Page:All the Year Round - Series 2 - Volume 1.djvu/629

, and the primary current directly proceeding from it.

Now, Faraday's discovery was, that this galvanic or primary current, at the moment when it to begins flow, and again at the moment when it ceases to flow, produces a secondary or induced, and perfectly independent current, in another conductor wound around the first, but not in contact with it. At the moment when the primary current begins to flow, the induced current passes in the same direction with it; but at the moment when the primary current ceases to flow, the induced current passes in the opposite direction. Instead of being, as in the primary current, continuous, the induced current is only momentary; and, in order to produce it at pleasure, it is necessary to have some contrivance by which to cut off and to restore the primary current as often as may be desired. As often as it is cut off, the reverse induced current passes; as often as it is restored, the direct induced current passes. The instrument used for this purpose is called a break, or contact breaker. It is placed in a gap in the primary or galvanic circuit, communicating with one extremity of the gap, and capable of being made to touch the other extremity also. When it touches, it is said to "make" contact, and, when it ceases to touch, it "breaks" contact.

Not only does the magnet, like the primary current, induce electricity, but a piece of soft iron is rendered magnetic during the passage of a primary current through a coil of wire surrounding it. If the iron be massive, it retains its magnetic quality for a few moments after the galvanic current ceases; but, if it be of small bulk, it gives up its magnetism immediately. In the manufacture of a "coil" for the display of induced electricity, all the foregoing facts are taken into account. The centre, or core, of the coil is formed of a bundle of soft iron wire. Around this is wound the wire for the primary current, and around this again the wire for the secondary current. When the ends of the primary wire are connected with the two poles of a galvanic battery, the core of iron wires becomes a core of magnets, and hence assists the primary current in inducing electricity in the secondary wire. When the ends of the primary wire are disconnected from the battery the core ceases to be magnetic, and the withdrawal of the magnet assists the cessation of the primary current in again inducing electricity in the secondary wire.

The largest induction coils hitherto made have been about a foot or fifteen inches in length, by about four inches in diameter. Seven miles has been about the extreme limit of length of the secondary wire; and nine inches the greatest length of spark that could be obtained. With these figures as standards of comparison, we approach the "monster coil" now under consideration.

In this, the central core of iron wires is composed of pieces each five feet long, and the thickness of knitting needles, the whole core being five inches in diameter. The primary wire is of copper, thirty-seven hundred and seventy yards in length. The secondary wire is also of copper, and is one hundred and fifty miles in length. The rods of the core are separated from one another, or insulated, by being wound round with cotton, and the primary wire is covered in a similar manner. The secondary wire is covered with silk; and all these coverings are required in order to force the current to keep within each wire, or to pass along its length, instead of escaping from it laterally to contiguous turns of the spiral. The whole apparatus is enclosed within cylinders of vulcanite, and is mounted on strong supports, themselves similarly covered. The ends of the secondary wire issue one from each extremity of the coil, and are connected to "terminals," one of which is a point, and the other a polished disc of metal. They stand on movable columns in front of the coil; and the wires, when necessary, can be detached from the terminals, and attached to any other apparatus that may be required. When the primary wire is connected with a powerful galvanic battery, and contact is made, the core becomes a bundle of magnets, and this bundle combines with the primary wire to induce an electric current in the secondary wire. When contact is broken, the primary current ceases to flow, the core loses its magnetism, and an electric current is again induced in the secondary wire. If the terminals be not too far apart, this induced current leaps across the space between them in the form of a visible spark or flash.

There is yet another piece of subsidiary apparatus, called the condenser. This consists of a number of small sheets of insulated tinfoil, connected together, and with the primary wire, to which they form a sort of loop circuit. The condenser is supposed to afford a safety-valve, or reservoir of space for the primary current, and a security against any injury being done to the