288 



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



electro-magnet by the interaction, when it was held npright, its poles 

 being completely covered by a closely fitting armature. And it was 

 also found that when thus set up in preparation for the formation of 

 a closed magnetic circuit, the magnetization was produced by a much 

 slower motion of the large armature than when the small magnet had 

 its circuit partly open. When the circuit was completely closed, if 

 the large armature were twisted oh by a slow equable motion, in such 

 a manner that both poles were uncovered at the same time, then the 

 small magnet could be made to sustain not only its own weight 

 (between 5 and 6 lbs.) but an additional 3 lbs. also. 



During the passage of the electric current, obtained by the forcing 

 open of the closed circuit, the fall of magnetism in the large magnet 

 itself is checked, the direction of the magnetic polarity remaining un- 

 changed, the current checking or opposing the fall being in the same 

 direction as that from the battery which caused the primary mag- 

 netization. If the ends of the helix wires are not connected together 

 this effect is not obtained. 



Electric currents, though of less intensity and quantity, can be 

 produced in the helices of electro-magnets, without altogether break- 

 ing up the closed magnetic circuits. For instance, with the 58 lb. 

 electro-magnet, the circuit being completely closed by its armature, 

 and the helices being connected with a galvanometer, a very slight 

 pull applied to the armature produces a current of electricity giving a 

 considerable deflection of the needle in the same direction as the 

 battery current ; and the stronger the pull the greater the deflection 

 of the galvanometer needle, up to the point at which the magnet is 

 lifted from the ground, after which no further motion of the needle is 

 produced, unless the magnet is subjected to additional strain. Thus, 

 hanging a 4 lb. weight upon the uplifted magnet, produced deflections 

 in the same direction as the pull on the armature, and on removal of 

 the weight produced reverse deflections. 



Trying the same set of experiments with a very small electro- 

 magnet, so that we might proceed to absolute rupture of the closed 

 magnetic circuit without danger to the galvanometer, we found that 

 the addition of successive weights to the magnet while hanging sus- 

 pended by its armature, produced successive deflections of the 

 galvanometer, the needle coming to rest at zero after each addition, 

 as in the case of the large magnet. 



When the maximum weight which the magnet was capable of sus- 

 taining was reached, and a real movement of the armature com- 

 menced, the induced current in the helix of the electro-magnet was 

 very greatly increased by the addition of even the smallest weight. 



From these experiments it may be inferred that in like manner as 

 the passage of an electric current round a bar of iron produces 

 elongation of the bar, so the elongation of the bar produces in its 



