246 Prof. Thomson on the Forces experienced by inductively 



which has been described, the ball is allowed to move with great 

 freedom in a straight line, but prevented from all other motion. 



11. In making the experiments described below, it is con- 

 venient to have two stops so arranged that the motion of the 

 arm may be kept within any desired limits, and manageable 

 in such a way, that by means of them the arm may be rapidly 

 brought to rest in any position. In general, before com- 

 mencing an experiment, the arm ought to be brought to rest 

 near one end of its course, and kept pressing very slightly 

 upon one of the stops by the torsion of the wire, which may 

 be suitably adjusted by the torsion- head, and the other stop 

 ought to be pushed away, so as to leave the arm free to move 

 in one direction. 



12. 'Experiment 1. — Place a common bar-magnet with either 

 pole, the south, for instance, near the ball of soft iron in its 

 line of motion, but on that side towards which it is prevented 

 from moving by the stop. Taking another bar-magnet of 

 considerably greater strength than the former, bring its north 

 pole gradually near the fixed south pole of the other, in the 

 continuation of the line of motion of the iron ball. When 

 this north pole reaches a certain position the arm will cease 

 to press on the stop, and if we push the north pole a little 

 nearer still, the arm will altogether leave the stop and take a 

 position of equilibrium, in which, after it is steadied (as may 

 easily be done by means of the stops), it will remain stable, 

 although the stops be removed entirely. If, by means of one 

 of the stopsj the ball be pushed to any distance farther from 

 the magnets than this position of stable equilibrium, it will 

 return towards it when left free. If it be drawn a little nearer 

 by means of the other stop, and, when left for a few seconds, 

 it be found to continue pressing upon the stop, then, when 

 the stop is removed, the ball will return to that position of 

 stable equilibrium. If, however, it be very slowly drawn still 

 nearer the magnets, when it reaches a certain position it will 

 cease to press on the stop ; and if after this it experience the 

 slightest agitation, or if it be drawn any nearer, it will leave 

 the stop and move up till it strikes the nearer magnet, in con- 

 tact with which it will almost immediately come to rest. It 

 thus appears that there is a position of unstable equilibrium 

 for the ball between the former stable position and the nearer 

 magnet. It is easy to arrange the torsion-head so that the 

 torsion of the suspending-thread or wire may have as little 

 effect as we please, by finding, by successive trials, either of 

 these positions of equilibrium, subject to the condition that, 

 when the magnets are removed, the torsion would not sensi- 

 bly disturb the arm from the position so found. 



