These effects are much more powerful when, instead of lengths of straight wire, or single circles of wires, we use two coils of wire, one of which, namely, that which conveys the primary currents, is placed in the axis of the other. It must be distinctly understood that the secondary currents are of momentary duration only; they are not produced at all while the battery is flowing, but only at the time of its commencement and cessation. If, however, we make the primary coil so that it can be slid in and out of the axis of the other, then while the primary current is continuously flowing, we can produce secondary currents in the other coil, by causing the coils to approach or recede from each other. As we bring the coils near each other, and slide the primary into the secondary, the current in the latter is inverse; when the one coil is receding from the other, it is direct. These mechanical actions are not produced without expenditure of force, for the approaching coils repel each other and the receding coils attract each other. The setting up of the battery current in the primary coil when placed within the other is equivalent to bringing it, with the current flowing, from an immense distance in an extremely small time. Similarly, when the battery current is broken, it is equivalent to an instantaneous recession. The effects, therefore, are proportionately powerful. It is found, also, and this we shall presently refer to more fully, that when, instead of the primary coil, a magnet is similarly moved into, or removed from, the axis of the secondary coil, currents in opposite directions are set up in the latter without any battery being used at all. The direction of these currents is the same as would be produced by a primary current that would form, in a piece of iron placed in the axis of the coil, an electro-magnet with poles similarly situated to those of the magnet so introduced or withdrawn. Hence, by placing a bar of soft iron in the axis of the primary coil, the secondary currents will be produced with increased force. When a long secondary coil, having the turns of its wire well insulated from each other, surrounds a primary coil provided with a core of soft iron, or still better, with a bundle of annealed iron wires, a series of powerful discharges, like those of a Leyden jar, may be obtained between the terminals of the secondary coil, when the battery contact is made and broken in rapid succession.
Fig. 267.—Ruhmkorff’s Coil.
Such induction coils have been very carefully and skilfully constructed by Ruhmkorff, and are therefore often called “Ruhmkorff’s Coils.” One of these is represented in Fig. [267]. A B is the coil, and the apparatus is provided with what is termed a condenser, which consists of layers of tin-foil placed between sheets of thick paper, and alternately connected so that one set communicates with one extremity of the primary coil, and the other with the other. This condenser is conveniently contained in the wooden base of the instrument. Its introduction has greatly increased the intensity of the secondary current, and sparks of 18 in. or 20 in. in length have been obtained in the place of very short ones.
It should be stated that of the two secondary currents, only one has sufficient intensity to traverse the secondary circuit when there is any break in its continuity. This is the direct secondary current, or that which is produced on breaking the primary circuit. The reason is that the commencing current in the primary circuit induces in the spires of its own coil an inverse current, and the battery current therefore attains its full strength gradually, but still in a very short time; while, on the cessation of the battery current, the same induction sends a wave of electricity through the primary coil in the same direction, and then the current ceases abruptly. Consequently, in the latter case, the induced electricity of the secondary coil is set in motion in much less time, and therefore possesses much greater intensity.
The magnetism of the iron core is usually made use of to break and make the current, by the attraction of a piece of iron attached to a spring, which, by moving towards the end of the core, separates from a point in connection with the battery, and, the current no longer flowing, the magnetism ceases, and the spring again brings back the iron and renews the contact.
Fig. 268.—Discharge through Rarefied Air.
By means of such coils many surprising effects have been produced. Perhaps one of the most beautiful experiments in the whole range of physical science is made by causing the discharges of the secondary coil to take place through an exhausted vessel in the manner represented in Fig. [268]. A beautiful light fills the interior of the vessel, and the terminals appear to glow with a strange radiance—one being surrounded with a kind of blue halo and another with a red. On reversing the direction of the currents, which is done by the little apparatus at the right-hand end of the coil in Fig. [267], the blue and the red radiance change places. Beautiful flashes of light may also be made to appear in the vessel, having the most marked resemblance to the streamers of the Aurora Borealis. When, instead of vessels almost free from common air, we repeat the experiment with tubes containing an extremely small residue of some other gas, such as hydrogen, carbonic acid, &c., the colour of the light and other appearances change Geissler’s tubes have already been spoken of in connection with the spectroscope; but, independently of that, the various beautiful appearances which such tubes have been made to present, by the introduction of fluorescent substances and other devices, render the induction coil an instrument of the highest interest to the scientific amateur. Then there are striking physiological and other effects which the coil is capable of producing. For instance, we are able by its instrumentality to produce from atmospheric air unlimited quantities of that singular modification of oxygen which is called ozone. The electricity of the coil has been used for firing mines, torpedoes and cannons, and for lighting the gas-burners of large buildings.
