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the ends of the wire to the battery-. If you apply iron filings or nails to either end of the bar, they will be immediately attracted; disconnect one of the w ires from the battery, the filings will drop off, and the iron will have lost it attractive power; connect the battery wire, attraction will again follow. From this you will perceive that if a bar of soft iron be surrounded with wire, it becomes

magnetic so long as an electric current passes through the wire. The polarity of this electro-magnet is dependent on the direction the wire is wound round it.

I nstead of inserting in the helix a bar of soft iron, insc-t a bar of steel: the steel, on passing a current through the wire, becomes also magnetic; remove the steel,and it will be found that, unlike the soft iron, it retains its magnetism.

The bar shows an electro-magnet of small power; they can, however, be made of great power, and able to support greater weights than natural or artificial magnets. The common form of electro-magnets is that of a horseshoe, with a large quantity of insulated wire wound in the same direction round each pole.*

Induction by Voltaic Currents.— If you complete a battery circuit, as in Fig. 14, and place near it a second wire, forming also a complete circuit but unconnected with the battery, it will be found that a current of electricity is induced in the second wire, but with this peculiarity, that the current is induced momentarily: when the battery circuit is closed, it almost immediately ceases; but on breaking the battery circuit, another current is perceived in the second wire, but in the opposite direction to the first. By increasing the quantity and proximity of the two wires, an induced current may be obtained of sufficient strength to produce sparks. Induction by Magnetism.—A similar result to the above may be obtained

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(without the use of a voltaic current) by a steel magnet. If a helix of wire (Fig. 26) be prepared and connected with a galvanometer, a deflection will be observed immediately a strong bar magnet is inserted within it—this current is of short duration; withdraw the magnet, and a current in the opposite direction will be observed.

Another method, and that now usually adopted for producing "magnetoelectricity" (as this kind of electricity is called), is by making a small horseshoe electro-magnet. On bringing it near the poles of a powerful permanent magnet, a strong current is induced in the wire of the electro-magnet. By fixing this electro-magnet on a pivot near the poles of the permanent magnet, and giving it a rapid rotation, intense currrents are induced.

• At page 667 will be found a description of the mode of construction of an clectro-nugnet .

Induction Coils.

An explanation of the above facts is rendered necessary before you could comprehend the construction of, or results obtained from, a Ruhmkorff's induction coil; for in this coil, to which I have been gradually leading you, you will find that induction by voltaic currents and by magnetism plays the most important part.

Now, we have seen that one wire induces a current in a contiguous wire; if a helix of a second wire of much smaller diameter than that just used be placed around the helix with the soft iron in it, it will be found that the currents induced in this wire on making and breaking battery circuit are greater in strength than when the bar is removed, the presence of the bar most materially increasing the induction. This rough arrangement is the foundation of an induction coil: the helix round the soft iron is the primary coil, the outside wire is the secondary coil. By increasing the size of the magnet and the lengths of the primary and secondary wires, instead of a faint spark, we obtain a flame of light many inches in length; but the principle of the coil remains the same.

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To M. Ruhmkorff is due the credit of perfecting the induction coil which now bears his name. The coil consists of the following parts:

1. The core of soft iron, or electro- 3. Secondary coil.

magnet. 4. Contact-breaker.

2. Primary coil. 5. Condenser.

In experimenting upon electricity there are no more beautiful experiments than those performed with Ruhmkorff's intensity coil. I therefore give my readers a description of a very useful coil, capable of carrying out a great variety of experiments. For this description I am indebted in a great measure to a small but very interesting volume on " Intensity Coils," by Dyer, recently published.

The core of soft iron consists of a bundle of small iron wires of No. 18 gauge; the wires should be straight and tied well together, the whole forming a bundle of £ in. diameter and 7J in. in length. The ends should be filed smooth and slightly varnished.

The Primary Coil is wound on a reel consisting of a cylinder of cartridgepaper (see section), having an ebonite disc at each end 3.J in. in diameter and 1J in. thick; the tube should be 7 in. long and I in. in diameter. Upon this is wound the primary wire; but before winding, to prevent damage to the reel, it should be filled up inside with a wooden roll. Two holes should be made in one of the dises to receive the ends of the primary wire, which should consist of about 1 lb. of cotton-covered wire, No. 15, measuring about 20 yards. One end of this wire should be passed through one of the holes in the disc and wound carefully on the reel to the opposite disc, and then back again, so as to form two layers; the end should then be passed through the remaining hole in the disc The layer should then be well and carefully varnished with several coats of shellac dissolved in spirits of wine; a second coat must not be put on until the first be dry. When dry, the primary wire must then be tightly covered with cartridge-paper passed several times round and gummed down. When dry, it must be varnished.

Secondary Coil.—In this, the opposite disc, one above and the other below, should be two holes for the ends of the secondary wire, which consists ol about 800 yds. (\ lb.) of No. 32 tt/t-covered copper wire. This wire requires great care in the handling, and the operation of putting it on is one of great delicacy. Pass the lower end through the lower of the holes, and then wind on carefully until the first layer is complete, and then having varnished it, and covered it with four or five layers of very fine gutta-percha sheet, a second layer may be wound on: the sheeting must not be put on until the vamish be dry. And so on, layer after layer, varnish and sheeting between, until the wire is finished and the coil complete; the end should then be passed through the other hole in the disc As each layer is put on, an observation should be taken with your galvanometer and battery, to see whether the wire is perfect and not broken. When the wire is entirely wound on, it should be protected by a series of coatings of gutta-percha, and with an outside wrapper of leather or other material. Very often, to increase the strength of the coil and decrease the danger of the small secondary wire breaking where it comes through the disc, it is soldered on to a larger copper wire, which is wound round the secondary, and forms its outer layer. Where the secondary wires come through the disc, they are usually made into a helix.

The Contact-breaker, or Interrupting Apparatus, performs a very valuable part in the performance of the coil; in fact, it is the arrangement whereby we are enabled to utilize the wonderful effects of induction. In our previous experiments we have seen that the effects of induction only took place on making or breaking battery connection: in order to effect this, we introduce the ingenious arrangement known as a " contact-breaker." There are various forms of it, but the principal remains the same—the invention of Dancer. In Fig. 28, fixed on a brass pillar, C, in connection with one end of the primary wire, is a spring with a small armature at the end of it, facing the bundle of wires, and at a small distance from them. Between the armature and the pillar holding the spring is a small pillar with a screw through the upper part, the end of which, projecting through, touches the spring, and regulates its distance from the iron bundle. The end of the screw, and the part of the spring it touches, should be faced with platinum. The pillar holding the screw is in connection with the terminial P, N being in connection with the other end of the primary wire. Now bring the two ends of a battery into connection with p N; a circuit will be at once established through the coil, the spring and the screw which rests against the spring. But we have seen that a current of electricity passing round soft iron makes it magnetic, so our bundle of iron wire becomes magnetic, and attracts to it the armature at the end of the spring. This action immediately results in severing the connection between the spring and the screw; the battery at once ceases to act; the iron wires are no longer magnetic; the spring armature immediately resumes its original position, but as soon as it does so the battery connection is again

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established, the armature attracted, the result being that the armature is kept oscillating between the two points so rapidly as to produce a loud humming sound and vivid sparks at the point of contact. We therefore obtain the result we wished, and our battery circuit being so rapidly made and broken, we are thereby enabled to provide a self-acting arrangement, in order that we might profit by the induction which takes place in the secondary wire, which experiment has shown to us only takes place on the making and breaking of battery contact. The more rapid the action, the greater frequency of the induced current.

The Condenser.—This is an invention of Fizeau, and was first applied by Ruhmkorff to his coil with great effect. It is, in fact, a peculiar arrangement of a Leyden jar, and consists of sheets of tinfoil insulated from each other by silk, paper, or paraffin. The original appeared to have been made of two large sheets of tinfoil, separated by an insulating coating, and then placed between two insulating coatings; they were rolled up into the desired size, and connected with the contact-breaker. For a coil of the dimensions I have described, Dyer gives the following as the capacity of the condenser:

"Fifty sheets of tinfoil, 5x5; sixty pieces of varnished paper, 7x5; and two mahogany boards, varnished on each side, but rather smaller. Upon the lowest board place five pieces of the varnished paper; upon that one piece of tinfoil, overlapping at one side about 1 in.; then paper; then a second piece of tinfoil, but overlapping at the other side; then paper, tinfoil, and so on to the end; care being taken that the alternate pieces of tinfoil overlap on the same side, that is, the odd numbers to the left or one side, the even numbers to the right or other side. When the last tinfoil is put on, place above it the remaining pieces of paper and the other mahogany board. It is then ready to be placed in a box under the base of the coil."

The various parts being constructed, it is necessary to fix the completed coil and the condenser upon a proper frame. A wooden base, large enough to hold the parts, should be constructed, fitted at one end with the contactbreaker (Fig. 28); at the other end, for receiving the wires of the secondary coil, should be two insulated supports, having at their upper ends two double binding-screws, the one for the coil wire, the other for the experimental wire. The condenser should be fixed under the base in a hollow, and tightly screwed in; but before doing so a connection should be made between the one set of plates and the pillar, C, holding the spring armature, and between the other set of plates and the pillar holding the adjusting-screw. These pillars, to facilitate their connection, should project through the wooden base. Connect the primary wires to c N, Fig. 28, and the secondary wires to the terminals at the other end.

This coil must not be worked with more than six cells, as it may be hopelessly damaged.

If you obtain a coil, or make one, similar to the above, or at any rate if you wish to try any experiments with the induction coil, I would recommend you to be careful, as the shocks are too strong to be taken with comfort, and use well-insulated wires to connect to the secondary terminals.

When you have finished any experiment, disconnect one of the battery-wires from the coil (you can arrive at this result by unscrewing the contact-breaker), and do not connect it on again until you have the secondary wires ready connected for another experiment.

Never keep the battery-wires connected unless you are performing some experiment, otherwise you allow the battery to run to waste.

Fig. 29 shows an instrument called a discharger, which is a very necessary addition to the induction coil.

On a small stand are fixed two insulated supports, having at their upper

end a small tube; beneath it a binding-screw for attaching the coil wire. Through this tube passes a small brass rod with an insulated handle; at the opposite end is a screw for attaching wires, or whatever may be required to F,G- "9" be experimented with. In the centre

of the stand is placed a small table of insulating material. By making the top of the supports movable, an improvement may be effected.

The following are a number of experiments that can be performed with an induction coil.

\st Experiment. — Connect the battery: an intensely vivid spark will be seen at the contact-breaker. This is called the primary spark, and appears continuous, though not so in reality: it is due to the making and breaking battery contact.

2nd Experiment.—Join two wires to the terminals of the secondary coil; bring them close together (hold only one in your hand): a magnificent spark

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