288 Mr. W. Baily on a Mode of 



In the other extreme case the number of pairs of electro- 

 magnets is reduced to two, and the number of batteries is also 

 reduced to two. Let the poles of one pair of magnets be called 

 a, af } and those of the other b, V , Then the arrangement of the 



pole as seen from above is y r, and the successive states of 



these poles will be 



1. 2. 3. 4. 5. 6. 7. 8. 1. 



NN ON SN SO SS OS NS NO NN 



SS SO SN ON NN NO NS OS SS 



N, S, representing north and south poles respectively, and 

 representing that the pole is not magnetic. It appears that 

 the change from 1 to 3 through 2 is nothing more than rever- 

 sing a af ; and the change from 3 to 5 consists in reversing 

 b V ' . Similarly we pass from 5 to 7 by reversing a a/, and 

 again from 7 to 1 by reversing b b' . The whole process is 

 thus shown to consist in reversing a a' and b V alternately. 



In passing from 8 to 1 we see by case (1) that the parts of 

 the disk over b, V are respectively repelled from a, a' \ and by 

 case (3) we see that the same parts are respectively attracted 

 to af, a. Again in passing from 1 to 2 we see by case (2) 

 that the parts above a } af are respectively attracted by b, V \ 

 and we also see by case (4) that the same parts are respec- 

 tively repelled by V b. 



The effect of each of these two changes is to make the disk 

 tend to rotate in the direction a b a! V . All the other changes 

 may be shown to have the same effect ; so that the disk will 

 rotate in the direction a b of V . 



If starting with the state (1) we reverse b V first, we should 

 have the series of states as follows : — 



1. 2. 3. 4. 

 NN NO NS OS 

 SS OS NS NO 



from which it may be easily shown that the disk would rotate 

 in the direction b a V af. 



The experiment with the four electromagnets may be readily 

 performed by means of a commutator which will reverse the 

 currents several times in a second ; and a considerable rotation 

 can be given to the disk. 



The commutator which I constructed for the experiment 

 consisted of a wheel of wood with a brass rim. This rim was 

 completely cut through in places equidistant from each other : 

 and ten tongues of thin copper pressed against the rim*. 



* The number of cuts must not be less than seven. I used eisrlit cuts. 



5. 



6. 



7. 



8. 



1. 



SS 



SO 



SN 



ON 



NN 



N T N 



ON 



SN 



SO 



SS 



