

1826.] MagnetismofMetatsfS^c. arising front their Rotation, 24§ 



Vice versdf if the plate be drawn along in its own plane, and 

 the magnet be free to move in a horizontal direction, the former 

 ought to drag the latter along in the same direction with a velo- 

 city continually accelerating, till they move on together with 

 equal velocities. 



It is manifest that, ceteris paribus, the greater the relative 

 velocity, the more will the pole developed in the plate lag; 

 behind the magnet, or the magnet (in the reverse case) behind 

 the pole. The more oblique, therefore, will be the action, and 

 the greater the resolved part of the force, and the velocity pro- 

 duced by it dato tempore. The same effect must also be produced 

 by an increase in the absolute force, or lifting power of the 

 magnet; so that in such experiments there is an advantage in 

 using large magnets which have great lifting powers, over small 

 ones with intense directive forces, and this is perfectly conso- 

 nant to experience. 



Hitherto we have only considered the case of rectihnear 

 motion. If we regard the magnetism of the plate as very tran- 

 sient, and the velocity moderate, the whole space occupied by 

 the magnetized portion of the plate will still be small, and 

 confined to the immediate neighbourhood of the point vertically 

 under the magnet. If the motion of the latter change its direc- 

 tion, the momentary pull communicated to the plate will always 

 be in the direction of a tangent to the curve described. If, 

 therefore, it describe a circle, it will tend at every instant to 

 impress a gyratory motion on the plate about a centre vertically 

 under the centre of its own motion, and vice versa, if the plate 

 be made to revolve about a centre, it will tend to drag the 

 magnet round with a continually accelerated motion, provided 

 its rectilinear recess from the centre of motion (or its centrifugal 

 force) be prevented by a proper mechanism. The former is the 

 case of a disc of copper suspended by its centre, and set in rota- 

 tion by a magnet revolving beneath it. The latter is that of 

 a compass-needle, or of our neutralised system of vertical mag- 

 nets suspended over a revolving disc of copper. A very pretty 

 illustration of the direction of these forces is obtained by 

 suspending a circular disc of zinc or copper from the end of a 

 counterbalanced arm, which is itself suspended by its middle, 

 thus constituting a kind of double balance of torsion. If the 

 length of the arm be so adjusted, that the circumference of the 

 disc shall be an exterior tangent to the circle described by the 

 poles of a revolving magnet, the whole disc will be swept round 

 in an orbit concentric with the motion of the magnet, while it at 

 the same time acquires a rotatory motion on its own centre ia 

 the contrary sense. The centrifugal force is here overcome by 

 the arm and the weight of the disc, and the velocity goes ort 

 accelerating till the increase of resistance puts a stop to further 

 accessions. 



