wires forming a straight "core e is usually inserted in the hollow of the primary coil в; 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 421

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 electricity; 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 were 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

W

FIG. 15.

through the "magnetic field," 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 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'.

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 a 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. It can 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

tricity by means of little pulsating drums immersed in a vessel of water; when two drums pulsate together in time they repel each other, even as two similarly electrified particles of matter repel each other; and when they pulsate out of time they attract each other, just as two dissimilarly electrified particles attract each other. May it not be then that the atoms of matter steeped in the ether are like these pulsating drums in the liquid water, and attract or repel each other according as their vibrations are harmonious or discordant?

THE first electric telegraph put to any practical use was a short line erected in 1833 by Gauss and Weber, the celebrated German physicists, to connect the Observatory with their physical cabinet at the University of Gottingen. In 1837 Messrs. Cooke and Wheatstone applied their needle telegraph instrument to a

D