is charged.  It can be discharged by joining its two poles with a wire, and letting the two opposite electricities on its plates rush together.  Now, the sudden discharge of the condenser C C through the primary coil P C enhances the inductive effect of the current.  The battery B, here shown by the conventional symbol [Electrical Symbol] where the thick dash is the negative and the thin dash the positive pole, is connected between the terminals T1 T2, and a commutator or pole-changer R, turned with a handle, permits the direction of the current to be reversed at will.

Figure 36 represents the exterior of an ordinary induction coil of the Ruhmkorff pattern, with its two coils, one over the other C, its commutator R, and its sparkling points D1D2, the whole being mounted on a mahogany base, which holds the condenser.

The intermittent, or rather alternating, currents from the secondary coil are often applied to the body in certain nervous disorders.  When sent through glass tubes filled with rarefied gases, sometimes called “Geissler tubes,” they elicit glows of many colours, vieing in beauty with the fleeting tints of the aurora polaris, which, indeed, is probably a similar effect of electrical discharges in the atmosphere.

The action of the induction is reversible.  We can not only send a current of low “pressure” from a generator of weak electromotive force through the primary coil, and thus excite a current of high pressure in the secondary coil, but we can send a current of high pressure through the secondary coil and provoke a current of low pressure in the primary coil The transformer or converter, a modified induction coil used in distributing electricity to electric lamps and motors, can not only transform a low pressure current into a high, but a high pressure current into a low.  As the high pressure currents are best able to overcome the resistance of the wire convening them, it is customary to transmit high pressure currents from the generator to the distant place where they are wanted by means of small wires, and there transform them into currents of the pressure required to light the lamps or drive the motors.

We come now to another consequence of Oersted’s great discovery, which is doubtless the most important of all, namely, the generation of electricity from magnetism, or, as it is usually called, magneto-electric induction.  In the year 1831 the illustrious Michael Faraday further succeeded in demonstrating that when a magnet M is thrust into a hollow coil of wire C, as shown in figure 37, a current of electricity is set up in the coil whilst the motion lasts.  When the magnet is withdrawn again another current is induced in the reverse direction to the first.  If the coil be closed through a small galvanometer G the movements of the needle to one side or the other will indicate these temporary currents.  It follows from the principle of action and reaction that if the magnet