Professor Joseph Henry (Journal Franklin Inst., 1886) employed a cylinder driven by clockwork, making ten revolutions per second. The surface was divided into 100 equal parts, each equal to 1/1000 second. The time marks were made by two galvanometer Henry. needles, when successive screens were broken by a shot. Henry also used an induction-coil spark to make the cylinder, the primary of the coil being in circuit with a battery and screen. This form of chronograph is in many respects similar to the instrument of Konstantinoff, which was constructed by L.F.C. Breguet and has been sometimes attributed to him (Comptes rendus, 1845). This chronograph consisted of a cylinder 1 metre in circumference and 0.36 metre long, driven by clockwork, the rotation being regulated by a governor provided with wings. A small carriage geared to the wheelwork traversed its length, carrying electromagnetic signals. The electric chronograph signal usually consists of a small armature (furnished with a style which marks a moving surface) moving in front of an electromagnet, the armature being suddenly pulled off the poles of the electromagnet by a spring when the circuit is broken (Journal of Physiology, ix. 408). The signals in Breguet’s instrument were in a circuit, including the screens and batteries of a gun range. The measurement of time depended on the regularity of rotation of the cylinder, on which each mm. represented 1/1000 second.
In the chronograph of A.J.A. Navez (1848) the time period is found by means of a pendulum held at a large angle from the vertical by an electromagnet, which is in circuit with a screen on the gun range. When the shot cuts this screen the circuit Navez. is broken and the pendulum liberated and set swinging. When the next screen on the range is broken by the shot, the position of the pendulum is recorded and the distance it has passed through measured on a divided arc. From this the time of traversing the space between the screens is deduced. By means of an instrument known as a disjunctor the instrumental time-loss or latency of the chronograph is determined.In Benton’s chronograph (1859) two Benton. pendulums are liberated, in the same manner as in the instrument of Navez, one on the cutting of the first screen, the other on the cutting of the second. The difference between the swings of the two pendulums gives the time period sought for. The disjunctor is also used in connexion with this instrument. In Vignotti’s chronograph (1857) again a pendulum is employed, furnished with a metal point, which moves close to paper impregnated with ferro-cyanide of potassium. The gun-range screens are included in the primary circuits of induction coils; when these circuits are broken a spark from the pointer marks the paper. From these marks the time of traverse of the shot between the screens is determined.
In the Bashforth chronograph a platform, arranged to descend slowly alongside of a vertical rotating cylinder, carries two markers, controlled by electromagnets, which describe a double spiral on the prepared surface of the cylinder. One Bashforth. electromagnet is in circuit with a clock, and the marker actuated by it marks seconds on the cylinder; the circuit of the other is completed through a series of contact pieces attached to the screens through which the shot passes in succession. On the gun range, when the shot reaches the first screen, it breaks a weighted cotton thread, which keeps a flexible wire in contact with a conductor. When the thread is broken by a shot, the wire leaves the conductor and almost immediately establishes the circuit through the next screen, by engaging with a second contact, the time of the rupture being recorded on the cylinder by the second marker. The velocity with which the cylinder rotates is such that the distance between successive clock marks indicating seconds is about 18 in.; hence the marks corresponding with the severance of a thread can be allotted their value in fractions of seconds with great accuracy. The times when the shot passes successive screens being thus recorded on the spiral described by the second marker, and the distance between each screen being known, the velocity of the shot can be calculated.
The chronoscope invented by Sir Andrew Noble is so well adapted to the measurement of very small intervals of time that it is usually employed to ascertain the velocity acquired by a shot at different parts of the bore in moving from a state of rest Noble. inside the gun. A series of “cutting plugs” is screwed into the sides of the gun at measured intervals, and in each is inserted a loop of wire which forms part of the primary circuit of an induction coil. On the passage of a shot this wire is severed by means of a small knife which projects into the bore and is actuated by the shot as it passes; the circuit being thus broken, a spark passes between the terminals of the secondary of the coil. There is a separate coil and circuit for each plug. The recording arrangement consists of a series of disks, one for each plug, mounted on one axle and rotating at a high angular velocity. The edges of these disks are covered with a coating of lamp-black, and the secondaries of the coils are caused to discharge against them, so that a minute spot burnt in the lamp-black of each disk indicates the moment of the cutting of the wire in the corresponding plug. Hence measurement of the distance between two successive spots gives the time occupied by the shot in moving over the portion of the bore between two successive plugs. By the aid of a vernier, readings are made to thousandths of an inch, and the peripheral velocity of the disks being 1100 in. a second, the machine indicates portions of time rather less than one-millionth of a second; it is, in fact, practically correct to hundred-thousandths of a second (Phil. Trans., 1875, pt. i.).
In the Le Boulengé chronograph (“Chronograph le Boulengé,” par M. Bréger, Commission de Gâvre, Sept. 1880) two screens are used. The wire of the first forms part of the circuit of an electromagnet which, so long as it is energized, supports Le Boulengé. a vertical rod called the “chronometer.” Hence when the circuit is broken by the passage of a shot through the screen this rod drops. The wire of the second screen conveys a current through another electromagnet which supports a much shorter rod. This “registrar,” as it is called, when released by the shot severing the wire of the second screen, falls on a disk which sets free a spring, and causes a horizontal knife to fly forward and nick a zinc tube with which the chronometer rod is sheathed. Hence the long rod will be falling for a certain time, while the shot is travelling between the two screens, before the short rod is released; and the longer the shot takes to travel this distance, the farther the long rod falls, and the higher up on it will be the nick made by the knife. A simple calculation connects the distance through which the rod falls with the time occupied by the shot in travelling over the distance between the screens, and thus its velocity ascertained. The nick made by the knife, if released while the chronometer rod is still suspended, is the zero point. If both rods are released simultaneously, as is done by breaking both circuits at once by means of a “disjunctor,” a certain time is consumed by the short rod in reaching the disk, setting free the spring and cutting a nick in the zinc; and during this time the long rod is falling into a recess in the stand deep enough to receive its full length. The instrument is so adjusted that the nick thus made is 4.435 in. above the zero point, corresponding to 0.15 sec. This is the disjunctor reading, and requires to be frequently corrected during experiments. The instrument was modified and improved by Colonel H.C. Holden, F.R.S. For further information respecting formulae relating to it see Text Book of Gunnery (1857).
The electric chronograph of the late H.S.S. Watkin consists of two long cylinders rotating on vertical axes, and between them a cylindrical weight, having a pointed head, is free to fall. The weight is furnished with an insulated wire which Watkin. passes through it at right angles to its longest axis. When the weight falls the ends of the insulated wire move very close to the surfaces of the cylinders which form part of a secondary circuit of an induction coil, the primary circuit of which is opened when a screen is ruptured by a shot. A minute mark is made by the induced spark on the smoked paper with which the cylinders are covered. The time period between events is deduced from the space fallen through by the weight, and by means of a scale, graduated for a given distance between the screens, the velocity of a shot is at once found. It may be noted that the method of release is such that the falling weight is not subjected, after it has begun to fall, to a diminishing magnetic field, which would be the case if it were directly supported by an electromagnet. An iron rod when falling from an electromagnet, during a minute portion of its fall, is subject to a diminishing force acting in the opposite sense to that of gravity, whereby its time of fall is slightly changed.
Colonel Sebert (Extraits du mémorial de l’artillerie de la marine) devised a chronograph to indicate graphically the motion of recoil of a cannon when fired. A pillar fixed to the ground at the side of the gun-carriage supported a tuning-fork, the Sebert. vibration of which was maintained electrically. The fork was provided with a tracing point attached to one of the prongs, and so adjusted that it drew its path on a polished sheet of smoke-blackened metal attached to the gun-carriage, which traversed past the tracing point when the gun ran back. The fork used made 500 complete vibrations per second. A central line was drawn through the curved path of the tracing point, and every entire vibration cut the straight line twice, the interval between each intersection equalling 1/1000 second. The diagram so produced gave ihe total time of the accelerated motion of recoil of the gun, the maximum velocity of recoil, and the rate of acceleration of recoil from the beginning to the end of the motion. By means of an instrument furnished with a microscope and micrometers, the length and amplitude, and the angle at which the curved line cut the central line, were measured. At each intersection (according to the inventor) the velocity could be deduced. The motion at any intersection being compounded of the greatest velocity of the fork, while passing through the midpoint of the vibration and the velocity of recoil, the tangent made by the curve with the straight line represents the ratio of the velocity of the fork to the velocity of recoil. If a be the amplitude of vibration, considered constant, v the velocity of the fork at the midpoint of its path, r the velocity of recoil, α the angle made by the tangent to the curve with the straight line at the point of intersection, and t the line of a complete vibration; then, v = 2πa/t; r = v/tan α.
F. Jervis-Smith’s tram chronograph (Patents, 1894, 1897, 1903) was devised for measuring periods of time varying from about one-fourth to one twenty-thousandth part of a second (Proc. Roy. Soc., 1889, 45, p. 452; The Tram Chronograph, by Jervis-Smith. F. Jervis-Smith, F.R.S.). It consists of a metal girder having a T-shaped end. This carries two parallel steel rails, the edges of which lie in the same vertical plane. The girder, which is slightly inclined to the horizontal plane, is geometrically supported, being carried at its end, and at the extremities of the T-piece, on a V-groove, trihedral hole and plane. A carriage or tram furnished with three grooved wheels runs on the rails, and a slightly smoked glass plate is attached to its vertical side. The tram in the original instrument was propelled by a falling weight, but in an improved form one or more spiral springs are employed. All time traces are made immediately after the propelling force has ceased to act. The tram is brought to rest by a gradually applied brake, consisting of two crossed leather bands stretched by two springs; a projection from the tram runs between the bands, and brings it to rest with but little lateral pressure. When, for certain physiological experiments, a low velocity of traverse is required, a heavy fly-wheel is mounted on the tram and geared to its wheels. A pillar also mounted geometrically, placed vertically in front of the carriage, carries the electromagnet style or signals and tuning-fork which can be brought into contact with the glass by means of a lever. Also styli are used which depend for their action on the displacement of one or more wires under tension or torsion carrying a current in a magnetic field, the condition being such that no magnetic lag due to iron armatures and cores exists. Two motions of a slide on the pillar, viz. of rotation and translation, allow a number of observations to be made. The traces are counted out on a sloping glass desk, and the time of flight of a projectile between two or more screens is found. When very close readings are required, they are made by means of a traversing geometric micrometer microscope. When the distance between the screens is known, and also the time of flight, the midpoint velocity is found by applying Bashforth’s formula. When the velocity of shot from a shot-gun has to be found, a thin wire stretched across the muzzle takes the place of the first screen, and a thin sheet of metal or cardboard carrying an electric contact, or a Branly coherer, the conductivity of which is restored by means of an induced current, takes the place of the second screen. The electric firing circuit is provided with a safety key attached by a cord to the man who loads the gun and prepares the electric fuse. The firing circuit is closed by inserting the key in a switch at the rear of the gun, thus preventing him from getting into the line of fire when the gun is fired by the chronograph. The tram, when the instrument is adjusted, has a practically constant velocity of traverse.
The polarizing photo-chronograph, designed and used by A.C. Crehore and G.O. Squier at the United States Artillery School (Trans. Amer. Inst. Elect. Eng. vol. 14, and Journal United States Artillery, 1895, 6, p. 271), depends for its Crehore-Squier. indications upon the rotation of a beam of light by a magnetic field, produced by a solenoidal current which is opened and closed by the passage of the projectile. The general arrangement is as follows:—A beam of light from an electric lamp traverses a lens, then a Nicol prism, next a glass cylinder furnished with plane glass ends and coiled with insulated wire, then an analyser and two lenses, finally impinging on a photographic plate to which rotation is given by an electric motor, the plane of rotation being perpendicular to the direction of the beam of light. The same plate also records the shadow of a pierced projection attached to a tuning-fork, light from the electric lamp being diverted by a mirror for this purpose. The solenoid used to produce a magnetic field across the glass cylinder, which is filled with carbon bisulphide, is in circuit with a dynamo, resistances, and the screens on the gun range. It is a well-known phenomenon in physics that when, with the above-mentioned combination of polarizing Nicol prism and analyser, the light is shut off by rotating the analyser, it is instantly restored when the carbon bisulphide is placed in a magnetic field. This phenomenon is utilized in this instrument. The projectile, by cutting the wire screens, causes the magnetic field to cease and light to pass. By means of an automatic switch the projectile, after cutting a screen, restores the electric circuit, so that successive records are registered. After a record has been made it is read by means of a micrometer microscope, the angle moved through by the photographic disk is found, and hence the time period between two events. In the photo-chronograph described in Untersuchungen über die Vibration des Gewehrlaufs, by C. Cranz and K.R. Koch (Munich, 1899), also note on the same, Nature, 61, p. 58, a sensitive plate moving in a straight line receives the record of the movement of the barrels of firearms when discharged. It was mainly used to determine the “angle or error of departure” in ballistics.
In a second chronograph by Watkin (“Chronographs and their Application to Gun Ballistics,” Proc. Roy. Inst., 1896), a metal drum, divided on its edge so that when a vernier is used a minute of angle may be read, is rotated rapidly by a motor at a Watkin. practically uniform speed. The points of a row of steel-pointed pins, screwed into a frame of ebonite, can be brought within 1/200 in. of the surface of the drum. Each pin is a part of the secondary circuit of an induction coil, the space between the pins and the drum forming spark-gaps. The drum is rubbed over with a weak solution of paraffin wax in benzol, which causes the markings produced by the sparks to be well defined. The records are read by means of a fine hair stretched along the drum and just clear of it, the dots being located under the hair by means of a lens. The velocity of rotation is found by obtaining spark marks, due to the primary circuits of two induction coils being successively broken by a weight falling and breaking the two electric circuits of the coils in succession at a known distance apart. This chronograph has been used for finding the velocity of projectiles after leaving the gun, and also for finding the rate at which a shot traverses the bore. For the latter purpose the shot successively cuts insulated wires fixed in plugs screwed into the gun at known intervals; each wire forms a part of the primary of an induction coil, and as each is cut a dot is made on the rotating drum by the induced spark.