290 



Lord Elphinstone and C. W. Vincent. [Mar. 18, 



in place of being 5° will be 8°, and so on in proportion to the amount of 

 force made use of. 



None of the above-mentioned effects could be shown with the small 

 magnets under pressure : and it was not found possible to produce a 

 recognisable current without actual movement of the armatures. 



Under certain circumstances the attractive force of electro-magnets 

 in closed magnetic circuit is found to increase with lapse of time. For 

 example : — A small (J shaped electro-magnet with limbs 6 inches 

 long, having a core of f -inch iron, and helices consisting of four layers 

 of No. 16 covered copper wire, when excited by four Bunsen cells, 

 supported as an armature a similar (J " shaped iron bar, but without a 

 helix upon it, this latter remained firmly attached after the voltaic 

 current had ceased, but the hanging on to it of an additional weight 

 of 3 lb. 6 oz. instantly wrenched it away from the electro-magnet, 

 and broke the closed magnetic circuit. 



The magnet was then re-excited, the armature being fixed to the 

 electro -magnet by being held in contact with the poles whilst an 

 electric current, of a few seconds' duration, passed through the circu- 

 lating wire. In place of immediately attempting to add any additional 

 weight, the two iron \J 's were left hanging face to face, in the form of 

 the link of a chain, for twenty-four hours, at the end of which time 

 the weight of 3 lbs. 6 oz. was hung on and sustained. Forty- eight 

 hours later, an additional weight of 3 lbs. 10 oz. was carefully added, 

 making in all 7 lbs. sustained. Twelve hours afterwards 1 lb. more 

 was added, bringing up the entire weight to 8 lbs. beyond that of the 

 armature ; this was suffered to remain for five days, when the system 

 was taken to pieces. 



On a subsequent occasion the same magnet sustained an entire 

 weight of 10 lbs. beyond that of the U -shaped armature, the weight 

 sustained being reached by beginning with an. amount well within 

 the sustaining power of the electro-magnet wire in closed circuit, and 

 increasing it by small additions made with intervening intervals of 

 time varying from twelve hours to several days. 



Another and smaller \J magnet was likewise experimented on ; this 

 weighed with its coils 3 lbs. 6 oz. Its armature was a strip of soft 

 iron completely covering the poles, and having a hook in the centre, to 

 which weights could be easily attached. 



This electro-magnet was excited by the passage, for a few seconds, 

 of the current from two one-pint bichromate cells. On breaking 

 battery contact, the armature failed to sustain 4 lbs. The electric 

 current was again sent round the electro-magnet, and the armature 

 was pressed against the poles, being carefully adjusted so as to cover 

 them completely, and at the same time to place the hook precisely an 

 the centre, so that the pull should be fair and equal when a weight 

 was hung upon it. By this careful manipulation, on breaking con- 



