manent magnet; and this fact was taken advantage of by Wilde, who constructed what was really a double machine, consisting of a smaller one with steel magnets and a Siemens armature, the current from which was passed through the large electro-magnets of the other, thus exciting a magnetic field of much greater intensity than was before possible. In this field another armature revolved, and furnished the current utilized in the external circuit.

This was hardly accomplished when a most interesting discovery was made, almost simultaneously, by Wheatstone and Siemens, which enabled them to dispense with the permanent magnets entirely. It consisted in utilizing the minute traces of magnetism which exist in all iron, for the production of feeble currents, which, in their turn, excite a more intense magnetization, and in this way the cores of the magnets are quickly charged to saturation. The first machine involving this principle was exhibited by Wheatstone to the Royal Society, on February 14, 1867, and it is not necessary to say that it excited the greatest interest. By a remarkable coincidence, such as occurs now and then in the development of a scientific principle, on the same day a paper was presented to the society by Siemens in which the same improvement in construction was de

scribed. The coils about the electro-magnet are either a part of, or a "shunt" from, the main circuit. When the armature is started, either the whole or a part of the current circulates about the field-magnets; and, although it may be feeble at first, by its effect in increasing the magnetism of the cores, it very soon reaches its maximum. The discovery of this method of developing and maintaining the "field" must always be regarded as of the highest importance.

The production of dynamo-electric machinery received a fresh impetus from the construction of a novel machine by Gramme of Paris, in 1870; from which time, in fact, it passes from the experimental to the industrial and commercial stage. Gramme's generator produced practically continuous currents of constant strength, and its merits were due to the introduction of a new form of armature. In the form and construction of this armature, however, Gramme was anticipated by Pacinotti, an Italian, who had constructed a machine on this principle in 1860, but it had remained undeveloped. This armature is the basis of those used in several modern dynamos. Its operation will be better understood after a little further consideration of the conditions under which induction takes place, as discovered by Faraday.

Reference has already been made to the fact, that, in defining these conditions, Faraday made use of the expression "lines of force;" and the study of electro-dynamic induction is greatly facilitated by an understanding of this far

Curves formed by iron filings in the field of a bar magnet.

reaching conception. Every one is familiar with the experiment of sprinkling iron filings upon a sheet of paper, card-board, or glass underneath which a magnet has been placed. The arrangement of these filings, especially when the card-board is lightly tapped to facilitate their movement into lines and curves about the

poles of the magnet, is a very striking and instructive phenomenon. In the first of his "Experimental Researches," Faraday speaks of moving a wire so as to "cut" the magnetic curves, and explains in a foot-note as follows:

By magnetic curves, I mean the lines of magnetic forces, however modified by the juxtaposition of poles, which would be depicted by iron filings, or those to which a very small magnetic needle would form a tangent.

From that time these hypothetical lines have been usefully employed in the development of the principles of electro-dynamics. According to this theory, every region in which a magnet would be in any way acted upon or influenced is to be considered as a field of magnetic force. The region immediately surrounding the earth is such a field, as is shown by the tendency of a magnetic needle to rest in a certain direction. All fields of force are pervaded by lines of force, which are lines along which the force acts, and are defined as above. If a very small magnetic needle, suspended so as to be free to move in all directions, be brought into the field of force surrounding a magnet, it will come to rest in, or, more accurately, tangent to, a line of force. If it be moved somewhat from its first position, its direction will, in general, change, showing that the lines of force are not parallel. Such

a needle may be used for exploring a field of force; and when the lines are traced out, it will be found, as shown in the experiment with the iron filings, that the lines of force apparently spring out of the poles of the magnet, or, as is often convenient, they may be imagined to come out of the north pole, and to reunite on entering the south pole. They will be found to be most numerous in the immediate vicinity of the pole; so that they may conveniently represent the two essential elements of a force, direction and intensity, the latter being measured by the number of lines cutting through a given area, as a square centimetre, taken at right angles to their direction.

Experiment with the iron filings, or with the small exploring needle, shows that the position of the poles of a magnet in relation to each other determines the form, and to some extent the number, of lines of force at a given point; and the introduction of a third pole in the immediate neighborhood will be found to modify them materially. If two magnets have the opposite poles placed near to each other, or if a magnet be bent so as to bring its poles near together, it will be found that, in the region directly between them, the lines of force are very numerous and nearly straight.

Now, Faraday's investigations proved that,