The correlation of the physical forces, heat, light, electricity, magnetism, and motion, is one of the most interesting subjects for study that can be suggested to the lover of science. The examination of the precise meaning of the term correlation, so ably considered by Professor Grove, indicates a necessary mutual or reciprocal dependence of one force on the other. Thus, electricity will produce heat, and vice versÂ; motion, such as friction, produces electricity, and the latter, by its attraction and repulsion, establishes itself as a source of motion. Electricity produces light, also magnetism, and contrariwise light is said to possess the power of magnetizing steel, whilst magnetism again produces light and electricity. Such are the intimate connexions that exist between these imponderable agents, and we may trace cause and effect and its reversal amongst these forces, until the mind is lost in the examination of the bewildering mazes, and is content to return to the beaten track and work out experimentally the practical truths. We have had occasion to notice in another part of this playbook the fact that a current of electricity causes the evolution of magnetism in its passage through various conducting media, and the truth has been specially illustrated by the various experiments in the chapter devoted to electro-magnetism. In commencing this portion of electrical science, we have no new terms to coin for the title of the discourse, as we merely reverse the other when we examine the nature and peculiarities of

MAGNETO-ELECTRICITY.

Clarke's magneto-electrical machine.

The source of the power must necessarily be a bar or horse-shoe shaped piece of steel permanently endowed with magnetism. If the former is thrust into a cylinder of wood or pasteboard, around which coils of covered copper wire have been carefully wound, so that the extremities communicate with a galvanometer, an immediate deflection of the needle occurs, which, however, quickly returns to its first position, but is again deflected in the opposite direction on the withdrawal of the steel magnet from the coil of copper wire. (Fig. 235.)

The rapid entrance and exit of the steel magnet in the helix of copper wire would be insufficient to produce any quantity of electricity, and the ingenuity of man has been taxed to arrange a method by which a magnet may be suddenly formed and destroyed inside a coil of insulated copper wire. The difficulty, however, has been surmounted by several ingenious contrivances, based on the principles first discovered by Faraday; and the one especially to be noticed is the revolution of a coil of copper wire enclosing a piece of soft iron, called the armature, before the poles of a powerful magnet. The first machine was invented by M. Hypolyte Pixii, of Paris, and in 1833, Mr. Saxton improved upon this machine, and three years afterwards, Mr. E. M. Clarke described a very ingenious modification of the electro-magnetic machine, which is depicted below. In this picture, the letter a is the permanent fixed horse-shoe magnets, which are very appropriately termed the battery magnets, because they take the position that would otherwise be occupied by a voltaic battery, and they are indeed the prime source of the electrical power that is evoked. d is the intensity armature which screws into a brass mandril seated between the poles of the magnets a, motion being communicated to it by the multiplying wheel, e. This armature or inductor has two coils of fine insulated copper wire of 1500 yards in length, coiled on its cylinders, the commencement of each coil being soldered to the bar d, from which projects a brass stem, also soldered into d, carrying the break-piece h, which is made fast in any position by a small binding-screw in a hollow brass cylinder to which the other terminations of the coils, f f, are soldered, these being insulated by a piece of hard wood attached to the brass stem. o is an iron wire spring pressing against one end of the hollow brass cylinder; p is a square brass pillar; q is a metal spring that rubs gently on the break-piece h; t is a copper wire for connecting the brass pieces with the wood l between them, and out of which p and o pass; r r are two handles of brass with metallic wires, the end of one being inserted into either of the brass pieces connected with p and o, and the other into the brass stem that carries the break-piece h, delivers a most severe shock directly the wheel is set in motion.

In Saxton's electro-magnetic machine, the permanent steel magnets are placed horizontally instead of perpendicularly, and are composed of six or more horse-shoe-shaped pieces of steel. The armatures, or inductors, or electro-magnets (for they consist of pieces of soft round iron with wire wound round them), are two in number, and are adapted to exhibit either quantity or intensity effects. The quantity armature is constructed of stout iron, and covered with thick insulating wire. The intensity armature is made of slighter iron, and covered with from one thousand to two thousand yards of fine copper wire coated with silk. The quantity armature is intended for the exhibition of results similar to those which are procurable from a voltaic battery, such as the magnetic spark, inducing magnetism in soft iron, heating platinum wire. The intensity armature is employed for the chemical decomposition of water and other bodies, and likewise for the administration of those terrible blows to the nervous system which cause strong men of the mildest deportment to become painfully excited, and to make those ejaculations which are so peculiar to the genus John Bull.

EXPERIMENTS WITH THE MAGNETO-ELECTRIC MACHINE.

First Experiment.

The decomposition of water by the passage of electricity from one platinum plate to another, has already been illustrated at page 198. The same fact may likewise be displayed by the following arrangement of the machine. (Fig. 236.)

a. Apparatus for decomposing water and collecting the gases separately. b b. Wires proceeding from the machine at m, n. q, works on the single break, h.