32 Mr. W. Sturgeon on the Distribution oj 



position of the needle be such that its deflections will corre- 

 spond with the motion of the plate when rotatinf^ to the right, 

 its deflection will again correspond with the motion of the disc 

 when the latter is caused to rotate to the left; consequently 

 the deflections in the first case will be contrary to the deflec- 

 tions in the latter case. The same law will be observed when 

 the position of the needle is such that the deflections are op- 

 posed to the direction of motion given to the disc; for if the 

 needle travel towards the left whilst the disc revolves towards 

 the right, it will travel towards the right when the revolution 

 of the plate is towards the left ; manifesting in all cases that 

 when the exciting magnet is stationary, the direction of the 

 force which impels the needle entirely depends upon the 

 direction of motion given to the disc. 



This law being understood, we have next to contemplate 

 the direction in which any selected pole of the needle travels 

 whilst the disc is in motion ; and a little reflexion will make 

 it readily appear that in whichever of the two positions (Ex- 

 periments 16. and 17.) the needle may be placed whilst the 

 plate is at rest, it will exhibit a constant tendency to assume 

 some determined new position when the motion given to the 

 plate is in 07ie certain direction ; and as constant a tendency 

 to take up some other determined new position when the rota- 

 tion of the plate is reversed. 



To simplify this point still more, we will first suppose the 

 needle to be placed as in fig. 14^. Plate I., 5 « being respectively the 

 south and north poles. The needle in this position will travel 

 in the same direction as the disc revolves (Experiment 16.) ; 

 and when the revolution of the disc is in the direction indi- 

 cated by the large exterior arrow, the south pole of the needle 

 will be deflected towards the right of the exciting magnet. 

 Again: Let the needle be placed as in fig. \S. s n as before 

 being the south and north poles respectively. In this case the 

 needle will travel in the opposite direction to that of the re- 

 volving disc (Experiment 17.); but in this, as in the former 

 case (as will be observed by comparing the two figures), when 

 the disc revolves in one and the same direction, as indicated by 

 the large exterior arrows, the south pole of the needle has a 

 constant tendency towards the right hand, as is shown by the 

 small arrows pointing out the direction of its course. Hence 

 the new position for the south pole of the needle, determined 

 by the forces excited in the disc by its revolving in this parti- 

 cular direction, is evidently on the right side of the exciting 

 magnet, or to the right of an observer with the apparatus 

 placed before him, as in fig. l-i and 15. 



This point being ascertained, the needle is now to be 



