CHRONOSCOPE, or CHRONOGRAPH, an instrument contrived to measure the dura tion of certain short-lived luminous ,phenomena, such as the electric spark, of which the eye itself can be no judge, owing to the persistence of impressions of light un the after the cause of seii,ation ha, (I Th, phenomenon is observed, by rein:Limn in a mirror, in such rapid motion that the itnage of the luminous object Would appear to describe a circle sup posing the luminosity, tq endure long enough. Should the phenomenon; be instantaneous, the image will appear as a mere point; should it last for an appreciable' time, theWage will /form an arc, greater or4ess of the cfrcle. The nature and operation of jronoscolles will be best understood by s few' scriptions.
This is mu of the most successful of all the pendulum in struments, where the: value of the time is ex pressed in arc. It m4 lit said to consist of two separate instruments, ! th pendulum instrument and the disjunctor. Th pendulum instrument is an upright elate o4 with a gradu ated arc, mounted on .a. stand, and supporting two pendulums, two, electromagnets, a pair of springs and the pivot upon which the escape ment system works. One of the pendulums is termed the chronometer pendulum and the other the register pendulum; and the mag nets are so adjusted one behind each pendulum, that when magnetized by a cur rent of electricity they will just sustain the bobs of their respective pendulums, into both of which a piece of soft iron is inserted. The disjunctor consists of a small stand' on which are two pieces of brass, each provided with a pressure-screw, a brass spring, fastened by another pressure-screw, and a cam to work the spring; the brass pieces have platinum points, separated• from each other by a short interval, and the spring has also a platinum point below it, which when pressed down by the action of the cam connects the two other points; thus connecting, when requisite, the circuits through the apparatus. The electric currents are ob tained by means of voltaic batteries, there being two circuits for an ordinary 'experiment, one passing through the magnet of the chronometer pendulum on the first screw, the other through the magnet of the register pendulum and the second screw; as both pass through the dis junctor, the simultaneous disjunction of both circuits can be effected by turning the releas ing spring, and so disconnecting the platinum points.
Noble The of action of this instrument consists in registering by means of electric currents upon a recording surface traveling at a uniform and very high speed, the precise instant at which a pro jectile passes certain defined points in the bore of a fire-arm. It consists of two portions:
First, the mechanical arrangement for obtain ing the necessary speed, and keeping that speed uniform; secondly, the electrical recording ar rangement. The first part of the instrument consists of a series of thin metal discs, each 36 inches in circumference, fixed at intervals upon a horizontal shaft, which is driven at a high speed by a heavy descending weight through a train of gearing multiplying 625 times. If the requisite speed of rotation were got up by the action of the falling weight alone, a considerable waste of time would ensue; to obviate this inconvenience the required velocity can be obtained with great rapidity by means of the handle. The speed usually attained in working this instrument is about 1,000 inches per second, linear velocity, at the circumference of the revolving disc, so that each inch traveled at that speed represents the one one-thousandth part of a second; and as the inch is subdivided by the vernier into a thousand parts, a linear representation at the circumference is thus ob tained of intervals of time as minute as the one one-millionth part of a second. As a small variation in speed would affect the relation be tween the several records obtained, the uni formity of rotation is ascertained on each occasion of experiment by three observations, one immediately before, one during and • one immediately after the experiment, the mean of three observations being taken for the average speed. Some idea may be conveyed of the minute intervals of time which can be meas ured, from the fact that the distances between the parts of a 10-inch gim at which the time records have been obtained are in some in stances only two and four-tenths inches, while the total time the projectile takes to reach the muzzle of the gun—a distance of 100 inches when fired with a full charge, is about the one-hundredth part of a second. By this means the time may be recorded which the projectile occupies, from the commencement of motion, in reaching different parts of the bore, and from these time records may be deduced the velocity with which • the projectile is passing through the different parts of the bore, and the pressures in the gun which corresponds to these velocities.