath it is an apparatus, L, holding in tension a cocked main spring, which carries a knife edge. After the relative altitudes of the two magnets have been adjusted, the knife edge is caused to make an indentation upon the zinc tube of the chronometer for a zero mark, and then the machine is ready for the record. The magnet holding the chronometer is excited by the circuit through the screen nearest to the gun, and the registrar magnet by the other circuit. When the first is ruptured the chronometer falls; when the second is ruptured the registrar falls, springing the knife edge and causing a cut to be made in the zinc tube. The space between this cut and the zero mark gives, by the formula for falling bodies, the time interval between the two ruptures. - The two machines described are capable of showing the velocities of projectiles with great precision, their mean errors not exceeding of a second. Col. Benton has used his machine without the aid of electricity. In place of electro-magnets to hold the pendulums and release them, he has employed springs kept tense by means of cotton threads. The threads, being ruptured by the passing shot, release the springs, which dismiss the pendulums.
The results so obtained are but little less accurate than those with electro-magnets. Velocimeters are also used for obtaining data by which the resistance of air to the motion of projectiles may be determined. There is no known method of computing it, and the only resource is to measure it directly. This the velocimeter enables us to do by showing how much a projectile is retarded in passing over a series of intervals. But for each set, and for each velocity, a separate trial must be made. The determination of the velocity through a succession of intervals requires a chronograph of much more complicated structure than the simple ones described, for it must record the times of rupture of half a dozen circuits, or of half a dozen ruptures of the same circuit. Usually the screens are all placed in the same circuit, which renews itself after each rupture, before the shot reaches the next screen. The recording device consists of a cylinder or disk revolving at a known rate, and receiving at the instants of rupture some visible marks from a stationary device or tracing point controlled by the current.
The most ingenious application of the velocimeter yet made is the measurement of the varying velocity of a projectile in the bore of a gun, which has been accomplished by means of a special apparatus devised by Capt. W. H. Noble of the British army, and also by a series of Le Boulenge chronographs.