Page:Popular Science Monthly Volume 3.djvu/603

Rh 100 turns. The outer end of the coil of one section forms the commencement of the first coil of the next section, and so on. The whole of the wire is therefore divided into 40 equal sections, being, however, continuous throughout.

To understand better how an uninterrupted current is produced, let us imagine a line to be drawn equatorially, or perpendicular to the lines of force between the poles of the horseshoe magnet, and dividing the ring armature into two parts; suppose, likewise, that to the two ends of one of the 40 coils two wires are soldered, the other ends of which are attached to a galvanometer. Now let the ring be intermittently revolved in one direction, so as to give to the said coil a



succession of movements of about 10 degrees, stopping each time to permit the galvanometer-needle to resume its normal position. It will then be seen that the whole time the coil is above the equatorial line the galvanometer-needle will be urged in the same direction, and the currents may be called positive. But, as soon as the said coil crosses the equatorial position, the currents generated in it will be negative, and in the opposite direction to what they were at the other half of the circle. This experiment shows that a reversal of the direction of movement carries with it a reversal of the direction of the current.

From this insight into what is produced in one of the sections, the general phenomena produced by the whole circle of coils are easily understood. The 20 sections which are on one side of the equatorial position are the source of positive currents; these may be of unequal intensity among themselves, but, for a uniform velocity of rotation, their sum is evidently constant; for, as one coil crosses the equatorial line from north to south, an opposite one comes up from south to north to take its place. On the other hand, the 20 sections which are on the other side of the equatorial line are the seat of negative currents, the sum of whose intensities is likewise constant, and equal to that of the positive currents.