producing Arago's Rotation. 289 



These tongues were in two groups of five ; and in each group 

 the distances between the contacts of the tongues with the rim 

 were half the distances between the cuts in the rim. The dis- 

 tance between the two groups must be greater than the dis- 

 tance between the contacts. 



Let A be a wire from the positive pole of our battery, 

 and A! A! f be wires from the negative pole of the same battery; 

 B a wire from the positive pole of the other battery, and B'B" 

 wires from the negative pole ; and let a a' be the ends of the 

 coil round one pair of electromagnets, and b V the ends of the 

 coil round the other pair. In the figures l.to 4 the wheel is 

 seen in four consecutive positions, with the tongues in contact 

 with it ; and the letters show with what wire each tongue is 

 connected. 



The contact of two tongues with the same section of the 

 ring puts the wires to them into electric connexion. 



The connexions are, in 



Fig. 1. Aa, A' a', Bb, B'V, 



Fig. 2. A a', A" a, Bb, B f b f , 



Fig. 3. A a', A" a, Bb', B"b 



Fig. 4. A a, A' a', Bb', B"b 



Fig. 1. A a, A' of, Bb, B r U. 



Hence in passing from fig. 1 to fig. 2 the current through 

 a a f is reversed ; in passing to fig. 3 that through b V is re- 

 versed. The current through a a! is reversed again in passing 

 to fig. 4, and that through b V is reversed again in passing to 

 fig. 1. The commutator is thus seen to reverse each pair of 

 magnets twice while rotating through the angle subtended by 

 one division of the rim ; so that with eight divisions one turn 

 of the wheel reverses each pair of magnets sixteen times. 



If the wheel is rotated in the opposite direction, the series 

 of magnetic states is obtained in the reverse order, and the disk 

 rotates in the opposite direction. But there is a better me- 

 thod of reversing the motion of the disk — which is, to introduce 

 an ordinary commutator into one of the circuits, either between 

 the battery and the wheel, or between the wheel and the mag- 

 nets. The reversal of this commutator reverses the motion, 

 while the wheel is rotated continuously in one direction. 

 Fig. 5 gives the arrangement of the connexions. A, B are 

 the batteries ; a, a' , b, V the electromagnets ; C tho ordinary 

 commutator ; and J) the wheel. 



It should be noted that the rotation of the disk is accom- 

 panied by the formation of induced currents whose intensity 

 depends on the velocity of rotation, and whose effect is to di- 

 minish the rotation. These opposing currents are got rid of 



