MA GNETO-ELE CTRIC ILL UMINA TION. 



587 



100 turns. The outer end of the coil of one section forms the com- 

 mencement 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 intermit- 

 tently revolved in one direction, so as to give to the said coil a succes- 



sion of movements of about 10 degrees, stojDping 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 cur- 

 rents 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 cur- 

 rents, the sum of whose intensities is likewise constant, and equal to 

 that of the positive currents. 



