



? 



91.] On Variational Electric and Magnetic Screening. 423 



If the (triple compound) magnet inside is reversed, by means of the 

 central wire and cross bar outside, shown in the diagram, the magneto- 

 meter outside is greatly affected while the copper shell is at rest ; 

 but scarcely affected perceptibly while the copper shell is rotating 

 rapidly. 



< 9. When the copper shell is a figure of revolution, the magnetic 

 force at any point of the space outside or inside is steady, whatever 

 be the speed of rotation ; but if the shell be not a figure of revolu- 

 tion, the steady force in the external space observable when the shell 

 is at rest becomes the resultant of the force due to a fixed magnet 

 intermediate between M and I compounded with an alternating force 

 with amplitude of alternation increasing to a maximum, and ulti- 

 mately diminishing to zero, as the angular velocity is increased with- 

 rut limit. 



10. If M be symmetrical, with reference to its northern and southern 

 polarity, on the two sides of a plane through the axis of rotation, I 

 becomes a null magnet, the ideal magnetic matter in every circle of 

 which it is constituted being annulled by equal quantities of positive 

 and negative magnetic matter being laid on it. Thus, when the rota- 



is sufficiently rapid, the magnetic force is annulled throughout 

 e space external to the shell. The transition from the steady force 

 of M to the final annulment of force, when the copper shell is symme- 

 trical round its axis of rotation, is, through a steadily diminishing 

 force, without alternations. When the shell is not symmetrical round 

 axis of rotation, the transition to zero is accompanied with alter- 

 ations as described in 8. 



11. When M is not symmetrical on the two sides of a plane through 

 .e axis of rotation, I is not null ; and the condition approximated to 



ugh external space with increasing speed of rotation is the force 

 e to I, which is an ideal magnet symmetrical round the axis of 

 itation. 



12. A very interesting simple experimental illustration of screening 

 inst magnetic force may be shown by a rotating disc with a fixed 



et held close to it on one side. A bar magnet held with its 

 netic axis bisected perpendicularly by a plane through the axis 

 rotation would, by sufficiently rapid rotation, have its magnetic 

 >rce almost perfectly annulled at points in the air as near as may 

 to it, on the other side of the disc, if the diameter of the disc 

 :ceeds considerably the length of the magnet. The magnetic force 

 the air close to the disc, on the side next to the magnet, will be 

 erywhere parallel to the surface of the disc. 



2 F 2 



