Discoveries and Inventions of the Nineteenth Century

just touch its surface during the whole time of its descent. Now let the surface of the glass be blackened in the flame of a candle. If the glass be allowed to slide down when the fork is not vibrating, the end of the bristle, by removing the lampblack from the surface as the glass falls, would trace out a vertical line. If, on the other hand, the blackened surface were itself not moved, but simply brought into contact with the end of the bristle, while the fork was sounding, there would be marked only a very short horizontal line, corresponding with the extent of the vibratory movements of the prong. When the glass is allowed to fall while the prong is in motion, the combination of the horizontal movements of the bristle, and the vertical one of the glass, will produce a waved line, which will exhibit perfectly regular curves if the glass has been moved with uniform velocity. It is plain that if the time taken by the glass to pass in front of the bristle were accurately known, the number of movements per second executed by the prong of 657the fork could be found. On the other hand, if the rate of vibration of the fork be known, the time occupied in the passage of the glass may accurately be read off. If this simple experiment be understood, the principle of the electric chronograph will be clear. Substitute for the sliding glass a cylinder covered with white glazed paper which has been coated with lampblack in the same manner; suppose the cylinder to revolve at a uniform rate, while a tuning-fork is similarly writing its vibrations on the surface of the paper, and let the same mechanism which turns the cylinder, slowly draw the sounding-fork along a straight slide parallel to the axis of the cylinder. The waved line will not form a complete circle on the surface of the cylinder, but will be traced out in a spiral, owing to the combined motions of the fork and the cylinder. As the number of movements per second of a vibrating body emitting a given note are accurately determined and perfectly regular, the waved line on the cylinder thus furnishes an exact measure of small intervals of time, the utility of which will presently be seen. Fig. 316.—The Electric Chronograph. Fig. 316 represents the apparatus as actually constructed. A is the cylinder covered with the blackened paper, and driven by clockwork contained in the case, B, the rate of movement being regulated by the conical pendulum at C, so as to be approximately uniform. D is a lever for starting and stopping the movement. The clockwork also causes the carriage, E, to slide along the bars, F. This carriage bears three electro-magnets, 658and to the armature of each a fine pointed strip of metal is attached so as just to touch the surface of the cylinder. The movement of the armatures takes place in a direction parallel to the axis of the cylinder, and they are acted on by independent currents, each electro-magnet having its own binding-screws for the attachment of wires. The armature of one of these is a steel bar, which vibrates in the manner of the prong of a tuning-fork, at a rate known by the note it gives out. Sometimes, for delicate experiments, the movements of this armature are checked and controlled by introducing into its electric circuit a contact-breaker, formed of a real tuning-fork, the vibrations of which are maintained by electro-magnets. Another electro-magnet on the carriage, E, can be connected with the pendulum of a standard clock, so that seconds may be also marked on the cylinder, as larger units in the division of the time. In an electric chronograph, which was exhibited at the International Exhibition of 1862, an ingenious and excellent mode of making and breaking the electric contacts from the standard pendulum was adopted. Two short vertical glass tubes, placed side by side, had each near its lower end a small horizontal branch; these branches were placed in a line with each other, with their ends in very close proximity. The larger tubes were filled with mercury which flowed into the horizontal branches, and the two streams joined in the narrow space between the ends of the tubes. Although the mercury was here unsupported by the glass, and surrounded by air only, it did not run down, for the space was so small that the cohesion of the mercury itself sufficed to keep the drop hanging between the two open ends of the tubes. A very thin sheet of mica was carried by an arm of the pendulum, so placed that at each complete oscillation the mica entered the space between the two tubes and divided the mercury, and thus all electrical communication between the two reservoirs was cut off. The mica remained during one beat of the pendulum in this position; and at the commencement of the return beat was withdrawn, allowing the divided columns of mercury to flow together again, and complete the electric circuit. This admirable make-and-break arrangement acted with the greatest regularity: the mercury was not spilt, as might have been expected, there was no friction, and any oxide formed by the spark which passed when the current was interrupted was removed by the mica. The pendulum provided with such an arrangement allows the current to pass, and interrupts it, during alternate seconds, and the result is that the cylinder is marked with a regularly divided broken line, thus, , the establishment and interruption of the current at the end of each second being marked with great sharpness and precision. The third electro-magnet of the apparatus represented in Fig. 316 is acted on by currents through the wires, G, H. The point attached to its armature traces a plain spiral line on the revolving cylinder, except at the instant when the current is established or interrupted. And the phenomenon to be timed is in some way made to accomplish the making and breaking of this circuit. This may be perhaps better understood by an example. It has sometimes happened that the boxes employed in the pneumatic dispatch stick fast in the tubes, and resist all efforts to dislodge them by manœuvres with the compressed or rarefied air, or other means. In such a case it becomes necessary to ascertain with tolerable accuracy the position of the obstruction, so that the tube may be cut at the right place and the obstacle removed. The known velocity of sound has been ingeniously used for this purpose; the electric chronograph being made 659the means of ascertaining, to a small fraction of a second, the time required for the report of a pistol to be propagated through the air of the tube and reflected back from the obstruction. An elastic membrane is spread over the open extremity of the pneumatic tube; this membrane has in the centre a little disc of platinum, in electric connection with one of the wires, G, H. The other wire passes to a galvanic cell or battery, and the return wire from the battery is connected with a platinum point, the distance of which from the disc can be adjusted with nicety by means of a screw, so that the circuit may be complete only when the platinum point and the platinum disc are in contact. The screw is adjusted so as to bring the point as near as possible to the disc without actual contact. The chronograph cylinder having been set in motion, a small pistol is fired into the pneumatic tube through a side opening. Sound-waves of alternate compression and rarefaction pass along the tube, and are reflected backwards and forwards many times in succession between the obstacle and the membrane. By these the membrane is alternately forced out and drawn in, making and breaking the electric contact accordingly, and thus causing the point of the electro-magnet to describe in the cylinder an indented line, the intervals of which indicate the time the sound requires to traverse the tube to the obstruction; and thus the position of the latter may be known with sufficient accuracy. A very interesting application of the electric chronograph is to the measurement of the velocities of projectiles. The science of gunnery has acquired an exactness unknown before electricity was made to carry messages from the cannon-ball in its swiftest flight, and to write the record of its own course. Instruments for thus measuring the velocities of projectiles have been contrived by several electricians, among whom Wheatstone appears to have been the first. The principle in most of these chronographs is precisely the same as that on which the apparatus represented in Fig. 316 is constructed. The action of the projectile which is electrically indicated is the severing of a slender wire, which is stretched from side to side of a wooden frame, so that it passes continuously backwards and forwards in parallel lines. Thus a kind of screen is formed, through which the missile must pass, and in its passage must rupture the wire. If the wire conveys a current of electricity, this current is therefore interrupted at the moment the ball passes. Sometimes the immediate effect of the breaking of the wire is mechanical, as in Wheatstone’s arrangement, where the wire is stretched by a weight over a series of pulleys, and attached to a contact-maker, which completes the circuit when it is set free by the rupture of the wire. A similar arrangement in the screens has been proposed by Mr. Siemens for the establishment of circuits in connection with charged Leyden jars, the sparks of the discharges being made to take place at the surface of a revolving cylinder of polished steel, where the place is shown by the spot they leave on the metal. M. Pouillet’s chronoscope dispenses with the revolving cylinder, and measures the duration of the current established by the projectile at one part of its course and cut off at another, by the arc through which the needle of a galvanometer is impelled. The instrument which has been most employed in this country by artillerists is that invented by Professor Bashforth. Its indications are extremely accurate, for readings may be taken to the two-thousandth part of a second. From ten to fifteen screens are placed in the path of the projectile at distances asunder which may vary from 15 ft. to 150 ft., but which, of course, are carefully measured. Each screen is formed of a wire 660carrying an independent current, which also circulates in the coils of an electro-magnet. The ten electro-magnets have styles attached to their armatures in such a manner that when all the currents are passing, ten parallel spiral lines are traced on the surface of a vertical revolving cylinder, about 12 in. long and 4 in. diameter. The cylinder is covered with a sheet of highly-glazed paper coated with lampblack; and when the sheet is removed, the lampblack may be fixed on the paper if desired, and the record made permanent. The equal intervals of time are marked on the same cylinder by another electro-magnet connected with a pendulum beating half-seconds. The axis of the revolving cylinder, &c., carries a heavy flywheel, which is set in motion by the hand at the rate of about three revolutions in two seconds, and the rest of the mechanism is thrown into gear just before the signal to fire is given. The arrangement of the electric connections at the screens is such that at the moment of the rupture of the wire, the circuit is broken for an instant only. At this moment, the iron of the corresponding electro-magnet ceasing to attract its armature, the latter is drawn away by a spring; but the re-establishment of the current immediately brings it back, and the style continues to trace the same spiral line as before. The passage of the ball through the screen is therefore marked by a little notch in the spiral line, thus, , and the point where the deviation begins