periment, each by its separate thread, 

 the induction of a simil ar magnetism upon 

 all of them will produce a mutual re- 

 pulsion among them, and they will of 

 course all diverge from one another. 

 But if the wires be made of steel, so as 

 to retain whatever magnetism may be 

 communicated to them, and a northern 

 polarity be given to the lower ends of 

 two of the wires, but a southern pola- 

 rity to the lower ends of the other two 

 wires, when each of these pairs is 

 kept apart, the wires will repel each 

 other ; but if both pairs are brought to- 

 gether, all the four wires will unite and 

 adhere together. The reason is that 

 those wires which have opposite polari- 

 ties attracting each other, unite to form 

 a pair in which the polarities are ba- 

 lanced, and the repulsion each had be- 

 fore exerted towards those which were 

 similar in the other pair, is now entirely 

 neutralized. The same thing will hap- 

 pen, however numerous are the pairs of 

 wires that are dissimilarly magnetized. 



(37.) In order that a bar of iron may 

 receive the combined inductive influence 

 of two magnets, it is not necessary that 

 they should all be situated in the same 

 line as in the example already given. 

 The same effect will result if the bar be 

 at right angles to the two magnets, as in 

 the following figure (18), provided the 



Fig. 18. 



MAGNETISM. ^ ^*?*^ V 



evident that an effect of an opposite kind 

 must result when the dissimilar poles of 



Fig. 19. Fig. 20. ^SlTTA * 



** * A. 



two ends be immediately acted upon re- 

 spectively by the poles of opposite deno- 

 minations of the magnets. The attrac- 

 tion of a bar in this situation is much 

 increased by the conspiring inductive in- 

 fluence of the two magnets ; and the 

 force exerted is more than double of that 

 by which it would have adhered to either 

 of the magnets when singly employed. 

 This may be verified by attaching the 

 scale of a balance to the bar (fg. 19,) 

 and adding weights till its adhesion to 

 the magnets is overcome : these weights 

 will be found to exceed the sum of the 

 weights which the two magnets would 

 have supported, by means of the adhe- 

 sion of the same iron bar, if they had 

 been applied separately. 



(38.) While such is the effect of the ap- 

 plication of the dissimilar poles of two 

 magnets to the ends of a bar of iron, 

 namely, that of conspiring to induce the 

 same kind of magnetism, it is likewise 



the magnets are both applied to the same 

 end of a bar. These poles, being of dif- 

 ferent kinds, will produce contrary ef- 

 fects ; their inductive influence will op- 

 pose, instead of assisting, each other, 

 and the magnetism induced on the bar 

 will be only that resulting from the dif- 

 ference, instead of the sum of their in- 

 tensities. If the bar be of some length, 

 and if the magnets be of equal strength, 

 and applied close to each other, their 

 actions upon the remoter parts of the 

 bar will be so nearly equal, that they 

 will almost entirely neutralize each 

 other, and no sensible degree of mag- 

 netism will be excited. Thus if while a 

 key is supported by a magnet as in fig. 

 20, we gradually bring down upon it a 

 second magnet, with its lower pole of the 

 opposite kind to the lower pole of the 

 first magnet, it will tend to induce in the 

 key a polarity of an opposite kind to 

 that which it has received from the first 

 magnet. In as far as it exerts that in- 

 fluence it diminishes this magnetism, 

 and consequently weakens the attrac- 

 tion. Another cause also operates in 

 diminishing the attraction. The polarity 

 induced upon the adhering end of the 

 key is of the contrary kind to that of the 

 pole of the first magnet; it is therefore 

 of the same kind with that of the second 

 magnet which is brought near it, and 

 which, therefore, as far as the key re- 

 tains its induced magnetism, must repel 

 it. Accordingly, it happens that when 

 the second magnet, if sufficiently power- 

 ful, is brought within a certain distance 

 from the upper end of the key, it de- 

 stroys its power of adhering to the mag- 

 net, and the key drops off. 



A similar counteraction of magnetic 

 induction will take place, when the other 

 pole of the second magnet, that is the 



