June 18,1875.] bVO [Cresson. 



THE EFFECT OF MAGNETIC AND GALVANIC FORCES UPON 



THE STRENGTH OF, AND DESTRUCTION OF IRON AND 



STEEL STRUCTURES. 



By Charles M. Cresson, M.D. 



(Bead before the American PMlosopMcal Society, June 18, 1875.) 



Bai's and Structures of Iron and Steel when allowed to remain at rest 

 for a considerable time acquire measurable magnetic polarity. 



Moderate percussion, alternations of heat and cold, exposure to the 

 rays of the sun, especially with a long axis of figure parallel, or nearly 

 coincident with a magnetic meridian of the earth, have a tendency to de- 

 velop and strengthen magnetic polarity. 



Thus, Iron Bridges, Iron Vessels upon the stocks in ]3rogress of con- 

 struction, and Iron Railway Tracks are particularly liable to acquire 

 magnetic polarity. 



It is asserted that the relative position of the long axis of Iron Ships 

 with reference to the magnetic meridian materially affects their polarity 

 and the facility of the correction of their compasses. 



If the keels of such vessels be laid on a North and South line, they are 

 supposed to acquire greater polarity, and to retain it more steadily than 

 when laid East and West. 



The evidence of an iron ship's polarity is exhibited to the greatest de- 

 gree, by comparison of its effects upon its comjDasses when the vessel is 

 sailing in an easterly or westerly direction. 



A consideration of the following facts seems to favor the conclusion 

 that magnetic bars of Iron should be better able to resist tensile strain 

 than those which are not magnetic. 



A thoroughly magnetic bar is one of which each end repels a pole of a 

 magnetic needle. The centre of such a bar is neutral, that is attracts 

 either end of a magnetic needle and repels neither. 



If we break such a bar in half, we are possessed of two magnetic bars ; 

 that end of the original bar which attracted the south end of a magnetic 

 needle continues to attract it, that which attracted the north end continues 

 to do so, whilst the two new ends which had formed the neutral centre of 

 the original bar, each acquires a polarity opposite to the other, and also 

 opposite to that possessed by its own opposite end. A continuance of this 

 process, that is, the fracturing of each half until we have obtained such 

 minute fragments of the bar as can be examined only under the micro- 

 scope, still produces perfectly polarized bars, possessing all of the mag- 

 netic characteristics of the original bar, with varying, attracting, and 

 repelling force according to some ratio of the relative length and thick- 

 ness of the fragments. 



Arguing upon this, we are led to the conclusion that a continuance of 

 this process must produce molecular magnets. 



If we place magnetic bars in contact with each other, the north and 

 south poles alternating and in contact with each other, we obtain a metallic 

 A. p. S. — VOL. XIV. 3z 



