208 



Prof. J. A. Ewing on the Molecular 



when an external force is applied. The model is very easily 

 made. Each magnet is a piece of steel wire about one tenth 

 of an inch in diameter and two inches long (fig. 1), bent in 



Fiir.T. 



the middle to bring the centre of gravity below the point of 

 support. The hole, or rather recess, for the pivot is made by 

 a centre punch : the pivot itself is a sewing-needle fixed 

 upright in a small base-plate which is punched out of a sheet 

 of lead. The bars swing with but little friction, and their 

 pole-strength is sufficient to make the mutual forces quite 

 mask the earth's directive force when they are set moderately 

 near one another. The group is arranged on a board on 

 which lines are drawn to facilitate regularity in grouping 

 when that is wanted, and the board slips into a large frame 

 or open-sided flat box wound round the top, bottom, and two 

 sides, with a coil through which an adjustable current may be 

 passed to expose the group to a nearly homogeneous external 

 magnetic force. The coil is wound in a single very open 

 layer, through which a sufficiently good view of the group 

 inside is obtained *. A liquid rheostat with a sliding terminal 

 is used to secure continuity in varying the magnetic force. 

 It is scarcely necessary to add that the magnetic force which 

 is applied to the group is too weak to have any material effect 

 on the magnetism of individual bars. It alters their align- 

 ment only, just as a magnetic force alters the alignment of 

 Weber's molecular magnets. 



When a number of these magnets are grouped either in a 

 regular pattern or at random, and are left after disturbance 



* In showing the experiments, the board with the magnets on it may, 

 of course, be placed in clear view abo T , e the coil ; the coil is then made 

 larger, or a stronger current is used. 



