176 Transactions of the American Institute. 



the center are yet more curved, and, following their direction, they 

 lead us to points on the other end of the magnet, corresponding to 

 those from which ihey emanated. But they all are curved, and have 

 their origin in the axis of the magnet, and return to points on the 

 other side of its center, symmetrically placed with those from which 

 they emanated. 



Viewing these lines, we are immediately struck with their analogy 

 to the rays of .light radiating from a luminous point, and would, there- 

 fore, naturally infer that the force expands itself in like manner, and 

 produces less and less effect on one and the same surface as we recede 

 from the magnetic pole. Experiment shows the truth of this conclu- 

 sion ; for we find that, when we quickly cut across these lines, always 

 making one and the same sweep, the force of the electric current in 

 the wire diminishes as we recede from the pole, just as the brightness 

 of a light diminishes as we recede from it. But, in the case of light, 

 if the rays are parallel, the brightness remains the same, whether we 

 are at one foot, a thousand feet, or several miles from the point of 

 origin ; so, in the case of the magnetic lines, if they are parallel, the 

 magnetic intensity, as evinced in the electric current, remains the 

 same at all distances. Also, if the rays of light converge, the bright- 

 ness increases as we approach their point of confluence ; so with the 

 magnetic rays, their magnetic effect on one and the same body increases 

 as we near the point of their convergence. Moreover, if a beam of 

 light of parallel rays has twice the diameter of another (from the 

 same source), then the quantity of light in the first will be four times 

 that in the second beam ; likewise is this the case with a bundle of 

 parallel rays emanating from the magnet, a beam of twice the diame- 

 ter of another giving a fourfold quantity of electricity. Thus is com- 

 pletely established an analogy between these magnetic lines and the 

 rays of light ; a fact easy to remember, and of great significance in all 

 reasonings and researches on magnetism. But we can establish the 

 main facts I have given for ourselves ; and this I now propose to do 

 by experiment. 



However, before we can attempt these experiments, it is necessary 

 that we should clearly understand the experimental method by which 

 we detect the presence of a current of electricity in a wire, deter- 

 mine what we call its direction, and ascertain its strength. All of 

 these facts, of such fundamental importance for our further progress, 

 are obtained by observing the deflections which a magnetic needle 

 experiences whenever a wire carrying one electric current is brought 

 in its vicinity. This most important discovery was made by CErsted, 



