beyond this, little. For though it may enlarge the luminous circle of our knowledge, it must also widen the dark boundaries of the Unknown. A little light but guides us to deeper mysteries; and even if we should find out all the secrets of molecular movement and structure, all the intersection of the working parts of matter, we are still confronted with the vast enigma, What is Matter? what is the stuff out of which Nature is made? That is a riddle which we can never solve.

Of the still more awful problem, Who made it? Science has also no solution, and can have none. It is neither experiment nor reason which can fully satisfy us here.

Revelation alone affords the true answer. It takes up the wondrous story where science is constrained by its natural limitations to stop, and finds the solution in Him "who is the image of the invisible God, the first-born of all creation, for in Him were all things created, in the heavens and upon the earth." The mystery of creation and the mystery of salvation alike centre in Christ. He it was "who His own self bare our sins in His body on the tree," and He it is who was in the beginning with God. All things were made by Him, and without Him was not any thing made that hath been made."-ED. R. T. S.

CHAPTER VIII.

THE DYNAMO-ELECTRIC MACHINE.

THE electric light was first discovered by Sir Humphrey Davy in the year 1800, and for the last fifty years or more electricians have tried to make it of practical service to the world. Their efforts, however, were not very successful, because the source of electricity they employed was the voltaic battery. The current from such a source was not steady enough, and, moreover, a very large and cumbrous battery was required. Recent improvements of the magnetoelectric generator of electricity have, however, enabled them to procure a constant current of electricity by means of mechanical power, and led to that development of the electric light which is the topic of the day.

All magneto-electric generators, whatever their peculiar forms may be, are based upon the great discovery of Faraday, that if a closed wire or conducting ring is moved across a magnetic space a current of electricity is generated in the wire. A magnetoelectric generator is simply the best apparatus that can be devised for applying this principle to the production of an electric current. A magnetic space is provided between the poles of two or more powerful magnets,

and ccils of wire are caused to traverse this magnetic space in such a way as to excite a current in them. The stronger the magnetism of the space, the longer the wire, and the quicker it is moved, the stronger will be the current excited. Therefore, the aim of inventors is to construct their machines with powerful magnets and coils of wire having many turns, and to rapidly rotate these coils through the magnetic "field" by mounting them on an axle driven at a rapid rate by means of a pulley and a running belt from some steam-engine or other motor. Now as each coil or bobbin of wire passes between the poles of the magnet, a transient current is generated in it; but as there are a number of bobbins rapidly following each other, each with its transient current, the joint effect of the whole is a practically continuous current. By means of a collecting device termed a "commutator," these parcel currents are gathered up one after another and led away as one steady current.

In the magneto-electric generator the magnets are of steel permanently magnetised; but a more powerful magnetic field can be obtained by employing electro-magnets, or cores of soft iron encircled with coils of insulated wire. To excite these magnets, however, a current of electricity is required to circulate in the coils surrounding the cores. This current may either be obtained from a supplementary generator, or it may be derived from the coils of the machine itself, for there is always enough "residual" or left magnetism in the core of the electro-magnets to form a weak magnetic field, which will excite feeble currents in the coils as they are driven through it. These feeble currents, diverted into the electro-magnet, help to

excite it still more and strengthen the magnetic field. The result is yet stronger currents in the coil, which in turn excite still stronger magnetism in the field, and thus, by a reciprocal process of give and take the electro-magnetism is built up to a high degree of intensity, and a powerful current is generated in the coils. Part of this powerful current can be led away by the commutators, for some external purpose, leaving part to feed the electro-magnets; or the whole current after it has fed the electro-magnets can be led away for other uses, such as the production of light in an electric lamp.

Fig. 52 is a skeleton diagram of a generator of this

kind drawn by Professor Silvanus P. Thompson, of Bristol, and it explains the action of the Gramme, Brush, and other machines. Here N and s are the soft iron pole pieces of the two electro-magnets, between which the armature A revolves. This armature consists of a ring or "core" of soft iron wound with

coils of wire, but for the sake of simplicity only twelve turns are shown, all the turns being in one circuit, and each of them connected by a wire to a metal bar of the commutator c, carried by, but insulated from, the axle x of the armature. Two metal brushes bb' rub on these bars as they rotate past them and tap the current generated in the coils as the armature revolves. It will be seen that one of the brushes b is connected by wire to the coils of one of the electro-magnets, the other brush being connected to one wire running to the lamp, while the other electro-magnet is connected to the other wire running to the lamp. Now, as the armature is rotated from left to right, in the direction of the large arrows, currents of electricity are generated in its coils in the direction of the small arrows, and these being collected by the brushes, pass through the electro-magnets, heightening their inductive power, and then flow out to the electric lamps.

One of the best dynamo-electric generators is that of M. Gramme, shown in Fig. 53. It consists of two powerful electro-magnets м м and м' м', furnishing a strong magnetic field between the poles N s. In this space a series of coils of wire A, wound on an iron ring or hoop, and technically called an "armature," are mounted on an axle and rapidly rotated by a belt, from a motor running on the pulley P. The currents thus generated in the coils are led to a series of copper bars set round the axle at c, and a pair of metal brushes bb' rub upon these bars and carry off the current as the axle revolves. Each pair of bars as it passes the brushes delivers up its charge of current, and these successive charges make up the