of the reflecting galvanometer, it is probable that the Atlantic Cable enterprise could not have been accomplished. With the help of the figure (fig. 284), the reader will require almost no description of this instrument It may be enough to say that a tiny needle attached to an extremely light glass mirror, the whole weighing only a grain and a half, hangs by a single filament of unspun silk within a coil of silk-covered copper wire, the inside diameter of the coil being only about three-eighths or half an inch. A beam of light from a gas or lamp flame, about a yard distant, passing through a small opening in a scale of cardboard, s, falls on the tiny mirror, M, and being reflected back, falls on the graduated part of the scale. The slightest motion of the mirror, M, will, by a well-known optical principle, be exhibited in greatly magnified motions of the spot of light upon the scale, S. The delicacy of such an instrument is per fectly marvellous. For strong currents it is common to use a galvanometer, involving the very same principle, and consisting simply of a single strip of copper bent into the shape of a circle, with a short magnetic needle poised or delicately suspended at the centre of the circle. The strength of the current can be proved both mathematically and experimentally to be proportional to what is called the tangent of the angle of deviation of the needle, which may be read off either by reference to a graduated card, or by reflecting a beam of light, as in the Thomson galvanometer. ELECTRO-DYNAMICS. 983. From the magnetic powers of the current already considered, we might naturally suppose that there must be some fundamental or simple laws of attraction and repulsion existing between straight currents in proximity. The precise exposition of these forms the division of this subject known as electro-dynamics, which would involve more mathematical details than are consistent with the plan of the present work. It may, however, be mentioned that currents passing along movable wires, or flexible metallic conductors of any sort, cause attraction or repulsion of these, according to their mutual directions. If the movable conductors be parallel, there is attraction when the currents flow in the same direction, and repulsion when they flow in opposite directions; if the currents are inclined to each other at an angle, there is attraction, if they both flow either towards or from the point of crossing, but repulsion if one flows from, and the other towards, this point. 746 Electro-magnetic Engines. The same laws of current action explain the mutual action of magnets and currents, when we adopt Ampere's view, that a magnet is a group of circular currents. It explains why two circular currents, such as currents flowing in a helix of copper-wire, will attract or repel cach other exactly like magnetized bars. It explains why the current deflects the magnetic needle when the needle is movable, and it also explains why a bar-magnet attracts or repels a movable conductor, or why a movable conductor may rotate about a magnet when a current passes through it in one direction, and rotate in an opposite way when the current is reversed, or why a current may make a magnet rotate round its own axis. All these experiments may be easily performed in a variety of ways, and they are most interesting, theoretically, as supporting Ampere's hypothesis, and the fundamental laws of electro-dynamics. ELECTRO-MAGNETIC ENGINES. 984. The discovery of the immense attractive power which can be developed instantaneously in soft iron by means of the galvanic current, and which can be as quickly stopped by the mere breaking of the galvanic circuit, led many to imagine that the glory of the invention of the steam-engine would speedily be dimmed by the invention of an electro-motive engine, which might be worked by the unseen power conveyed by a single wire from a distant battery. Many ingenious machines have indeed been devised, whereby a practically useful motive power might be thus obtained. They are either in the form of a reciprocating beam, whose ends are alternately attracted by two electro-magnets, the electric current being cut off from each magnet as soon as the beam is attracted by it—this alternating motion, as in the steam-engine, can, by a simple crank, be readily converted into a continuous rotatory one-or they are in the form of a series of radial arms of soft iron attached to a flywheel, with one or more electro-magnets, so disposed that a spring admits the current to pass when an arm is approaching the poles of the magnet, but the current is cut off the moment the arm is close to the magnet. The power of the latter is thus cut off, the momentum of the fly-wheel keeps up the motion until the spring again closes the circuit, and the next soft-iron arm is within attracting distance of the magnet. It has been proposed to apply such motors to the driving of sewing-machines and small industries, and even to the propulsion of steamboats; but as the electro-motive power is derived from the Electric Clocks. 747 oxidation of zinc in the cells of the battery, and the consumption of zinc is proportional to the work done by the battery, the high price of this metal is an insurmountable obstacle in the way of its use as an economic source of power. Its use would be probably fifty times more costly than the use of coal, though, doubtless, there are circumstances where, economy being of little object, such electromotors would be more convenient than steam power. A much more practically useful application of electro-magnetism has been to clock-controlling, and to the construction of electric chronoscopes. 985. Amid the multitude of ingenious contrivances for applying electricity to the driving or regulating of clocks, the two most important and approved inventions have been those of Mr. Bain and Mr. Jones. The principle of Bain's electric clock, invented in 1840, may be thus described :-The bob of the pendulum is an electromagnet or coil of insulated wire, with a short, hollow, soft-iron core, the extremities of the wire being connected with the two suspending springs of the pendulum. On each side of the bob is a permanent steel magnetic bar, with the two like poles facing each other, and so placed that the pendulum swings partly over each without touching. By a simple contrivance at the top of the pendulum, a current sent through the coil of the bob from a local battery, which causes attraction between one of the steel magnets and the face of the bob next it of opposite polarity, is reversed when the pendulum has reached the extremity of its swing; repulsion between similar poles is the result, and the pendulum swings to the opposite side, until a break and reversal of the current once more reverse its motion. With this arrangement no driving weight or springs are required, and it was intended that a simple and cheap battery, consisting merely of a plate of zinc and a plate of copper buried in the damp earth, should suffice to keep up the oscillation of the pendulum and the motion of the clockwork. In practice, however, it has been found that such a driving power is unreliable for regularity, and not to be compared with the regularity of a falling weight or an un winding spring. Mr. Jones has improved very much on Bain's principle by using the current merely to regulate the motion of an ordinary clock. He attaches a pendulum with electric break and make arrangements quite the same as those of Mr. Bain, to a common clock driven by weights or by springs; and he arranges a standard or governing clock, which is supposed to keep practically perfect time, and which 748 Induced Electric Currents. may be situated at any distance, as in an astronomical observatory, so that at stated intervals the pendulum of the standard clock shall touch a spring and complete a battery circuit, which shall send a current through the distant electro-magnetic pendulum. If the latter be not in proper position when the electric wave passes through it, it receives an impulse which suffices to give it the needed acceleration or retardation, and keep it up to time. In this way one, or, in fact, any number of ordinary clocks may be made to possess all the regularity of the best astronomical clock. A clock of this kind is now in use in almost all the large towns of the kingdom for giving true time; and it produces the most satisfactory results. 986. The practically instantaneous passage of the electric current has also been applied to the determination of extremely short intervals of time, as, for instance, the time taken by a cannon ball in its passage between different stages of its course. The ball is made to break wires in its passage, and so interrupt electric circuits which govern the action of electro-magnets; these electro-magnets hold marking pointers against a cylinder turned by clock-work at a certain uniform rate, and the interval between the release of the pointers and the end of the indicating lines can thus be estimated with very great nicety. The times taken by a cannon ball to pass along the different parts of the bore of the cannon have even been found by this means, and the rate of acceleration of the expansive force produced by the explosion of gunpowder has in this way been estimated. INDUCED ELECTRIC CURRENTS. 987. The illustrious Faraday, to whom the science of electricity owes so much, discovered, in 1830, that when a wire, whose ends were connected with a galvanometer, such as we have described in Art. 981, was brought near another wire through which a voltaic current was passing, there was a slight affection of the galvanometer needle. When the wires were separated there was again a slight indication, as of a momentary electric wave having passed through the closed galvanometer circuit. Further experiments regarding this phenomenon revealed that it was a case of current induction or electric influence at a distance, somewhat analogous to that already considered under the head of frictional electricity (Art. 939). Unlike Ruhmkorff's Induction Coil. 749 the latter, however, this current induction was but momentary in its effects, taking place only when a galvanic circuit was brought near or removed from another metallic circuit, or when a galvanic tircuit was broken or commenced in the presence of the other closed circuit, for it is evidently the same thing to break the galvanic circuit as to remove it to a distance. It was shortly discovered that the induced or secondary current, as it was called, was opposite in direction to the inducing or primary current when the latter current was completed or brought near the former; and in the same direction as the primary when the latter was broken. Further, the nearer the two wires could be brought without metallic contact, the stronger was the induced current found to become; and the effect was very greatly intensified by winding the two wires, insulated by being overspun with silk, side by side on a reel or bob bin, as shown in fig. 285. Thus, if E F be the ends of the one wire, which are connected with a voltaic battery, and C D, the extremities of the second or secondary wire, it is found Fig. 285. by rapidly making and breaking the current flowing through E F (as may be done by interposing a file in the circuit and drawing the end, E, across the teeth of the file) that a perfect stream of induced currents, alternately in opposite directions, flows between the points, C D, when placed near each other. The induced electricity was found to partake more of the nature of high tension or frictional than of the massive current electricity. It will charge a Leyden jar very rapidly, will give most powerful shocks, and will produce all the beautiful luminous effects of the friction electrical machine. The best form of apparatus for producing these effects is that known as Ruhmkorff's Induction Coil. 988. Figure 286 represents one of the most compact and convenient forms of this coil, as given by the original constructor himself. The essential parts of it are an iron core, or core of iron wires, bound firmly together, and seen protruding at the nearer end of the instrument, in the figure. On a bobbin with glass ends cemented upon it, and into which this core fits, is wound, first, a coil of thick insu |