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ing circuit; while B' is the “secondary or induced circuit. If, then, the poles of a voltaic battery are connected to the ends of the coil through a device for making and breaking the connection between them, every time the connection is made a pulse or jet of current will traverse the coil B' in a contrary direction to the flow of the current in B; and every time the connection between the battery and B is broken, a similar jet or current will traverse the coil B' in the

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same direction as that of the current which had just been flowing in B. Consequently, by rapidly making and breaking the connection between B and the battery, a rapid succession of contrary jets or pulses of electricity is set up in B', and as these jets are of a much more intense nature than the current in B, if we keep the two ends of the coil B' almost but not quite touching, we shall get a stream of sparks between the points of the wire. The sparks will be strongest when B is

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inside B', but their strength is only moderated by separating the coil further apart. The apparatus shown is known as the “sledge” coil, because B' can be slid along a graduated scale = 1, so as to moderate the sparks to different degrees of intensity.

The interruption of the current in the "primary' coil is effected automatically by means of an electromagnetic contact breaker shown at D and E. The wires from the voltaic battery come to binding-screws A a', one of which A' is connected to one end of the coil B through the coils of a double electro-magnet D and the wire i; while the other terminal A is connected to the other end of the coil B through the standard a, the arm E, and the screw v. Now the current flows from one pole of the battery to a, then up the standard a, along

A the arm E, and up the screw v with which it is in contact, through the coil B, and back to the other pole of the battery through the coils D of the electro-magnet. But in traversing the coils Dit magnetises the iron core in these coils, and thus attracts a piece of soft iron or “armature” at the end of E, and pulls the latter down, thereby breaking the contact between E

This interrupts the current in the coil B, and demagnetises the poles D. The arm E therefore springs back into contact with v, and the current is again estab

, lished in B, only to be interrupted and established as before. This automatic making and breaking of the current in B goes on as long as the poles of the battery are connected to the terminals A A', and hence a corresponding succession of alternating currents is maintained in B'. These result in a continual stream of sparks between the terminals of that coil.

To increase the inductive effect, a bundle of iron

and v.

wires forming a straight“ core c is usually inserted in the hollow of the primary coil B; and sometimes the magnetism excited in this core is used to attract the armature E, and break the circuit instead of the magnetism excited in the extra magnet D. A metal point dipping and re-dipping into a pool of mercury is also employed instead of the metal arm E touching and untouching the contact screw v. These, however, are differences of detail. So also is the insertion of the device known as a “condenser” in the primary circuit, to enhance the inductive effect on breaking the circuit, and subdue it on making the circuit, thereby throwing into relief the sparks due to the opening circuit, and to a certain extent suppressing the others. Herr Ruhmkorff has likewise added a simple and useful device, termed a “commutator,” for reversing the battery current in the primary circuit at will. It consists in connecting the poles of the battery to two brass cheeks on a small ivory barrel, which is mounted on a level axle, so that by turning it to one side or the other the cheeks make contact either way with two vertical sp;ings connected to the ends of the primary.

Various patterns of induction coils are now made by instrument makers for medical purposes; it being found that the gentle “shocks” produced by sending

“ the stream of induced currents from the secondary coil through the body have a stimulating effect in cases of paralysis. Induction coils are also used in telephony, as we shall see later on. In experimental physics they are employed to produce the variegated glows of the electric discharge in rarefied gases, and Mr. Spottiswoode, President of the Royal Society, has constructed one for researches of this kind, which gives a spark 423

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inches long, when worked with 30 Grove cells. The secondary coil of this giant “Inductorium contains 280 miles of wire.

The induction coil is based on the phenomenon of electro-dynamic induction or the induction of moving clectricity; but Faraday mated this discovery by also finding that the motion of a magnet near a wire induced a current in the latter. This action is called

. magneto-electric induction, and it does not matter whether the magnet moves and the wire is kept still or the wire moves and the magnet is kept still. All that is necessary is that there should be a relative motion between the two, and that the wire should as it wero pass through the magnetic space, or “field,” between the two poles of the magnet. The strength of the current developed in this way depends of course on the power of the magnet and the resistance of the wire employed; but with the same magnet and wire the current is stronger the quicker the wire is moved

through the “magnetic field,” w

and the fairer it crosses the field at right angles to the line joining the two poles of the magnet. Thus, if n s, Fig. 15, are the two poles of a magnet, and w a wire passing through between

them, the current induced in FIG. 15.

it, shown by the arrow, will

be stronger when it traverses the “lines” in the magnetic field at right angles in this way than when it crosses at a slant, as shown by the dotted line w'.

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This twin discovery of the immortal physicist was pregnant with innumerable inventions, and especially those machines in which electricity is generated from mechanical power applied to rotating magnets. These will have to be considered in a separate chapter.

We have now reviewed some of the principal facts of electricity, and in succeeding pages we shall describe the chief applications which have been made of them. We may naturally inquire, what is this mysterious agent which is accomplishing so many wonders ? The wisest electrician of our day can only shake his head and confess his ignorance of the answer. There have been many theories of electricity, but none of these can yet be taken for the truth. It has been called a "fluid," but it is not now regarded as matter at all; it has been called “ form of energy," but there are reasons for believing that even this very vague definition is incorrect. We know, however, that it is universal, or seemingly so, and that it is connected with every kind of physical change, from the rotting of a withered leaf to the outbursts on the surface of the sun. The whole earth is evidently charged with it, and it is visible in the comet's tail as well as the Aurora Borealis. be transformed into heat, light, magnetism, motion, and hence the true secret of it is evidently to be sought within the deeps of Nature. Professor Challis, of Cambridge, long ago surmised it to be due to the elasticity of the ether, which is more than believed to pervade all bodies; and if the recent experiments of Professor Bjerknes, of Christiania, yield the proper clue, electricity is nothing more than a peculiar wave motion of the ether. Professor Bjerknes imitates all the attractions and repulsions of magnetism and elec

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