Construction of Galvanic Magnets, 125 



the magnet, then, which will diminish the weight of A, to that 

 quantity which shall be sufficient for the conduction of the mag- 

 netic power between the poles and enable them to react upon 

 each other, will prove of great advantage in the construction of 

 these magnets by diminishing the weight of iron while this 

 power remains the same. This has been partially attempted by 

 Mr. Davenport, who made the iron in his magnets rather thinner 

 near the center, and heavier at the poles. But the same effect 

 may be much more advantageously produced by giving the iron 

 the form seen in Fig. 2. Two straight pieces of iron, 

 B and C, are welded to a smaller flat bar A, which *=' ' 



is large enough to conduct the inductive effects of 

 the two extremities upon each other, and by being 

 much lighter than B and C, lessens the weight of 

 the whole magnet. There are several advantages 

 resulting from this form. The sides, B and C, be- 

 ing straight, are capable of being wound more evenly and more 

 compactly ; for in curved magnets, the winding upon the outer 

 or convex side, must necessarily be more loose than that upon 

 the inner side of the iron, and upon this account a greater quan- 

 tity of wire can be put upon the same weight of iron when 

 straight. 



That the attractive force of a magnet is increased by the re- 

 action of the two extremities upon each other, may be easily 

 proved by the following experiment. Let 

 AB, CD, Fig. 3, be two magnets, having y, 



D 



the poles A and C of different names. The 



attractive force of the lower extremities will 



be increased if A and C be joined by a bar 



of soft iron E. But this increase will by no b 



means equal the sura of the separate action 



of the four poles, A, B, C, D, or in other words, A will not impart 



to D a force equal to that which it exerts upon the bar E, nor in 



like manner will C impart an equal force to B. So that the 



greater the active surface, the greater must be the power of the 



magnet. 



Again, although the magnetic power of the parts B and C in 

 the galvanic magnet Fig 1, is conveyed by conduction through 

 the parts D and E, yet as the conducting power of the softest iron 

 is very imperfect, the force exerted by the parts B and C through 

 D and E, is very far short of that which they would be able to 



