272 Prof. Tyndall on the Nature of the Force hij ivliicli 



wlien the arrangement was that shown in fig. 41, it was drawn 

 towards them. 



But it does not sufficiently express the facts to say that the 

 bar is capable of two different states of excitement ; it must be 

 added, that both states exist simultaneously in the excited bar. 

 We have already proved that the state necessary for the action 

 of one pole is not that which enables an opposite pole to produce 

 the same action; hence, when the two ends of the bar are 

 attracted or repelled, at the same time, by two opposite poles, it 

 is a proof that these two ends are in difi'erent states. But if this 

 be correct, we can test our conclusion by reversing one of the 

 poles; the direction of its force being thereby changed, it ought 

 to hold the other pole in check and prevent all motion in the 

 bar. This is the case : if, in any one of the instances cited, the 

 polarity of either of the cores be altered ; if the south be con- 

 verted into a north, or the north into a south pole, thus making 

 both poles of the same quality, the repulsion of the one is so 

 nearly balanced by the attraction of the other, that the bar 

 remains without motion towards either of them. 



To carry the argument a step further, let us fix our attention 

 for an instant upon fig. 40. The end of the bar nearest to the 

 reader is repelled by a south pole ; the same end ought to be 

 attracted by a north pole. In like manner, the end of the bar 

 most distant from the reader is repelled by a north pole, and 

 hence the state of that end ought to fit it for attraction by a 

 south pole. If, therefore, our reasoning be correct, when we 

 place a north pole opposite to the lower end of the bar, and on 

 the same side of it as the upper north pole, and a south pole 

 opposite the upper end of the bar and on the same side of it as 

 the lower south pole, the simultaneous action of these four poles 

 ought to be more prompt and energetic than when only two poles 

 are used. This arrangement is shown in Plate V. : the two 

 poles to the right of the bismuth bar must be of the same name, 

 and the two to the left of the bar of the opposite quality. If 

 those to the right be both north, those to the left must be both 

 south, and vice versa. The current reverser for the magnets 

 appears in front, that for the helix is hidden by the figure. The 

 above conclusion is perfectly verified by experiments with this 

 apparatus, and the twofold deflection of the bismuth bar is ex- 

 hibited with remarkable energy*. 



The bar used in these cases is far heavier than those commonly 



* These experiments, ami almost iill the others mcutioiied in this memoir, 

 may be exhibited in tlie lecture-room. By attaching indexes of wood to 

 the bars of bismuth, and protecting; tlie indexes from aii- currents by glass 

 shades, the motions may be made visible to several hiuidreds at once. See 

 a description of a Polyinagnet, Phil. Mag. June 1855. — J. T. 



