teeth; this ring is supported by four arms or vertical pieces of brass B B (Fig. 27) which connect it to the shaft of the machine. Between the teeth small triangular wooden prisms are placed, and into the hollow spaces between the prisms and the teeth coils of silk-covered copper wire are fitted. This arrangement aims at obtaining between the iron teeth of the wheel a perfect insulation of the bobbins thus formed. In all the bobbins the wire is coiled in the same sense, and each of them is formed of nine spirals. Two consecutive bobbins are separated from one another by an iron tooth of the wheel, and by the small triangular wooden prism. On leaving one bobbin to construct the following one, I fix the end of the copper wire to the piece of wood which separates the two bobbins.

"On the axle which carries the wheel thus constructed I have grouped all the wires. One extremity of these wires forms the end of one bobbin, and the other the commencement of the following bobbin, and this is effected by causing them to pass through holes made for this purpose in a wooden collar centred on the same axis, and then attaching them to the commutator which is likewise mounted on the axis.

"This commutator consists of a small wooden cylinder having on its circumference two rows of sockets, into which fit sixteen brass pieces, eight into the upper, and eight into the lower sockets, all concentric with the wooden cylinder, over which they slightly protrude, and whose thickness separates one row from the other.

"Each of these brass pieces is soldered to the two ends of wire which correspond to two consecutive bobbins, so that all the bobbins communicate with one another, each of them being connected to the following one by a con

ductor, part of which is formed by one of the brass pieces of the commutator. If, therefore, two of these brass pieces are placed in communication with a battery by means of two metallic rollers G, the current on dividing will traverse the helix on either side of the point from whence start the ends of the wires attached to the brass pieces which communicate with the rollers, and the magnetic poles will appear in the iron of the circle on the diameter perpendicular to A A'. On these poles act the poles of a fixed electro-magnet, which determine the rotation of the transversal electro-magnet round its axis, since in the transversal electro-magnet, when it is in motion, the poles are always reproduced in the fixed positions which correspond to the communications with. the battery."

Pacinotti's claims to priority are no longer a matter of dispute, and it is to be regretted that he did not fully realize the importance of his discovery, leaving to Gramme the practical and industrial development of his principle.

Gramme's machine, however, in spite of the priority claimed by Pacinotti, has a real and absolutely personal value, justified, moreover, by its many successful applications which we shall describe in this volume.

To understand the action of Gramme's machine, let us recapitulate the simplest experiment of induction. Let us suppose a magnetized bar, one metre long, and a spiral conducting wire in reciprocal motion; if the spiral is brought near the bar, an induction current is developed in the former. Let us suppose that the bar penetrates into the spiral by a series of successive, uniform movements. It will be observed that an induction current corresponds to each of these movements. All these currents are of the same sense, until the

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spiral arrives at the neutral line; they alter their sense if the movement continues in the same direction beyond the neutral point, the iron ring is magnetized, and the magnetism is distributed in it in the following way :—

A and B (Fig. 28) are the poles, M and M' the neutral points.

By turning the ring, this distribution of magnetism is not altered; the poles are fixed in space, although the ring is displaced, because the coercive force of the soft iron of which it is made is zero, or can be neglected. The effect is,

therefore, the same as if the iron were immovable, the wire spirals alone moving over a magnetized bar.

In each of the elementary coils, therefore, a current is developed which, starting from one of the poles A, remains direct up to the neutral line M, takes the inverse sense from M to B, keeps that sense from B to the neutral line M', and becomes direct again from M' to the pole A. The current, therefore, remains of the same sense from one neutral point to the other.

All the coils which, at a given moment, are in the upper semicircle, are all at the same time traversed by currents of direct sense, which are joined for tension; the coils of the lower semicircle are traversed by currents of inverse

sense, also associated for tension. A state of equilibrium is thus established, and the system can be compared to two series of batteries, equal in number and in power, acting in opposition. By putting the two extremities of an external circuit in communication with the opposite common poles of the two batteries, these batteries are associated for quantity; in the same way, by establishing

Fig. 29.-Gramme's laboratory machine, with ordinary magnets. collectors on the neutral points of the ring, the currents developed in the ring can be collected. These collectors are formed of copper brushes, which rub against a series of radiating pieces R (Fig. 28) insulated from one another, and each connected with the end-piece of one coil and the beginning piece of the next. These currents are therefore, by the action of the collectors, transferred from the soldering of the two coils to the piece R.