1891.] on Electric arid Magnetic Screening. 355 



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

 wire and cross bar outside, shown in the diagram, the magnetometer 

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

 scarcely affected perceptibly while the copper shell is rotating 

 rapidly. 



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 revolution, 

 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 inter- 

 mediate between M and I compounded with an alternating force with 

 amplitude of alternation increasing to a maximum, and ultimately 

 diminishing to zero, as the angular velocity is increased without 

 limit. 



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- 

 tion is sufficiently rapid, the magnetic force is annulled throughout 

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

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

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

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

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

 nations as described above. 



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

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

 through external space with increasing speed of rotation is the force 

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

 rotation. 



A very interesting simple experimental illustration of screening 

 against magnetic force may be shown by a rotating disc with a fi.xed 

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

 magnetic axis bisected perpendicularly by a plane through the axis of 

 rotation would, by sufficiently rapid rotation, have its magnetic force 

 almost perfectly annulled at points in the air as near as may be to it, 

 on the other side of the disc, if the diameter of the disc exceeds con- 

 siderably the length of the magnet. The magnetic force in the air 

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

 parallel to the surface of the disc. 



[W. T.] 



